Notes: The Art of Stargazing Month 4: May - June 2013 · -6-The Art of Stargazing – Month 4...

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Notes: The Art of Stargazing

Month 4: May - June 2013

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The Art of Stargazing – Month 4

Contents – The Art of Stargazing (Month 3)

What You Will Learn This Month .................................................................................................. 4

Science of Astronomy – Stars, Part II ............................................................................................. 4

Overview ..................................................................................................................................... 4

How Stars Form .......................................................................................................................... 5

The Structure of a Star ................................................................................................................ 8

Star Colors and Temperatures ..................................................................................................... 9

Stellar Classification ................................................................................................................. 13

The HR Diagram ....................................................................................................................... 14

Tour of the Night Sky – Virgo, Corvus, Crater, Hercules ............................................................ 17

Overview ................................................................................................................................... 17

The Constellation Virgo ............................................................................................................ 18

The Constellations Corvus and Crater ...................................................................................... 20

The Constellation Libra ............................................................................................................ 20

The Constellation Hercules ....................................................................................................... 21

The Constellation Draco ........................................................................................................... 23

Observing Techniques – Telescopes ............................................................................................. 24

Overview ................................................................................................................................... 24

General Guidelines.................................................................................................................... 25

Refracting Telescopes ............................................................................................................... 26

Apochromatic and ED Refractors ............................................................................................. 29

Newtonian Reflecting Telescopes (and Dobsonian Telescopes) .............................................. 31

Schmidt-Cassegrain Telescopes................................................................................................ 34

Maksutov-Cassegrain Telescopes ............................................................................................. 36

Summary of Telescope Pros and Cons ..................................................................................... 39

Solar System Observing – Saturn ................................................................................................. 40

Overview ................................................................................................................................... 40

Structure and Composition ....................................................................................................... 41

The Ring System ....................................................................................................................... 44

Moons ....................................................................................................................................... 45

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Observing Tips for Saturn ......................................................................................................... 48

The Deep Sky This Month ............................................................................................................ 50

Overview ................................................................................................................................... 50

Porrima ...................................................................................................................................... 51

M104 (“Sombrero Galaxy”) ..................................................................................................... 51

M87 ........................................................................................................................................... 53

Markarian’s Chain .................................................................................................................... 55

NGC 4216 ................................................................................................................................. 57

M13 (“Great Cluster in Hercules”) ........................................................................................... 57

M92 ........................................................................................................................................... 59

Alpha Herculis .......................................................................................................................... 60

NGC 6543 (“Cat’s Eye Nebula”) .............................................................................................. 61

What You Have Learned This Month ........................................................................................... 63

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What You Will Learn This Month In this fourth month of The Art of Stargazing, we continue to look at the workings of stars.

We’ll look at the overall structure and composition of stars and how they form from tenuous and

massive clouds of cold gas and dust. Then we’ll discover how astronomers analyze this light to

figure out the temperature and size of stars. And since astronomers, like all scientists, love to

classify things, we’ll learn about the basic stellar classification system and how it’s linked to

temperature and color. After you go through this section, you’ll be able to look at any star in the

sky and estimate its temperature, mass, and state of evolution.

In the sky tours this month, we continue our tour of the relatively star-sparse constellations of

northern spring. We’ll look at the zodiacal constellations Virgo and Libra, as well as the tiny

constellations Corvus and Crater. And we’ll look at the majestic constellations Hercules and

Draco. As for the deep sky, we continue with a few of the finest galaxies visible in a small

scope, as well as two excellent globular clusters and a young planetary nebula. Where possible,

you’ll see images that will help you know what you’ll see visually in a small telescope.

And speaking of telescopes… this month we take a detailed look at the main configurations of

telescopes available to amateur astronomers. This section will help when choosing a telescope,

or when simply following ads and articles about telescopes in the astronomy magazines. You’ll

also know the strengths and weaknesses of each telescope design, and you’ll be able to choose

the best telescope to suit your personal situation.

We also take a detailed look at the planet Saturn. The ringed planet is still well positioned for

viewing through June and July this year, so now is the time to examine the planet closely. You

will learn how to use a telescope to see this planet, a planet which turns many casual stargazers

into dedicated amateur astronomers.

Let’s get into it!

Science of Astronomy – Stars, Part II

Overview

Last month, you journeyed briefly into the world of nuclear physics to learn the answer to the

ancient mystery of why stars shine. You learned about nuclear fusion, the amazingly efficient

process by which hydrogen is transmuted into helium in the core of a star, and how it releases

energy according to Einstein’s famous equation: E=mc2. And you learned that our Sun, a mid-

sized star, has enough hydrogen to shine for some 10 billion years.

This month, we continue our look at the science of stars to pick up practical details of why stars

shine with different colors and have vastly different lifetimes. And you’ll learn about a powerful

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tool astronomers use to classify stars according to their mass, temperature and age. Once you

work through this material, you will see stars in a new way, with a clear understanding of how

they work and some of their key properties. Rather than taking the fun and mystery out of

stargazing, this knowledge will help you enjoy it that much more.

How Stars Form

In our galaxy, the interstellar medium—the space between stars— contains billowing clouds of

gas and dust that drift and rotate around the galactic center. The concentration of gas and dust in

the interstellar medium (ISM) is very low, more than a hundred trillion times less dense that

Earth’s atmosphere. But since the galaxy is so large, there is billions of Suns worth of mass in

the space between the stars.

A dark cloud of cold gas and dust which harbors new star formation

The gravity from passing stars and star clusters causes pressure waves to move through the gas

of the ISM, which results in denser clouds of gas in some regions compared to others. These

dense regions themselves, because they contain more mass, have more gravity. So they slowly

pull in more gas over the course of millions of years and grow denser and more massive still.

These growing gas clouds start out cold, but they begin to heat up as the gas particles move

faster under the influence of the cloud’s increasing gravity. In this way, gravitational energy is

turned into heat.

The gas clouds which come together in this way have enough mass to form thousands of stars

and span hundreds of light years. Within the cloud, however, smaller regions about a light-year

across and with the mass of 10-100 sun-like stars themselves collapse, grow denser, and heat up.

These smaller regions are called Bok globules. In time, the centers of these globules can grow to

temperatures of thousands, then millions, of degrees. If a globule has enough mass, its core gets

hot enough to begin the process of nuclear fusion. An enormous amount of energy is then

released, enough to push back against the force of gravity. The cloud stops collapsing and

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instead begins to radiate infrared and visible light, and pushes away much of the remaining gas

and dust. During this stage, which lasts perhaps 100,000 years, the nascent star is called a

protostar. In many cases, a disk of cooler gas and dust remains around the star. Planets,

asteroids, and comets may form within this disk. At first, as star’s nuclear furnace settles down,

the star’s brightness may vary. But over hundreds of thousands to millions of years, the star

settles down into an adult stage where it burns its nuclear fuel steadily.

Astronomers have shown with computer models that these globules of collapsing gas and dust

which form stars may break into two or three associated clouds, each of which forms a separate

star. This explains why the majority stars in the Milky Way are paired or in groups of multiple

stars.

Bok globules within an emission nebula

In many cases, a gas cloud collapses but contains insufficient mass to generate heat for nuclear

fusion. The result is a failed star. Lower-mass clouds instead form planets of one to up to

thirteen times Jupiter’s mass. Such planets are gas giants and radiate more energy than they

receive. But they are not stars. Slightly more massive objects, which again radiate energy but do

not sustain nuclear fusion, are called brown dwarfs. Such objects were considered by

astronomers since the 1960’s, but were only first observed in 1994. Brown dwarfs live in the no-

man’s land between planets and stars. None are visible in amateur telescopes.

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A simplified view of star formation (credit: HowStuffWorks.com)

As little pockets of stars form, their energy sets gas in the larger cloud aglow. Light from the

stars knocks away the electrons from the protons in the hydrogen atoms in the larger cloud. As

the electrons cool down and recombine with the protons, they radiate light at characteristic

wavelengths, especially at 656 nm (red light) and 486 nm (blue-green light). We see this pink

and green light from the large glowing clouds of forming stars. The Orion Nebula is one of the

most famous examples of a gas cloud set aglow by new stars forming within. These clouds are

called emission nebulae because they emit light at a characteristic wavelength. They are also

sometimes called by a more evocative name: stellar nurseries. In time, as the newly formed

stars in the gas cloud push away the remaining gas and dust, a group of dozens or hundreds of

young stars remain behind as an open star cluster. The Beehive and Hyades, which you have

met already, are examples.

An emission nebula in which hundreds of newly formed stars cause hydrogen gas to glow

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The Structure of a Star

Most of the energy production through nuclear fusion in a star occurs in its core. This central

region has a radius of 1/20 to 1/10 of the star’s total radius. The core has a temperature of 10-20

million K or more and is more than ten times as dense as lead, so we can’t see it directly. Nor

can we see the blazing hot radiative region around the core which is full of hot protons and

electrons and light. The interior of a star is so dense, in fact, that it takes light more than 50

million years of bouncing around inside a star before it makes it to the surface and escapes into

space. The energy from the Sun falling onto your vegetable garden was produced in the core of

the Sun millions of years ago!

In some stars, the radiative region is surrounded by a convective region which churns material

from deep in the star out towards the surface. In the outer regions of the convective zone, the

temperature and pressure of the hot gas drops enough for light to quickly escape into space. The

thin outer skin of glowing gas in all stars is called the photosphere, and it’s the part of a star we

can see directly. The pressure is lower in the photosphere and so is the temperature. While the

interior of a star has a temperature of millions of degrees, the photosphere of a star is typically

between 4,000 K and 20,000 K. The photosphere of our Sun has an average temperature of

about 5,700 K.

(Note: The symbol “K” refers to Kelvin, which is the scale astronomers use to measure

temperature. A difference in temperature of 1 K is the same as the difference of 1 oC. But in the

Kelvin scale, water freezes at 273 K, which is 0 oC. Absolute zero, when matter is as cold as it

can ever get, is at 0 K or -273 oC. At high temperatures, Kelvin and Celsius are about the same:

1,000,273 K is the same as 1,000,000 oC, for example).

The structure of a Sun-like star

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Star Colors and Temperatures

Last month, you read about the great prediction of August Comte, who said in 1835 that the

composition of stars in an example of knowledge forever beyond our reach. By the 1860’s, he

was proven wrong, as astronomers began to analyze the light from stars at a range of different

colors by, for example, passing a star’s light through a prism. This is the science of spectroscopy.

Almost everything we know about stars comes from measuring their spectra, which is a measure

of how much light of each color comes out of a star.

In the late 19th

and early 20th

centuries, at the same time astronomers were measuring the spectra

of stars, physicists were learning about the spectra of glowing bodies of solid matter in the

laboratory. They found the temperature of a glowing lump of material in the lab was related to

its temperature. Hotter material glowed at shorter wavelengths than cooler material. Material

that glowed mostly red for example, was cooler than material that glowed yellow, which in turn

was cooler than material that glowed white when heated. And material heated to glow at blue

wavelengths was the hottest of all. In time, physicists could link the spectrum of glowing

material in the lab with its temperature. The image below gives you an idea of how this all

works. Glowing material of temperature 7000 K has a spectrum that peaks at about 450 nm

(blue). Material of temperature 5,000 K peaks at about 550 nm (yellow). And material of

temperature 3,000 K peaks at infrared wavelengths of about 1000 nm. Infrared is light that is

beyond the red, so it has wavelengths longer than red light.

Don’t be too confused by these ideas. They are probably familiar to you to some extent. A

glowing piece of iron can be “red hot”, for example, when heated in a forge. If it gets even

hotter, it can be “white hot”. If it could get hotter still, it would be “blue hot”, though most

metals will vaporize before they start glowing blue.

Examples of how much light is emitted from heated material at four temperatures.

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The black line shows the spectrum of the Sun; the green line shows the spectrum of an ideal

glowing body of solid material at a temperature 5777 K

As another example, the spectrum below compares the spectra of three stars, the hot star Spica in

the constellation Virgo, our Sun, and the cool but very large red star Antares in the constellation

Scorpius. Remember the temperatures here refer to the photospheres of the stars, the visible

surfaces, not the cores.

Now you can head outside and look for the color differences between stars and understand what

it means. Red stars are the coolest, with a surface temperature of about 3500-4,000 K. The blue

stars are the hottest with temperatures of 20,000K or more. And in between are the white stars

with temperatures of about 10,000K. In the case of white stars, their spectra peak in the green at

about 500 nm. But they also emit light at other wavelengths… in the red, blue, and so on… and

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their combined light appears white to our eyes. So there are stars that emit a lot of green light,

but you need a spectroscope to separate the green from the other colors.

So that’s how astronomers use spectroscopy to reveal the temperature of stars. But spectroscopy

can also reveal the chemical composition of stars. Here’s how…

As you’ve seen already, solid objects, or very dense balls of hot gas like stars, will emit light

continuously at a range of wavelengths (see images above). The brightness of the light changes

over wavelength depending on the temperature of the material, but there are no gaps in the

spectrum… it is continuous. All hot dense objects of the same temperature emit the same kind of

light. It’s impossible to tell the composition of the material by looking at the spectrum.

With cooler and more tenuous gases, it is different. The great German scientists Gustav Kirchoff

and Robert Bunsen (of Bunsen-burner fame) discovered that when tenuous gases are heated (in a

flame, for example) they emit light only at very specific wavelengths that are completely unique

to the type of material in the gas. They also discovered that when continuous light passes

through cooler gas, the gas absorbs light at very specific wavelengths that are completely unique

to the type of material in the gas, and at the same wavelengths at which the gas emits light when

heated or burned in a flame. The image below shows you the three situations: solid hot body,

solid hot body shining through cooler less dense gas, and cooler gas emitting light itself.

Continuous spectrum from a hot solid body (top); specific wavelengths emitted by a cooler

glowing gas (middle); and specific wavelengths absorbed by a cooler gas (bottom)

Here’s an example of emission spectra of five elements. Each element always sends out the

same wavelengths.

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Light emitted by hydrogen, sodium, helium, neon, and mercury gas

And here’s absorption and emission spectra of hydrogen… see how the wavelengths of emitted

light and absorbed light are the same? This is the “fingerprint” of hydrogen. Each element has

its own spectral fingerprint.

Absorption and emission spectrum of hydrogen gas

Once again, these lab-based measurements in the 1860’s and 1870’s were timely, because

astronomers were measuring spectra of the Sun, stars, and even some nebulae. What they found

was amazing... these celestial objects had the same spectral “fingerprints” as Earthbound

elements, especially hydrogen, with traces of iron, sodium, and so on. They even found a new

element that was not yet identified on Earth. It was named helium, after the Sun, and it wasn’t

discovered on Earth until 1895.

Here’s a simplified version of the spectrum of the Sun, with identification of each of the spectral

“fingerprints”. As you can see, while the Sun and most stars are made mostly of hydrogen, there

are traces of heavier elements such as iron, oxygen, and sodium.

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The spectrum of the Sun, with absorption lines from heavier elements identified

Stellar Classification

As astronomers measured more and more spectra of stars, they started to notice some similarities

and differences. Some stars had very strong signatures of hydrogen, some not. Some had lots of

heavier elements like iron, magnesium, some not. And so on.

Scientists love to classify things, so they began to classify stars by the strength by which

hydrogen gas absorbed light at particular wavelengths. A group of astronomers at Harvard

University in the first years of the 20th

century developed a system in which stars were labeled

consecutively from type A to N in order of decreasing hydrogen absorption.

As astronomers and physicists learned more about atoms through the 1920’s and 1930’s, they

discovered the Harvard classification system really just described the temperature of a star’s

atmosphere. They discovered the O-type stars are hotter than B-type stars, and B-type stars are

hotter than A-type stars, and so on. So they reordered and simplified their star classification

system to, in order of hotter to cooler: O, B, A, F, G, K, and M. This is the Harvard spectral

classification system, which is still used today. Here’s a summary of the classification system

according to temperature and spectrum…

Spectral Type Temperature (K) Color H Absorption Examples

O 30,000-60,000 blue weak Alnitak, Mintaka

B 10,000-30,000 blue-white medium Rigel, Spica

A 7,500-10,000 White strong Vega, Sirius

F 6,000-7,500 white medium Procyon, Canopus

G 5,300-6,000 yellow-

white

weak Capella, Sun

K 4,000-5,300 yellow-

orange

very weak Arcturus

M 2,500-4,000 orange-red very weak Barnard’s star, Antares

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For your interest, the image below shows you sample spectra of each type of star. You don’t

need to remember these, but it gives you an idea of what’s going on with the strengths of the

hydrogen absorption.

Absorption spectra of each main type of star

Within each type, there are finer subdivisions from 0-9. So there are G5-type stars, O9-type

stars, M2-type stars, and so on. As we move through the tours of the night sky, you will start to

hear the spectral types of each star so you can understand the nature of these stars.

It may seem like a lot to remember. But there is a famous mnemonic astronomy students have

used for years to remember O-B-A-F-G-K-M in order of hotter to cooler. It is “Oh Be A Fine

Girl (or Guy), Kiss Me”. Or my personal favorite alternative: Only Bored Astronomers Find

Gratification Knowing Mnemonics. Try it… it works!

The HR Diagram

Now let’s look at perhaps the most famous tool used by astronomers to understand the stars.

Around 1911-1913, the Danish astronomer Ejnar Hertzsprung and American Henry Norris

Russell were studying star clusters in which all the stars are roughly the same age and distance,

and they noticed a clear and surprising relationship between the brightness and

temperature/color/spectral class of the stars in the clusters. When they plotted the brightness and

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temperature of each star on a graph, Hertzsprung and Russell came up with something that looks

like this:

As you can see, the stars are not randomly scattered here. Most stars lie on a narrow band from

upper left to lower right, which means there is a very strong relationship between the brightness

and color (or temperature and spectral class) of many stars. Brighter stars tend to be bluer and

hotter, while fainter stars were redder and cooler. There were a few stragglers that defy this

relationship, but most stars lie along this band, which is called the main sequence.

Although Hertzsprung and Russell didn’t know it at the time, stars on the main sequence are

stars in their youth and middle age during which they burn hydrogen in their cores. All stars

spend most of their lives on the main sequence. At one time, astronomers believed stars evolved

along the main sequence, moving from hotter to cooler as they expelled energy over their

lifetimes. But this is not how it works. Once a star “turns on” by burning hydrogen through

nuclear fusion, it settles onto a particular spot on the main sequence and stays there until the

hydrogen runs out.

This famous diagram is now called the “Hertzsprung-Russell diagram” or simply the “H-R

diagram” after its inventors.

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Here’s a more graphical version of the HR diagram that includes many well-known stars,

including our Sun. The spectral types are across the bottom axis, and the brightness of the stars

is on the vertical axis along the left side. In this version of the diagram, brightness is spelled out

such that our Sun has a brightness of 1.0, and the spectral types are according to O-B-A… etc.

as well as temperature. Brightness can also be expressed in terms of absolute magnitude (which

you learned about in Month 3). As you can see, our Sun is a G-type star on the main sequence,

the star Bellatrix in Orion is a B-type star, and the star Sirius is an A-type star, also both on the

main sequence. Bellatrix has a brightness some 6,000 times that of our Sun.

Another view of the HR diagram showing many well-known stars

What causes a star to be at a particular place on the main sequence on the HR diagram?

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In a word… mass. The mass of a star when it first turns on governs the temperature of the core

and how fast it burns its fuel. More massive stars have hotter cores and so burn their fuel much

faster than less massive stars. Although they have more mass to burn, the burn rate is even

greater, so the most massive stars have relatively short lives. If you have 10x more money than

your neighbor, for example, but you spend it 100x faster, you will run out of money before he

does. The bright blue star B-type Bellatrix has 8x the mass of our G-type Sun, but it will only

live for about 100 million years, about 100x less than the expected lifetime of our Sun. Smaller

M-type stars have perhaps 1/10 as much mass as our Sun, but astronomers calculate they may

burn for 200 billion years!

Now as you can see from both H diagrams above, there are stars that are red but also extremely

bright. They are not on the main sequence. That’s where it gets interesting. These stars are

evolved stars, stars which have stopped burning hydrogen in their cores and which are now

burning heavier elements like helium, carbon, and so on. They are also called late-stage stars

because they are late in their lifecycles and are on the way to their demise. These stars have also

swollen in physical size as they burn heavier elements, and for that reason, they are called giants

or even supergiants. Next month, we’ll have a look at these stars and examine how all stars

move towards the end of their lives.

But now, let’s apply the understanding you’ve gained by having a look at some interesting stars

in the night sky this month.

Tour of the Night Sky – Virgo, Corvus, Crater, Hercules

Overview

Let’s continue a look at the relatively sparse skies of northern-hemisphere spring. Leo, which

you met last month, moves past the meridian by nightfall in late May and June. Böotes and

Corona Borealis are nearly overhead, as is the grand dipper-shape of Ursa Major. The faint stars

of Hydra snake across the sky low in the south. And for observers south of 35oN latitude, a few

of the stars of the far-southern constellation Centaurus show themselves just above the southern

horizon.

This month, we look eastward to the large but dim constellation Virgo, below which lie the two

smaller constellations Corvus and Crater. And you get a first look at the northern summer

constellations Libra, Hercules, and Draco. While none of these constellations have many bright

stars, they do hold a number of excellent deep-sky objects for a small telescope, so it helps to

know how to navigate this part of the sky.

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The Constellation Virgo

The constellation Virgo lies to the south and east of Leo. It contains a few interesting foreground

stars. But it’s mostly known for what lies beyond these stars, namely, galaxies. There are

dozens of relatively bright galaxies in Virgo, and hundreds more visible in large amateur

telescopes. Unlike Orion or its neighbor Leo, Virgo is not a bright constellation. But it’s big…

the second largest in the sky by area. Only Hydra is larger.

The constellation is usually associated with Dike, the Greek goddess of justice, daughter of Zeus

and Themis. She lived in the early days of the Olympian gods, when Zeus’s father Cronos ruled.

It was a golden age of mankind, a time before sickness and war, winter and death.

When Zeus wrested power from Cronos, things went downhill. The seasons grew extreme, and

mankind grew restless and warlike and failed to honor the gods. Dike warned mortals that things

would get worse (she was right) and she turned her back on man and became a recluse in the

hills of Greece. When she could stand no more of the world’s troubles, she cast herself into the

sky in the zodiac next to Libra, the Scales (of justice).

Other ancient legends associate Virgo with Persephone, the daughter of Demeter, the goddess of

the harvest. Because the stars of Virgo appear before sunrise late in the northern summer, many

other cultures, including the Babylonians link Virgo with crops and fertility. In India Virgo was

called Kauni, mother of the great god Krishna.

The constellation Virgo lies in the direction of the north galactic pole, far from the starry band of

the Milky Way, so there are only a few bright stars here. Spica, a blue-white beauty of a star, is

the brightest in the constellation. From the northern hemisphere, you can easily find Spica by

following the arc of the handle of the Big Dipper first to Arcturus, then on to Spica, also known

as α (alpha) Virginis.

Spica is the 15th

brightest star in the sky. At a distance of 280 light years, the star is intrinsically

bright at a magnitude of -3.4. Spica is actually two stars, each very bright B-type stars, that

revolve around each other every four days. They are too close to separate directly in any

telescope. The brighter of the pair is some 12,000x brighter than our Sun and eight times

brighter than its companion. Still on the main sequence or close to it, the brighter star lies far to

the upper left of the HR diagram. Spica is one of the closest stars that will one day explode at

the end of its life as a supernova, one of the biggest explosions in the universe. When this

happens millions of years from now, the star will grow to become as bright as a quarter Moon in

our skies for several weeks. Before it becomes a supernova, Spica will swell to become an

enormous red supergiant star much like the star Betelgeuse in Orion. More about this next

month…

Other stars in Virgo include:

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Zavijava – A white main sequence star not much larger than the Sun.

Vindemiatrix – Epsilon (ε) Virginis is also known as the “grape gatherer because the first

appearance of the star before sunrise in August marks the beginning of the vintage. This star may

be physically associated with the Hyades star cluster in Taurus.

Porrima – Also known as γ (gamma) Virginis, this is a lovely double star just 39 light years

away. The pair of F-type main sequence stars revolves about each other in 169 years, which

means you can see, in a good backyard telescope, the motion of the stars over the course of a

decade or so. Until 1995, the stars split easily in a small scope. Then by 2005, they moved too

close together to resolve in a small scope. They’ve now begun to separate again. This month,

and over the next few years, try to resolve them in your own telescope at high magnification.

They are separated by a little less than 2”.

The constellations Virgo, Corvus, and Crater, south and east of Leo

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While Virgo lacks bright stars, it holds no shortage of galaxies. Within the “arms” of the

maiden, especially between the stars Vindemiatrix and Denebola in Leo, you can find dozens of

galaxies with a small telescope. Most belong to the massive Virgo Cluster, the nearest large

galaxy cluster to the Milky Way. We’ll visit a few of these galaxies shortly.

The Constellations Corvus and Crater

Look next just south of Virgo at Corvus and Crater, the Crow and the Cup, two smaller

constellations with distinctive shapes. You will find that the stars δ (Delta, or Gienah) and η

(Eta), in the upper left-hand corner of the quadrilateral figure of Corvus, make a striking

appearance. The little star zeta (ζ) is a very pretty double for binoculars. There is a very faint pair

of stars close below and to the right of Beta (β) Corvi. This pair is a good test for binoculars.

The stars γ and δ Corvi point the way to Spica. Corvus is a constellation where the stars are not

labeled according to order of brightness. Its brightest star is γ Corvi (Achiba), a blue-white B8-

type star.

Crater means “cup” in Latin, and the constellation represents the cup of the great god Apollo.

Unlike Corvus, this group at least resembles its namesake. This small star group is worth

sweeping over for the pretty combinations of stars to be found in it. The brightest star, Alkes or

α Crateris, is an orange K-type star just evolving off the main sequence.

One of many legends associated with these two constellations tells of a crow stopping on his way

to fetch water for Apollo to accompany his snack of figs. The lazy crow refused to get the water,

telling Apollo that a snake, Hydra, kept him from the water, even taking the effort to capture a

snake in his talons for proof. Apollo caught the lie, however, and flung the crow (Corvus), cup

(Crater), and the snake (Hydra) into the sky. He further punished the lazy bird by making sure

that it would forever be thirsty, both in real life and in the heavens, by placing the Cup just out of

reach.

Both Corvus and Crater are ancient constellations and took their place among Ptolemy’s original

48.

The Constellation Libra

The constellation Libra, lying east of Virgo, does not contain much to attract your attention. Its

two main stars, β Librae (Zubeneschamali) and α Librae (Zubenelgenubi), may be readily

recognized as an irregular quadrilateral east of Spica and west of a line of three stars which make

up the claws of the constellation Scorpius. The upper of the two, Zubeneschamali (“zoo-BEN-

esh-ah-MAL-ee”), has a singular blue-white tint, while the lower one, Zubenelgenubi (“zoo-

BEN-el-je-NEW-bee”), is a very pretty double star for binoculars.

The constellation of Libra appears to have been added later than the other eleven members of the

zodiac. Its two chief stars at one time marked the extended claws of Scorpius, which were

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afterward cut off (perhaps because the monster proved too horrible even for its inventors) to

form Libra. The name of the stars Zubeneschamali and Zubenelgenubi derive from the Arabic

names for “northern claw” and “southern claw”.

As its name signifies, Libra represents a balance, or scale, and this fact seems to refer to the

invention of the constellation at least three hundred years B.C., when the September equinox

occurred at the moment when the sun was just crossing the western border of the constellation.

The equal length of days and nights at that season suggests the idea of a balance. Because of the

precession of the Earth’s polar axis, the September equinox now lies in Virgo, and in the year

2439, it will move into Leo.

The constellation Libra

The Constellation Hercules

Now let’s visit the celestial incarnation of the greatest of classical heroes, Hercules. With the

famous “Keystone” of four stars and a pair of splendid globular clusters, the constellation

Hercules is a regular stop for even casual stargazers from June through August. Hercules lies

high overhead in the north and well over the northern horizon in the southern hemisphere, so its

sights are accessible to stargazers all over the world.

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Hercules is one of Ptolemy’s original 48 constellations. But this star group was known earlier,

much earlier than even classical Greek civilization. The Greeks first knew these stars as

Engonasin, the “kneeling one”, and indeed these stars do suggest the form of a kneeling man

bent by toil and fatigue.

Eratosthenes first linked this tired kneeling celestial figure to Hercules. And why not? Hercules,

though blessed with immortality and unmatched strength, was burdened with the famous Twelve

Labors he undertook as penance for slaying his own sons in a fit of delusional rage set upon him

by Hera, who constantly blighted the life of the mighty hero. Hera hated Hercules because he

was the offspring of her husband, Zeus, and the wise and beautiful mortal Alcmene.

Hercules ranks as the 5th largest constellation in the sky, yet it has no bright stars. But that

doesn’t mean it’s hard to find. It lies between the semicircular shape of Corona Borealis and the

small parallelogram shape of Lyra and its brilliant blue-white star Vega. Look for the four-sided

Keystone asterism formed by pi (π) , eta (η) , zeta (ζ) , and epsilon (ε) Herculis.

Now trace out the rest of the constellation. The stars beta (β) and delta (δ) Herculis mark the

shoulders of the hero, and his arms extend towards Lyra. His left knee and shin, which are

pressed on the head of Draco, the Dragon, are marked by theta (θ) and iota (ι) Herculis.

The constellation Hercules

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The constellation’s brightest star is called Rasalgethi (“rah-sell-GAYTH-ee”). It’s a 3rd-

magnitude red-giant star found in the south end of the group, right next to 2nd-magnitude

Rasalhague, the brightest star in the constellation Ophiuchus. Rasalgethi is a superb double star

for a small telescope, and you’ll meet it up close shortly. As you will the famous Hercules

Globular Cluster, also known as M13 and its only slightly more modest neighbor M92, the other

bright globular cluster in Hercules.

The constellation Hercules also lends its name to a galaxy cluster. The Hercules Cluster lies

some 500 million light years away, and is one component of the even larger Hercules

Supercluster. You’ll need a big scope to glimpse any of these distant galaxies.

The Constellation Draco

Finally this month we look to the long and winding constellation Draco. This group winds

between the Big and Little Dippers. The tip of its tail lies just above the bowl of the Big Dipper,

while the small quadrilateral of its head lies near one of the feet of Hercules.

Which is fitting, since Draco is linked with the myth of the Twelve Labors of Heracles (the

Greek name for Hercules). In the myth, Draco represents the dragon Ladon, which guarded the

golden apples in the gardens of the Hesperides, the daughters of the Titan Atlas. The golden

apple tree was given as a wedding present to Hera when she married Zeus. Hera placed Ladon

around the tree so the Hesperides would not pick its apples.

So as part of his Labors, Heracles was asked to pick golden apples from the tree. He killed Ladon

with poisoned arrows and took the apples. Saddened by the dragon’s death, Hera placed him in

the sky. Draco is usually depicted coiled around the North Pole, with one foot of Heracles on its

head.

The Romans had a different story in which Draco was one of the Giant Titans who battled with

the Olympian gods for ten years. Draco was killed in battle by the goddess Minerva and placed

into the sky, where it froze around the North Pole.

To follow Draco, find the tip of the tail marked by the red-orange M-type Giausar (“JAW-sahr”)

by following the star Merak through Dubhe (in the bowl of the Big Dippter) about a distance

equal to the spacing between these two stars. Then follow the body of Draco to Thuban, to

Edasich (“ED-a-sick”) , then to a series of stars under the bowl of the Little Dipper. The head of

the beast consists of the stars Grumium, Eltanin (γ Draconis), Rastaban (β Draconis), and ν (nu)

Draconis.

The orange K-type star Eltanin is the brightest star in the constellation, but Thuban is the most

famous. It was once the North Star from around 2,800 B.C. to 1,900 B.C. As the Earth’s axis

slowly precessed (see Month 2), the north celestial pole moved closer to Kochab in Ursa Minor

and, in time, to Polaris.

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The winding constellation Draco

Observing Techniques – Telescopes

Overview

Last month in The Art of Stargazing, you learned about the basics of binoculars, essential tools

that every stargazer should understand and use from time to time. Some stargazers rely

completely on binoculars for their observations and enjoyment of the night sky. But most

amateur astronomers eventually consider the purchase and use of at least one telescope. So this

month we survey the types of telescopes available to amateur stargazers, and make some

recommendations of which telescopes to consider.

Many beginners believe the main purpose of a telescope is magnify objects, to make them

bigger, and that more magnification is better. This is not true. The main purpose of a telescope

is to collect light and bring this light a focus. Of course, this focused light can be magnified and

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seen with your eyes. Or it can be sliced and diced with a spectroscope, or cast onto film or its

electronic equivalent to make images, or measured and counted with photometers. The more

light a telescope collects, the brighter the image it can make, and the more useful it is to an

astronomer. But larger telescopes are bigger, more expensive, and more difficult to manufacture

and maintain than smaller instruments. This is the basic tradeoff to consider with a telescope,

either for professional or amateur use.

Here’s some good news: this is the best time in history to buy a telescope for amateur astronomy.

In the past 30 years, the market has made available an enormous selection of good-quality

instrumentation at relatively affordable prices. While a good telescope may not appear

inexpensive, in real terms the prices of telescopes are as low as they have ever been. An

excellent scope and mount can be purchased for the same price as a laptop computer. And while

the computer is designed to last for just a few years, a good telescope can last a lifetime.

Now here’s the bad news. As with breakfast cereals at a supermarket, the wide range of choices

of the sizes and types of telescopes has led to confusion and overwhelm for amateur stargazers

looking to invest in an instrument. But here’s something that will take the pressure of any

purchasing decision: there is no single type of telescope that’s best for all observers and for all

types of observing. Every telescope has benefits and drawbacks, as you are about to learn. It’s

simply a matter of understanding the pros and cons of each type of telescope, and matching it to

your personal interests and situation.

General Guidelines

Picking a telescope is a big decision. Before we get into the pros and cons of each type of

telescope, here are some rules of thumb to keep in mind so you get the best equipment to fit your

needs.

First of all, don’t buy a cheap department-store telescope that advertises amazingly-high

magnification (>300x), and which is mounted on a spidery tripod that causes the image of any

object to shake and wobble until you feel woozy. You will regret such a purchase… guaranteed.

You can spend a few hundred dollars on a good scope, or a few thousand on a great scope. But

as a guideline, you shouldn’t buy a telescope that costs less than $300. You’ll be disappointed

with the results. If you can’t afford that much, save a few more dollars, or stick with binoculars

for now.

Don’t worry about magnification. A telescope’s most important feature is aperture… the

diameter of its light-collecting lens or mirror. A telescope with a larger aperture collects more

light, gives you a brighter image, and lets you see finer detail. It’s the laws of physics. I suggest

your first scope should have an aperture of at least 80-90 mm. Otherwise, the images you see

will be too dim and fuzzy.

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Bigger aperture means higher cost and a bulkier telescope. I’ll leave budgeting up to you. But

keep in mind the size and weight of a telescope, and how far you have to move it to your main

observing site. Some excellent beginner scopes might be 4-5 feet long, 8-9 inches wide, and

have two main parts, each of which weighs 30 lbs or so. Can you store and move all that? If

not, consider a more compact telescope. You might give up some aperture, and smaller scopes

can be more expensive, inch-per-inch of aperture. But a big scope isn’t any good if you can’t use

it.

Now let’s get to some specifics…

Refracting Telescopes

The first type of telescope devised was the refracting telescope, or more simply, the refractor.

Refractors, which are perhaps the most recognizable form of telescope, use a lens mounted at the

front of a long tube to collect light and bend (or refract) it to a focus. While simple glass lenses

for magnification date back to ancient Egypt, and eyeglasses were used widely by the 13th

century A.D., it was not until the late 1500’s that Dutchman Hans Lippershey looked through

two lenses at once and found they caused distant objects to appear larger. This was the first

refracting telescope.

Lippershey’s telescope magnified 3 times (or 3x). The earliest refractors were used by mariners

and other terrestrial observers. In 1609, Galileo built a telescope that magnified 30x and became

the first to turn such a device to the sky. Refractors were the only type of telescope until Isaac

Newton invented the reflecting telescope in the late 17th

century.

An early refracting telescope

Every refractor has two main parts: a large lens to collect and focus light from a distant object

and a second smaller lens to magnify the image of the first lens. As with all telescopes, the light-

collecting ability of a refractor increases as the square of the diameter of the objective. A

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refractor with an objective lens of diameter 100 mm, for example, collects four times as much

light as a refractor with a 50 mm lens.

The length over which an objective lens focuses light to a point is called the focal length, shown

below as F. And the focal ratio of a refractor (or any telescope) is simply the focal length

divided by the diameter of the objective lens. A refractor with a lens of 100 mm diameter and a

focal length of 700 mm has a focal ratio of f/7. (Note: These definitions hold for all types of

telescopes, reflecting or refracting).

A second lens is used to magnify the image of the objective lens. This second, smaller lens is

usually called an eyepiece. Most eyepieces today are actually several lenses mounted together in

a small barrel. The eyepiece also has a focal length, shown in the image above as f. The

magnification of the telescope of focal length F with an eyepiece of focal length f is simply F/f.

So a refractor with focal length 700 mm and an eyepiece of focal length 10 mm will give a

magnification of 70x. Again, this relation is true for all types of telescopes.

The first refractors had a single objective lens ground to a convex shape and with a spherical

curvature. Such simple lenses cause a lot of image distortion. And they focus red, green, and

blue light rays at different planes, a problem called “chromatic aberration”, which looks like

rings of color around bright objects. To minimize these problems, early telescopes needed large

focal ratios, which meant even telescopes with small objective lenses were enormously long…

ten to twenty feet or more with just a 2” lens as the objective! In the mid-1750s, lawyer and

amateur telescope maker Chester Moore invented a refractor with a two-lens objective, each of

which was made from a different kind of glass to partially correct for chromatic aberration.

These types of objective lenses are called achromatic. Today, all refractors have at least two

lenses in the objective to correct for spherical and chromatic aberration.

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The light path of a refracting telescope

Achromatic refractors still have some chromatic aberration. This means when you look at the

Moon and brighter stars and planets, you will see an aura of blue or purple around the object,

which tends to detract from the view. The problem is especially obvious at focal ratios smaller

than f/10. It also worsens with larger apertures. That’s why achromatic refractors tend to have

smaller apertures of less than 100mm to 150 mm and focal ratios of f/8 or larger, so such

telescopes tend to be quite long for the aperture. A 100 mm refractor of focal ratio f/12 is more

than 1200 mm long, which can be unwieldy to mount, transport, and store. Worst of all, an

aperture of 100 mm (4 inches) is quite small by today’s standards, so faint celestial objects are

quite dim in such a telescope if they can be seen at all.

Unlike reflecting telescopes, refractors have the great advantage of having no central obstruction.

This results in very sharp images and high contrast. Also unlike reflectors, refractors are

mechanically simple and quite robust and need very infrequent optical adjustment and alignment.

So they are simple and easy to use compared to many reflectors.

A good achromatic refractor with focal ratio of f/10 to f/15 can give very sharp images and large

images of the Moon and planets and double stars. Such telescopes are less useful for observing

large, spread-out objects like star clusters and rich star fields along the Milky Way.

A two-element achromatic telescope objective lens

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Another tip for choosing refractors… you should avoid “short tube” achromats… so called “grab

and go” achromatic refractor telescopes with less focal ratios of less than f/8. They are small and

inexpensive, but they tend to give appalling amounts of false color when looking at bright

objects.

When choosing an achromatic refractor, look for instruments with at least 70 mm aperture and

focal ratio longer than f/10. With smaller apertures, the images will appear too dim. An aperture

of 90-100 mm is preferable, but these instruments tend to get longer and heavier.

A good achromatic refractor used to be quite expensive in real terms back in the 1950’s through

1970’s. Today, with modern manufacturing methods in Asian factories, prices for decent

refractors from Orion, Celestron, and Meade are as low as they have ever been. Still, if you pay

anything less than US$200-$300, you are likely wasting your money. New telescopes that cost

less are available, but tend to have poor optics and shaky mounts. In North America, a good 90

mm achromatic refractor on a passable mount costs a little less than $300. This is considered a

respectable beginner telescope.

An achromatic refractor with an aperture of 100 mm

Apochromatic and ED Refractors

In the past 25 years, telescope makers have learned to use new types of glass based on fluorite

and other compounds to create objective lenses that almost completely correct for any false color

and chromatic aberration, even at focal ratios of f/6 or f/7. Telescopes that use these lenses are

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called“apochromatic refractors”. The prefix “apo” is from Greek meaning “away from” for

“separate from”.

Apochromatic refractors, or “apos” as they are sometimes called, are fantastic telescopes which

give razor sharp images and superb contrast. They are favored by astrophotographers and visual

observers of the moon, planets, and wide-angle star fields of the Milky Way. Manufacturers of

high-quality apos include Televue, TMB, Vixen, Takahashi, and Astro-Physics.

But apos have a downside… cost. Top-notch apochromats are shockingly expensive, up to

$1000 per inch of aperture, and that’s just for the optical tube without a mount or finder. And of

course, even if you spend $4,000 on a 4” apochromatic refractor, you still only have a 4”

telescope, which means fainter objects remain beyond reach for visual use. But these telescopes

are renowned not for light-gathering capability, but for rendering crisp, clear, high-contrast

views of stars, star clusters, brighter nebulae, and the Moon and planets. For $4,000, for

example, you can get an 18” reflecting telescope that will show you thousands more deep-sky

objects, especially faint galaxies and galaxy clusters. But of course, an 18” telescope is large and

difficult to use, and it will easily fill up half a small garage, while you can carry many 80 mm to

100 mm apos as luggage on an airplane. So it all depends on what you’re after.

The most expensive apos use three or four elements in the objective lenses to deliver the best

images for astrophotography. As an example, the Telvue NP-101 has a 101 mm 4-element

objective lens, with two more corrective lenses near the eyepiece. It is a favorite of

astrophotographers and comes with a price tag of close to $4,000 (which does not include a

finderscope or eyepiece or mount). Similar telescopes from Takahashi and Astrophysics are

even more expensive.

In the past few years, however, telescope makers have developed simpler objective lenses that

are nearly apochromatic. These lenses are often called “ED”, for extra-low dispersion. The cost

of telescopes made with ED lenses tends to be far more reasonable than pure apos. For example,

you can get a decent 4-inch ED refractor (optical tube only) that gives you tack-sharp images of

planets and wonderful contrast on binary stars and stunning wide-field views of the Milky Way.

The cost? Just $1200, roughly, in North America. Such telescopes are available from Orion,

Astro-Tech, Williams Optics, and Stellarvue, for example. At this price, the objective lenses

have just two elements, but that works fine for visual use. Scopes with three-element ED

objectives, which work better for astrophotography, are available for about $2,200 (for a 100 mm

objective), still far less than a pure apo. Two and three-element ED scopes with 80-90 mm

objectives are also available at slightly lower costs.

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A Televue 85 mm apochromatic refractor and carrying case

If you are considering a refractor and can afford an apo or ED, they are highly recommended.

They are just as easy to use as their less-expensive achromatic brethren, but they give far better

images. I have two small ED refractors (66 mm and 102 mm objectives) and I turn to them often

for pleasing wide-field views of star clusters, star fields, and larger nebulae. With good short-

focal length eyepieces, these scopes also give sharp views of the Moon and planets, so they are

quite versatile. Unlike true apochromats, ED refractors do give a little false color (the blue and

purple aura around brighter objects), but it is far better than simpler achromatic refractors.

Many apos come without mounts, so you will need to select a mount separately. We will discuss

telescope mounts next month.

Newtonian Reflecting Telescopes (and Dobsonian Telescopes)

Now let’s turn to the other major type of telescope, the reflecting telescope, which uses a

carefully-shaped mirror to bring light to a focal point. The telescope with the most light-

collecting power for your money is the venerable Newtonian reflector, invented by Sir Isaac

Newton himself in 1689. If you love to see the fainter deep-sky objects, as well as the Moon and

planets, then this may be the scope for you.

The idea behind a Newtonian reflector is simple. Light falls on a curved primary mirror held at

the bottom of a long tube. The mirror focuses the light back to a point at the top of the tube

where a small flat secondary mirror directs the light towards an eyepiece at the side of the

telescope. This way, the observer’s head won’t get in the way of the starlight falling on the

mirror. For beginners, this arrangement is a little non-intuitive at first.

Newtonian reflectors are far less expensive than refractors of the same aperture because only one

optical surface needs to be ground and figured. In a refractor with a two-element objective, four

surfaces need to be figured, and this takes more time and adds cost.

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The light path of a Newtonian reflector telescope.

The main practical drawback with a Newtonian is size and weight. A 12” Newtonian optical tube

is more than 5 feet long and weighs 80 lbs (without the mount). For some, that is too much to

handle and transport, especially if you live in an apartment or have to transport the telescope

long distances to dark sky.

Newtonians also need frequent adjustment, or collimation, of the secondary mirror. This simple

procedure takes a few extra minutes at the beginning of each observing session once you get the

hang of it. But it can be a little intimidating the first few times you try it.

Less expensive and smaller Newtonians used to have mirrors with a spherical curvature. Today,

most such scopes have the superior parabolic curvatures which bring objects to a focus across the

entire field of view. But because there are no additional lenses or mirrors to correct the image in

a Newtonian, so you may detect an optical aberration called coma. This makes stars appear

wedge-shaped at the edge of the field of view. But it’s not a big deal if the focal ratio is bigger

than f/6 or so.

The classic 8” Newt on a bulky equatorial mount was a staple of serious stargazers in the 1960’s

and 1970’s. These scopes were big and heavy and hard to set up, and they were largely replaced

by the much more convenient Schmidt-Cassegrain in the 1980’s (see next section). But

Newtonians came back into style in a big way in the 1980’s when telescope makers

commercialized a design by the former monk and astronomy popularizer John Dobson. The

“Dobsonian” telescope is just an inexpensive large-aperture Newtonian on a simple altazimuth

mount. Dobsonians can’t be beat for purely visual deep-space observing. With huge mirrors (up

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to 36” in diameter) these new-age Newtonians are called “light buckets” because of their

immense light-collecting ability.

Because of its simple design a Dobsonian is the least expensive telescope per inch of aperture. A

quality 12” telescope on an altazimuth mount sells for less than $1000, while a good 8” sells for

about $500 in North America, including electronics in the mount to help you find celestial

objects. Orion, Meade, and Skywatcher make good mass-market Dobsonians from 4” to 16” and

beyond. Higher-end boutique manufacturers like Obsession make more expensive Dobsonians

with high-quality mounts and optics. Obsession scopes start at 12” and go as large as 25”

aperture. These are big scopes.

For serious beginners on a budget, a Dobsonian reflector with 6” or 8” aperture is perhaps the

best all-around telescope for under $500. With these scopes, you can see objects as faint as 13th

magnitude, yet you can also get good high-magnification views of brighter objects like the

Moon, planets, and double stars. The only caveat… these telescopes are not small or light. The

optical tube of an 8” (200 mm) Dob will be about 4 feet long and weigh about 40 lbs. The mount

will add another 30-40 lbs. Before you invest in such an instrument, make sure you are

comfortable moving and storing it. Not just once or twice, but night after night. A nice big

scope won’t do you much good if you don’t use it!

An 8” Dobsonian from Orion Telescopes.

One more thing… to get the most out of a large reflector on faint deep-sky objects, you need

dark sky. In a big city, the brightness of the background sky may exceed the surface brightness

of the objects you want to see. In that case, the biggest telescope in the world won’t help you.

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But the larger aperture and longer focal length of Newtonians do help in achieving higher

magnification, which helps in darkening the background sky. So don’t give up on Newtonians if

you have to endure light pollution. Simply remember that you may only get the most out of such

a telescope when you leave the city.

Schmidt-Cassegrain Telescopes

A Newtonian telescope has a big drawback: its simple optical system uses a curved mirror and a

flat diagonal mirror to direct light to an eyepiece. That means the length of Newtonian is roughly

equal to the focal length, so you get a long and heavy telescope with a big and awkward mount,

even with relatively modest apertures.

But there are other kinds of reflecting telescopes. Shortly after Newton developed his reflector,

an obscure French Catholic priest named Laurent Cassegrain invented a reflector that used two

mirrors to fold a long optical path into a shorter tube. Many amateur reflectors, and nearly all

professional telescopes, now use a variation of the Cassegrain design.

Long after Cassegrain, in 1930, the optician Bernard Schmidt added a new twist to Cassegrain’s

design. Instead of a parabolically-figure mirror, Schmidt used a simple spherical mirror, which

was easier to make, but which induced “spherical aberration” that made it impossible to achieve

a tight focus. Then Schmidt added a specially-figured lens at the front of the tube to correct for

spherical aberration and regain the ability to achieve a good focus. At the focal plane, on the

back side of the corrector lens, he placed a piece of film. This layout is a Schmidt camera, and

it’s still used for imaging wide-field views of the sky.

Finally, in 1946, an architect and artist named Roger Hayward replaced the film with a convex

mirror behind the corrector lens to send light out the back of the tube to an eyepiece or a camera.

This is what we now call a Schmidt-Cassegrain telescope (SCT). A company called Celestron

built on this design and developed manufacturing techniques to produce SCT’s in large

quantities that revolutionized amateur astronomy starting in the 1970’s.

SCT’s have the advantage of large apertures and long focal lengths, but the light path is folded

into a much smaller physical length because of the design of the primary and curved secondary

mirrors. An 8-inch SCT like those sold by Celestron have a focal length of 80 inches (2000 mm)

packed tube less than 18-inches (450 mm) long. The tube weighs about 12 lbs. It’s pretty

impressive.

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Optical path of a Schmidt-Cassegrain telescope

Schmidt-Cassegrains have something for everyone. Telescope manufacturers love them because

the spherical mirrors and corrector lenses are easy to make compared to parabolic mirrors for

Newtonians. Casual observers love them because they are portable AND they have a relatively

large aperture to help see faint deep-sky objects. And astrophotographers love them because

they’re easy to mount and guide, they lend themselves to narrow field imaging and, with

additional telecompressing optics, to wide field imaging.

A Schmidt-Cassegrain telescope from Celestron

SCT’s are not perfect at anything but they’re pretty good at everything. Their biggest advantage

is portability… an SCT packs a lot of aperture into a small package. However, it has a narrow

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field of view… a big drawback if you like rich-field views of star clouds. Because of the large

secondary mirror, you won’t get the same sharp contrast on the Moon and planets with an SCT

as you would with a refractor. And an SCT is twice the price of a Newtonian of the same

aperture.

Celestron was the first to market with SCT’s in the early 1970’s. Meade followed a few years

later. Today, the product line of Celestron especially has expanded to include basic SCT’s on

one-arm mounts for casual visual observing. This is the NexStar series (see above). An 8”

NexStar runs for about $1,200 in North America. The Celestron CPC series has a heavier two-

arm mount which makes for better observing at high magnification. An 8” CPC runs about

$2,000 in North America. For serious observers and astrophotographers, there is the CGEM

version which includes equatorial mounts for $2,200. And there’s the new Edge-series of SCT’s

from Celestron that are intended for astrophotographers only. An 8” SCT Edge and mount goes

for $2,500.

Meade has similar product lines. Their LS-6 and LS-8 series SCT’s are particularly interesting

as they include a computer, software, and GPS that aligns the telescope to the night sky without

any user input. The simplest version of the LS-6 goes for about $1000 in North America. Meade

also has systems with equatorial mounts and advanced optical design to compete with the

Celestron Edge.

All price estimations for the models mentioned above include a mount, tripod, electronics, and

motors to move the telescope to selected celestial objects. In addition to the popular and useful

8” aperture, there are also 6”, 10”, 11”, 12”, and even 14” and 16” SCT’s available from

Celestron and Meade.

While Dobsonian’s are simple and inexpensive, and often appeal to stargazers on a budget,

SCT’s are more versatile and are easier to store and transport. They are an excellent choice for

stargazers with larger budgets or who need more portability. Also, because of the location of the

eyepiece at the end of a short tube, SCT’s are usually easier to look through compared to

Dobsonians or Newtonians, which often require bending over and contorting one’s neck at

uncomfortable angles. Older observers often prefer SCT’s to Dobs and Newtonians for this

reason.

Maksutov-Cassegrain Telescopes

In the mid-20th

century, the Russian optician Dmitri Maksutov invented another variation of the

Cassegrain design. The so-called Maksutov-Cassegrain is a robust and useful design for small-

aperture telescopes. They are a godsend for urban observers who need a compact scope with a

go-to mount and good optics.

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Modern Maks are much like Schmidt-Cassegrains. They have a spherical mirror to collect light

and a curved lens up front to correct for aberrations. But the corrector lens on a Mak has a simple

spherical curve which is easy to manufacture. And the secondary mirror is simply a thin layer of

aluminum deposited on the back of the lens. This mirror cannot be adjusted, so unlike a

Newtonian or Schmidt, a Mak requires no alignment.

The optical path of a Maksutov-Cassegrain telescope

The downside of the Mak’s optics? To keep aberrations small, Maks are made with long focal

ratios… typically f/12 to f/15. That means you get a higher magnification with a particular

eyepiece and a narrower field of view than with an f/10 Schmidt or f/6 Newtonian. So Maks

aren’t great if you want wide, sweeping views of the Milky Way. They’re much better for small

objects that require high magnification like planets, the Moon, double stars, globular clusters,

and planetary nebulae. Many observers choose Mak-Cass telescopes instead of achromatic

refractors because they provide a long focal length and sharp image, but are more compact and

do not show chromatic aberration.

A Maksutov-Cassegrain optical tube, showing the mirror on the back of the front corrector lens

-38-


Maks are great for urban observers for two reasons. They are compact and easy to transport. And

the higher magnification will darken the washed-out city sky and bring out more contrast in a

deep-sky object. The corrector lens on a Mak is quite thick, which means Maks get heavy at

higher apertures because the lenses become massive. That’s why you won’t find commercially-

made Maks with apertures larger than 7 inches. Because they’re rugged and robust, Maksutov-

Cassegrains are used in harsh environments in industrial and military applications. And more

than a few field photographs in National Geographic have been made with Maks.

The most famous (and expensive) Makustov telescope is the Questar. First made in 1954, a

Questar is like a fine Swiss watch. These scopes have superb mechanics and razor-sharp optics

almost without aberration. Questars are widely used for terrestrial observing and nature

photography. And NASA used Questar telescopes on its early space missions. Questars with

aperture of 3.5” and 7” are still sold today, and cost from between $4,500 and $10,000.

But you absolutely don’t need to spend that much. Maks made a comeback about 20 years ago

largely because of the success of Meade’s ETX-90 and ETX-125 telescopes. The ETX-125 had

an aperture of 125 mm and sold with mount and tripod for about $1000. This telescope is no

longer sold in the 125 mm aperture, but the 90 mm aperture is still available for about $500.

Celestron has a 4” NexStar version also available for about $500.

Orion also sells 90 mm and 127 mm Mak-Cass scopes for less than $500 (optical tube only), as

well as a more serious version with a 180 mm (6”) aperture (see images above and below). The

180 mm version goes for about $1,300 in North America for the optical tube only. It has a focal

ratio of f/15 and a focal length of 2700 mm (106”) in a tube only 560 mm (22”) long.

One last word about a term you should know. Dioptrics is a fancy name for optical instruments

that use lenses; catoptrics is a term for instruments that use mirrors. Both Schmidt-Cassegrain

and Maksutov-Cassegrain telescopes use curved mirrors and lenses to bring starlight to a focus,

so they are sometimes called catadioptric telescopes.

-39-


The same 180 mm Mak-Cass telescope shown above, but on an equatorial mount

Summary of Telescope Pros and Cons

We’ve covered a lot about telescopes, yet it is only an introduction to the wide range of options

available to stargazers today. The table below sums up the main points about each type of

telescope.

Type Objective

Type

Typical

Apertures

(mm)

Pros Cons

Starting

Price

(with

mount)

Comments

Achromatic

refractor

Lens 70 – 150 Low cost; easy to use;

rugged

Chromatic

aberration at f/8 or

lower

$300 Good small beginner scope, but a

small Dob is a better bet if the

extra size can be accomodated

Apochromatic

and ED

refractors

Lens 60 – 127 Excellent image

sharpness and contrast;

little chromatic

aberration

High cost limits

aperture

$2,000 Superb instruments; a small ED

on a solid mount is an excellent

complement to a larger telescope

Newtonian/Dob Mirror 100-400 Low cost; ease of

mounting (Dob);

versatile

Large; requires

collimation;

awkward viewing position

$300 Excellent value and versatility; an

8” Dob with a computer will last

new stargazers for many years if they can handle the large size

Schmidt

Cassegrain

Catadioptric 100-400 Compact and versatile;

good for imaging

Large secondary

mirror

$1000 Good all around telescope; better

choice than Dobs if portability is a must

Maksutov

Cassegrain

Catadioptric 90-180 Compact; sharp images Narrow field of

view; takes a long

time to cool down

$500 Portable, but do not offer a wide

field of view; not as versatile as

SCT

-40-


Solar System Observing – Saturn

Overview

The planet Saturn reached opposition on April 26 and remains low in the southeastern sky in the

mid-evening hours. The planet is still well-positioned for viewing with a small telescope over

the next month or two. So let’s have a closer look at this fascinating and beautiful planet.

Saturn

Saturn’s position on June 10, 2013. The planet lies in the constellation Libra.

-41-


Saturn is the sixth planet from the Sun and the second-largest planet in our solar system after

Jupiter. The planet takes its name from the Roman god of agriculture and time, Saturn, whom

the ancient Greeks called Cronus.

At its equator, Saturn has a diameter of about 120,600 km, about 9.5 times the diameter of the

Earth, while at the poles is spans about 8.5 Earth diameters. The planet has a volume of 760

Earths and a mass of about 95 Earths. Its density, as many schoolchildren learn, is 30% less than

that of water. So if Saturn sat in a big bathtub full of water, it would float!

The planet averages about 9.5 astronomical units (AU) from the Sun, or about 1.4 billion km. At

this distance, it revolves around the Sun every 29.5 years. So it makes a complete trip around the

Sun a little more than twice during an average human lifespan.

Like Jupiter, Saturn rotates quickly, once every 10.57 hours. During the course of a night, you

see an appreciable fraction of the entire planet. Also like Jupiter, the planet’s non-solid nature

means its quick rotation flattens it into an oblate spheroid, with a larger diameter at the equator

than at the poles.

Like Jupiter, Saturn has been visited by several spacecraft, most recently by NASA’s Cassini

mission. The Cassini spacecraft has been in orbit around Saturn since 2004 and sent back

thousands of measurements and spectacular images of the planet and its rings and moons. You

can see a short retrospective video on the video page of The Art of Stargazing here:

http://www.oneminuteastronomer.com/art-of-stargazing/video-clips/

Structure and Composition

Like Jupiter, Saturn is a gas giant, with no solid surface. The planet’s core consists mostly of

iron, nickel and silicates, all squeezed by gravity to a blazing hot temperature of some 11,000 K.

And like Jupiter, Saturn radiates about 2.5x more heat than it receives from the Sun.

Saturn’s core is surrounded by a deep layer of liquid metallic hydrogen, around which lies a

layer of compressed liquid hydrogen and helium. Its icy outer gaseous layers also consist of

about 96% hydrogen and 3% helium, with traces of heavier molecular gases like ethane,

methane, acetylene, and ammonia.

http://www.oneminuteastronomer.com/art-of-stargazing/video-clips/

-42-


The belts of Saturn

The clouds of Saturn, the part we see directly from Earth, have a pale sand color and are far less

colorful than Jupiter’s cloud layers. The color is caused by frozen crystals of ammonia in the

upper atmosphere and ammonium hydrosulfide in the lower layers. The temperature in the upper

reaches of the clouds is as low at 100 K (-173 oC). The planet’s faint bands, the belts and zones,

have a nomenclature very similar to those of Jupiter (see image below). North on Saturn is the

side that faces the north celestial pole as seen from Earth.

Basic nomenclature of Saturn’s belts and zones

The wind speeds in the outer atmosphere reach up to 1,800 km/h, faster than on Jupiter, and

these winds stir up some interesting phenomena including many fascinating and transient

features that pop up from time to time in its upper atmosphere, and many puzzling permanent

-43-


features. In 1990, amateur and professional astronomers watched a white oval well up from deep

in the planet’s atmosphere and float to the surface, then dissipate slowly over the course of many

weeks. This short-lived storm seems to happen approximately once every Saturnian year when

summer peaks in the planet’s northern hemisphere. A similar storm was observed in 1903, 1933,

and 1960. If the pattern holds, the next storm will occur around the year 2020. The cause of this

phenomenon is unknown.

Another unrelated storm welled up in Saturn’s atmosphere in 2010. It started as a small white

spot, then spread out across the planet as it caught the high differential winds in the upper

atmosphere. The storm was imaged by the Cassini spacecraft (see below) and by Earthbound

professional and amateur observers for many months. It was even visible visually to observers

with small telescopes and steady skies.

The dispersal of a white “storm” cloud on Saturn in 2010-2011

Saturn also has a very strange hexagonal feature in its atmosphere near its north pole. It was first

discovered by the Voyager spacecraft, but it can now be seen in detailed images from amateur

telescopes. The hexagonal spans about 14,000 km on side, bigger than Earth, and seems to rotate

with the planet’s magnetic field.

-44-


The hexagonal cloud patter near Saturn’s north pole, as image from Cassini

The Hubble Space Telescope and the Cassini spacecraft have imaged a hurricane-like vortex

near the planet’s south pole. The storm has a clearly-defined eye, a feature which is not

observed in vortices on Jupiter, for example. This “south pole vortex”, which is larger than

Earth, may also be a permanent feature.

The south polar vortex in Saturn’s atmosphere

The Ring System

Saturn is perhaps most famous for its splendid system of rings. Jupiter and Uranus also have

rings, but Saturn’s system is by far the brightest and most intricate. The rings extend from about

7,000 km to 120,000 km above the equator, yet are only about 20 meters thick. They consist

-45-


mostly of small particles of water ice and bits of frozen organic compounds. The particles range

in size from fine dust-like grains to boulder-sized pieces no more than 10 meters in diameter.

The origin of Saturn’s rings is unknown. They may be been formed by material left over from

Saturn’s formation some 4.5 billion years ago. Or they may be the remnants of a small moon

destroyed by tidal forces when it wandered too close to the planet. In this case, the rings may be

quite new.

The ring system is amazingly complex. There are dozens of tiny “shepherd” moons among the

rings which split the rings into bands and finer subrings that astronomers are still sorting out. But

from our perspective as backyard stargazers, the rings are split into three main sections: the outer

A-ring, the middle B-ring, and the darker inner C-ring. Between the A and B rings lies the dark

gap called the Cassini division named after the 17th

-century astronomer who first noticed it. This

division is frequently visible in small telescopes. The smaller Encke division in the outer reaches

of the A-ring is a much more challenging sight.

Detail in the rings of Saturn. The A, B, and C ring and the Cassini and Encke divisions are

visible in amateur telescopes.

Moons

Saturn has more than 200 moons, 62 of which have well-determined orbits. The remainder are

small bodies… moonlets… that bob and weave through the planet’s elaborate ring system.

The 62 moons of Saturn are a fascinating mix of objects, ranging from tiny bodies discovered by

close-in spacecraft to the enormous moon Titan, which you can see in a pair of binoculars from

your backyard. Fifty-three of Saturn’s moons are named, and thirteen have a diameter larger

-46-


than 50 km. Just seven of Saturn’s moons are large enough to pull themselves into a spherical

shape.

The largest moons of Saturn, shown in relative size

Of this large collection of moons, nine were discovered with telescopes before the age of

spaceflight. They are, in increasing distance from Saturn: Mimas, Enceladus, Tethys, Dione,

Rhea, Titan, Hyperion, Iapetus, and Phoebe. The moons were named after the Titans of ancient

Greek legend.

Titan is by far the largest of Saturn’s moons, making up nearly 96% of the mass of all bodies

orbiting Saturn, including the rings. Titan is the second-largest moon in the solar system after

Jupiter’s Ganymede, with twice the mass of Earths’ moon. Titan is larger than the planet

Mercury.

Titan is also the only Moon with a substantial atmosphere, which is made mostly of nitrogen

along with traces of methane and other organic molecules. Titan also has lakes of liquid

hydrocarbons; the presence of organic molecules suggests Titan may have the right chemistry for

simple forms of life to develop. Like Earth’s moon, Titan is tidally locked to Saturn such that

its rotational period of 15 days and 22 hours is the same as its orbital period.

You can easily see Titan in binoculars or small telescope. Larger scopes resolve the planet into a

disk and reveal the orange color of the moon’s dense atmosphere.

-47-


Saturn’s largest moon Titan

Each of Saturn’s smaller moons has its own story. Iapetus is one of the most unusual of Saturn’s

moons. One hemisphere of the moon is dark as soot while the other is more reflective. The

moon varies in brightness by a factor of six as it orbits Saturn, and you can see this effect in a

modest telescope.

Mimas, the closest of the larger moons, is just 400 km across. It sports a baleful impact crater

nearly a third of the moon’s diameter, which suggests Mimas was nearly shattered upon collision

with another small moon.

Mimas

Enceladus, the sixth-largest moon of Saturn, is also an intriguing satellite. It is geologically

active and ejects plumes of water into orbit around Saturn. These plumes may be the source of

the innermost ring of Saturn. Enceladus also has liquid water under its southern polar region.

-48-


Saturn eclipsing the Sun as seen from NASA’s Cassini spacecraft.

Observing Tips for Saturn

Many casual observers get hooked on amateur astronomy after a first look at Saturn through a

telescope. More than a few have looked through my small refractor on a night of good seeing

and asked of Saturn, “Is it real?”

Oh yes, it’s real. And incredibly beautiful… the color, the proportions, the apparent 3D

perspective of this grand icy world. It is arguably the finest sight accessible with a small

telescope.

But Saturn can also frustrate many new stargazers. The visual image of the planet in a telescope

is often small. And if the atmosphere is not steady, the image tends to ripple and blur the

delicate details in the clouds and the rings, so it’s never as clear as you see in professional

images. Here’s some hard-won expertise about how to get the best view of this grand planet…

There are four aspects you need to consider to get the best views of Saturn: your optics, the

atmosphere, your observing skill, and your expectations.

Binoculars of 10-12x will show Saturn as a tiny, slightly non-circular disk, and they show Titan

as a tiny point. But that’s about it. To clearly see the rings of Saturn, you will need a telescope.

Which telescope? Really, any telescope will give you a good view of the planet. As mentioned

in the section this month on telescopes, refractors of longer focal length tend to give larger high-

contrast images of planets. Reflectors tend to have a little less contrast because of the central

obstruction of the secondary mirror. But if they are well collimated, reflectors do a fine job with

planets. Most of the best amateur planet imagers, for example, use SCT’s and Newtonians

because these telescopes are available in higher apertures, and higher apertures enable higher

resolution.

-49-


First, before you get started, make sure your telescope is aligned and cooled down to ambient

temperature. If you just take it from a warm house into a cool night, there will be eddies of air in

the telescope tube and movement of the mirror surface that will badly degrade the view until the

temperature of the scope equlibriates with the rest of its surroundings. It will take between 20-60

minutes for the scope to settle down, depending on the size of the mirror and lenses and so on.

Also, it helps to wait until Saturn is as high in the sky as possible before you observe. This year

and for the next decade, Saturn is south of the ecliptic and will never rise very high for northern-

hemisphere observers.

Don’t expect a Hubble-like image. Despite its beauty, Saturn appears quite small in a telescope.

The disk is only 20″ across, about 1/3 the apparent size of Jupiter at its closest. The rings extend

farther, about 45-50”, which makes the planet appear larger but even with the rings it’s never

larger than Jupiter at opposition. You can never see Saturn through a telescope quite as well as

you would like to.

Once you get the planet in view, pop a low-power eyepiece in your scope. At 25x, you’ll see

Saturn as non-circular, and 50-60x should reveal the rings and the planet’s disk.

Now move to at least 100x and take in the view. The image will appear larger but a little fainter

and possibly a little fuzzier. But keep moving to higher magnification until the image gets too

fuzzy or faint. The optimum magnification depends on your telescope and seeing conditions. In

steady sky with a high-quality scope at 300-400x, the sight of Saturn is, in the words of one

amateur astronomer, “pretty enough to make a grown man cry”. But it’s not often you can use

that much magnification. You need to experiment each night to determine the optimum

magnification that will give you the best trade-off between image size, sharpness, and brightness.

And yet you can see a lot of detail on Saturn, even though it may be frustratingly small. Even

nights when the air isn’t so steady, wait for moments of good seeing when the image will

suddenly sharpen and jump out at you like a tiny hologram. It’s darned impressive.

More than most planets, Saturn displays a striking 3-D effect caused by the darkened edges of

the disk and, when you can see them, the shadows cast by the rings on the planet. The rings are

about 17o from edge-on this year and cast a striking shadow towards the side of the planet

opposite the Sun.

The rings are the main attraction of course. At 100-150x, look for the Cassini division, the dark

gap between the A and B rings. It will be visible in reasonably steady sky. Now, can you

discern the difference in brightness between the two rings? Most observers agree the outer ‘A’

ring is fainter than the inner ‘B’ ring. If you have rock-steady sky and an 8-inch scope, look for

the elusive Encke division, another gap near the outer edge of the A-ring. At least 300x will be

needed to see this faint gap.

-50-


Like Jupiter, Saturn has a complex system of cloud bands visible with a small scope. The image

above shows how they’re labeled. These pale whitish-yellow bands are by no means as obvious

as Jupiter’s, but they are visible through most scopes. A yellow filter threaded into the back of

your eyepiece may help bring them out a little.

And there are the moons of Saturn. The brightest is Titan, as mentioned, which you can see with

binoculars. A 6-inch or larger scope may show the color of the dense yellow-orange clouds on

this massive moon. The clouds hide the entire surface of Titan. Which is too bad, because lakes

of liquid hydrocarbons are spread across this planet-like world. With a telescope of 4-inch

aperture, and dark sky, you can also find the moons Iapetus, Rhea, Dione, and Tethys, all of

which are approximately magnitude 10-11. It’s hard to tell one from the another. Here’s a link

for an online javascript tool at Sky and Telescope to help you find which moon is which:

http://www.skyandtelescope.com/observing/objects/javascript/saturn_moons

Simply enter your date, and it will render a small diagram to help you find which moon is in

which position around the planet.

Sky and Telescope also has apps for the iPhone and iPad to help you track Saturn’s moons.

Saturn and its largest moon Titan as they appear in a small telescope at 150x or so.

The Deep Sky This Month

Overview

This month we look at a good range of objects: faint galaxies, resplendent star clusters, two

superb double stars, and a fine planetary nebula. Compared to last month, most of these objects

are quite easy to see. And they are favorites of most experienced stargazers. As you master the

art of stargazing, you will return to these objects year after year.

http://www.skyandtelescope.com/observing/objects/javascript/saturn_moons

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Porrima

Astronomers best know the sprawling constellation Virgo for its rich trove of distant galaxies.

But Virgo holds a much closer object, the fine binary star Porrima, one of the few stars close

enough to show detectable motion during the brief span of a human lifetime.

Porrima, also known as γ (gamma) Virginis, lies at the southern end of a large arc of stars which

starts at Vindemiatrix, then swoops south to delta Virginis, then Porrima itself, then to Zaniah

and Zavijava (see map below). This arc of stars represents the “arms of the maiden” Virgo.

Porrima lies just 39 light years away. The stars revolve about each other in 169 years, which

means you can see, in a good backyard telescope, the motion of the stars over the course of a

decade or so. Until 1995, the stars split easily in a small scope. Then they moved too close

together to resolve. Since 2010, the stars have slowly begun to separate once again. In 2013, the

stars moved to a separation of 1.7”, enough to resolve in most telescopes, but only on nights of

very steady seeing. Try 150x or more to glimpse this challenging pair. It’s a good test of your

optics and your observing ability.

Each star in the Porrima system is about magnitude 3.6, and each is a white F-type main

sequence star just a little bigger and brighter and hotter than our Sun.

See the map below to locate Porrima and the next target, M104.

M104 (“Sombrero Galaxy”)

Striking in photographs, the Sombrero galaxy offers a fine, if unusual, example of an edge-on

spiral that looks like the classical Mexican headwear worn by members of a mariachi band. This

is a lovely object, a little like NGC 4565 in Coma Berenices but with a huge and brilliant central

bulge, likely caused by the machinations of a massive black hole in the center.

At magnitude 9.0, the Sombrero Galaxy is located 11.5° west of Spica and 5.5° northeast of eta

Corvi. The galaxy lies 20o from the main Virgo Cluster, but it’s likely an outlier of the cluster.

M104 is visible in 7x50 binoculars in good sky. In a 3” or 4” scope at low power, the galaxy

presents itself as a small oval glow with a star-like core. Increase to 75-100x to get a much better

view and get a glimpse of the dark dust lane that runs through the middle and splits the galaxy

into two unequal sections.

Charles Messier included M104 as an addendum to his famous list of 103 non-stellar objects in a

handwritten note. But it was not officially included in his famous list until 1921.

-52-


M104 was the first galaxy in which rotational motion was measured. By looking at the differing

redshifts of stars at each edge of the galaxy, it was shown that one edge turns towards us and

another turns away.

Location of double star Porrima and the Sombrero Galaxy (M104) in Virgo

Less than a hundred years ago, astronomers were uncertain about the nature of M104 and other

spiral “nebulae” as they then called them. Some thought they were relatively closeby objects in

our own galaxy. Others thought they were separate star systems in their own right. Working at

Lowell Observatory in 1912, the astronomer Vesto Slipher made the astonishing discovery that

the Sombrero was hurtling away from Earth at 700 miles per second, faster than anything in our

galaxy. This supported the view that these “nebulae” were outside our own galaxy, a view

confirmed in 1923 when the distance to the Andromeda galaxy was determined by Edwin

Hubble.

-53-


M104, the “Sombrero Galaxy” in Virgo

M87

The region between the star Denebola in Leo and Vindemiatrix in Virgo is packed with the

galaxies of the so-called Virgo Cluster, the nearest major galaxy cluster to the Milky Way.

There are some 2,000 galaxies here accessible to professional telescopes, and perhaps a hundred

visible in a 6-inch scope in dark sky. M87 is perhaps the dominant galaxy of the Virgo Cluster

and lies close to the center of mass of the mighty cluster. There’s nothing subtle about the giant

elliptical galaxy… no faint spiral structure or dust lanes to see at all. The excitement lies in

contemplating the stupendous size of M87 and the scope of the violent physical processes going

on in its core.

M87 is close by two other bright elliptical galaxies, M84 and M86. These three ellipticals are

the brightest fuzzy objects in this patch of sky. M87 lies just 1.5o east-southeast of M84 and

M86 at the tip of Markarian’s Chain (see the next section). The map below helps you get

located.

M87 is brighter and all by itself, while M86 and M84 lie side by side. A larger telescope gives a

brighter view of these galaxies but reveals no more detail. At magnitude 8.8, M87 is visible in

binoculars or the smallest telescope. This giant galaxy spans a diameter of 120,000 light years,

about the same as the Milky Way. But M87 is a spheroid, not a flat spiral. So it contains far more

stars… as many as 2.7 trillion solar masses by some estimates.

Aside from its immensity, M87 is known for a needle-like jet that’s blasted 5,000 light years into

intergalactic space, presumably by a disturbance caused by a black hole in its core. The central

black hole is monstrous… with an estimated mass of 2-3 billion suns! Radio astronomers see the

active center of M87 as a strong radio source called “Virgo A”. Keen observers under perfect

seeing conditions can see the jet in very large scopes of 20” to 30” aperture.

-54-


M87 and the galaxies of Markarian’s Chain

M87 also boasts a spectacular collection of globular clusters, up to 15,000, compared to the

Milky Way’s count of 200 or so. However, none are visible with backyard scopes.

If you have dark sky, look for another galaxy just 9’ (less than 1/6 of a degree) southwest of the

center of M87. This is 11th-magnitude NGC 4478, a small companion galaxy to the supergiant

M87. The pair lie about 53 million light years away.

-55-


Elliptical galaxy M87, and the associated group known as Markarian’s Chain

Markarian’s Chain as it might appear in a 4”-6” telescope at 50-80x. M86 and M84 are at

lower right. M87 is not in this image; it lies off image to the lower left

Markarian’s Chain

Finding galaxies between Denebola and Vindemiatrix is a little easier if you use a faint T-shaped

group of stars about 1/3 of the way from Denebola to Vindemiatrix. See the map above for the

location of this “T”, which spans about 2o. The brightest star in this group is 5th-magnitude 6

Comae Berenices, which is visible to the unaided eye in dark sky. Some 1.5o southeast of the

-56-


southern tip of the vertical bar of this “T” lie the two 9th

-magnitude galaxies, M84 and M86,

mentioned above. They mark the western end of a graceful arc of galaxies called “Markarian’s

Chain”, which arcs northeastward from M84 for 1.5o from Virgo into Coma Berenices, and

which includes at least seven galaxies. The group takes its name from astronomer Benjamin E.

Markarian who in 1961 published a paper that described his investigations of the dynamics of the

Virgo Cluster.

The chain looks best at lowest magnification in a large wide-field telescope like a 10” or 12”

Dobsonian. But a 4” scope at 50-75x in dark sky shows most of the chain, though you need to

look carefully for the fainter members. The brightest (and westernmost) members of the chain

are the side-by-side pair of elliptical galaxies M84 and M86. The pair lies almost exactly halfway

between Denebola and Vindemiatrix. M84 presents a 9th-magnitude oval glow, extended

slightly northwest to southeast, and a bright core. It’s an easy sight in a 3” or 4” scope.

M86 is just 1/4 degree to the east of M84. It’s about the same brightness. It’s slightly more

elongated in the same direction as M84, though its nucleus appears even brighter. Directly north

of M86 by 1/10 of a degree is the challenging 12th-magnitude galaxy NGC 4402. You’ll need at

least a 6” or 8” scope to pick it out of the background.

Turn your glance eastward along the chain to the two galaxies known as “The Eyes”. The

brighter and larger of the two is NGC 4438, just north of which is NGC 4435. These galaxies are

easily seen in a 4” scope in good sky. Recent observations of adjacent hot gas at X-ray

wavelengths suggest these galaxies collided 100 million years ago.

Now look 1/3 of a degree northeast of The Eyes to spot lenticular galaxy NGC 4461 and the

fainter elongated spiral galaxy NGC 4458. The pair is a challenge in a 4” scope, but pop out

nicely in a larger scope.

The last step in Markarian’s Chain takes you over the border into Coma Berenices to the

respectably bright galaxies NGC 4473 and NGC 4477. The former is an elliptical galaxy

extended into an east-west oval. The latter is a barred spiral galaxy, though you’ll see no

indication of this in a backyard scope. Both are about 10th magnitude and easy to see in a 4”

scope.

Most of the galaxies in Markarian’s Chain lie about 50 million light years away and are

gravitationally related and move together through space.

This chain is the most difficult sight on this month’s tour, but once you find M84 and M86, you

will at least have a starting point to find this famous group. Keep at it… if you have a 4”-6”

scope and dark sky, you will eventually succeed in seeing most of its members. Averted vision

and good dark adaptation are a must! A 4” scope of f/8 or slower at low magnification will show

all Markarian’s chain and M87 in the same field of view.

-57-


NGC 4216

Now to a lesser observed galaxy, the beautiful spiral NGC 4216 just 2o due south of 6 Comae

Berenices, the brightest star in our “T” guidepost mentioned above. This spiral is just a few

degrees from edge-on, but that’s enough for long-exposure images to reveal much detail,

including a bright central nucleus, spiral arms, and hints of a dust lane.

NGC 4216 (middle) as it appears in an 8” telescope. Fainter galaxies NGC 4222 (upper left)

and NGC 4206 (lower right) may be visible under optimum conditions in a 12” or larger scope.

In the late 19th century, this galaxy was of interest to astronomers who were trying to determine

the nature of these “spiral nebulae” as they were called. They photographed the spiral arms of

this galaxy and others to directly detect rotational motion. None was detected. The galaxies do

rotate, but they are much farther away than 19th-century astronomers could imagine, so such

rotation is hard to see directly. Astronomers now detect rotation in the arms of spiral galaxies by

observing the Doppler shifts of spectral lines from the stars within the arms.

In a 4” to 6” scope at 75x the galaxy is a slender white sliver about 5’ long. It has a bright core

and sharp edge, at least with averted vision. The dust lanes are very difficult to see with a 6”

scope. More aperture is needed. While the galaxy appears small, it’s actually as large as our

Milky Way, with a diameter of 95,000 light years and a mass of about 200 billion suns. The

galaxy lies about 55 million light years away.

M13 (“Great Cluster in Hercules”)

Now let’s leave galaxies behind for a while and get a first look at the fine objects coming into

view during the northern summer months. The most famous of all globular cluster north of the

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celestial equator lies here in the constellation Hercules. Cataloged as Messier 13, it’s sometimes

simply called the “Great Cluster” or the “Great Cluster in Hercules”.

Globular cluster Messier 13 in Hercules. Galaxy NGC 6207 is visible at lower left.

M13 competes with M5 in Serpens as the brightest globular cluster north of the celestial equator.

(You will see M5 next month). To many, M13 has a finer appearance in a small telescope, likely

as a result of its looser arrangement of stars. It’s also fairly easy to find, located between the stars

eta (η) and zeta (ζ) Herculis on the western side of the “Keystone” of Hercules.

At magnitude 5.8, the cluster is easily visible in binoculars or finder as a distinctly un-starlike

smudge. In dark sky the cluster is visible without optics. In a 4” scope at 100x or so, the cluster

resolves into a halo of stars about 20’ across, with a gradual brightening towards the core. An 8”

scope resolves the cluster all the way to the core, revealing an amazing arrangement of tiny

pinprick white and yellow stars of 11th and 12th magnitude. Take a quick glance to the side with

averted vision, and hundreds more stars snap into view in an almost shocking manner.

The cluster contains some 500,000 stars with an age of 13 billion years, not much younger than

the universe itself.

Like most globulars, M13 lies out of the plane of the Milky Way. So we’re looking here into

deep intergalactic space… all the individual stars are foreground stars in our own galaxy. The

spiral galaxy in NGC 6207, an 11th

-magnitude object some 45 million light years away, lies

about one degree to the northeast of M13. It may be visible in a 10-inch or larger scope. In very

detailed images of M13, you can see dozens more galaxies in the background.

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While M13 lies a distant 25,000 light years from Earth, author Kurt Vonnegut wrote, accurately,

“Every passing hour brings the Solar System forty-three thousand miles closer to Globular

Cluster M13 in Hercules — and still there are some misfits who insist that there is no such thing

as progress.”

M92

Hercules has another excellent globular cluster. But M92 is overshadowed by its beautiful and

famous sister M13 and it sits alone and nearly forgotten in the northern reaches of Hercules.

A comparative view of M13 (left) and M92 (right) as they appear in a 6” scope at 120x

If M92 were in most other constellations, it would rank as a showpiece object. This is a splendid

cluster in a small scope. At moderate magnification in a 4” scope, it easily resolves into

individual stars in the halo, and a larger scope reveals a core of fine diamond dust. The cluster

appears somewhat oval compared to M13, and perhaps slightly more concentrated near the

center. M92 is about the same distance away as M13, so its slightly smaller apparent size is a

consequence of its truly smaller diameter. The cluster is located north of the Keystone, about 7o

north of pi (π) Herc. It forms a nearly equilateral triangle with pi and eta with each side

measuring 7o or so. In very dark and clear sky, those with keen eyesight may see the magnitude

6.5 cluster without optical aid.

The image above shows M13 and M92 captured with the same telescope and same conditions.

As you can see, M92 holds up well to its more famous neighbor.

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The location of M92, M13, and double star alpha Herc, or Rasalgethi (bottom) in the

constellation Hercules

Alpha Herculis

Down at the bottom of Hercules, which is actually the great hero’s arms, lies the star alpha

Herculis. If you’re trying to learn your Flamsteed designations, the star is also known at 64

Herculis. This star’s proper name is Rasalgethi (“rah-sell-GAYTH-ee”), which means “Head of

the Kneeler”. This is a splendid star for visual observation. The components have a stirring color

contrast of orange and blue-green… a very pretty sight.

You’ll need a scope at 100x to resolve these two: they’re separated by about 4.6” (arc-seconds),

which is fairly tight. These stars take nearly 3,600 years to revolve around each other.

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The brighter star of alpha Herculis is an enormous M-type red giant that’s 400 times the diameter

of the Sun and some 800 times as bright. The pair have magnitudes of 3.5 and 5.4 roughly, and

are some 380 light-years away. The fainter G-type star is itself a closely-space double with an

invisible white dwarf companion. You might recall that G-type stars are yellow-white, but the

visual contrast of this star with its orange-red companion makes it appear greenish-blue to many

observers. What do you see?

Rasalgethi (alpha Herculi)

NGC 6543 (“Cat’s Eye Nebula”)

Now look up, way up, to the winding constellation Draco. There are many fine sights in this

constellation, but let’s pick the most famous, the excellent though small planetary nebula NGC

6543. Because of its appearance in detailed photographs, this nebula is sometimes called the

“Cat’s Eye Nebula”.

The Cat’s Eye is a tiny planetary. It appears star-like at 20x, though its greenish-blue appearance

may distinguish it from the surrounding stars. Increase magnification to 75x or more and the disk

of the nebula is resolved. The central star of NGC 6543, the star that’s throwing off the nebula, is

fairly bright, and most telescopes reveal it easily with modest magnification. At 125x or more,

the nebula shows some texture, including a somewhat darker inner region and a brighter ring

around the outside. The shape is slightly oval, and the color is quite pleasing compared to the

white stars in the background. Like most small planetary nebulae, the Cat’s Eye responds well to

high magnification if the sky is steady.

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The red circle marks the position of NGC 6543, the Cat’s Eye, about halfway between delta (δ)

and zeta (ζ) Draconis

The Cat’s Eye played an important role in astronomical history. It was the first to be examined in

a spectroscope by English amateur William Huggins in 1864. Huggins was shocked to find the

nebula had a totally different spectrum from any star. Its spectrum displayed a very small number

of emission lines at about 500 nm... the blue-green region of the visible spectrum. This led

Huggins to conclude the nebula was composed of a glowing gas, but he could not associate the

spectrum with any Earthly element. So the mysterious element was named “nebulium”. It turns

out the blue-green spectral lines are indeed from an Earthly element-- doubly ionized oxygen, or

OIII-- but it took some time for laboratory-based spectroscopists to figure this out because

oxygen must be in an extremely rarified state to emit light at this wavelength.

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A detailed view of the Cat’s Eye Nebula (NGC 6543)

What You Have Learned This Month Once you work through the notes and tours of The Art of Stargazing this month, you will have

learned:

Sky Tours

The legends, layout, and position of the constellations Virgo, Libra, Corvus, Crater,

Hercules, and Draco

The bright stars Spica, Porrima, and Vindemiatrix; the stars Zubenalgenubi and

Zubenalschemali in Libra, and the star Thuban in Draco, which was once the North Star

Science of Astronomy

How stars form out of large, massive, but very tenuous and cold gas clouds

Why stars have different colors, and how the color of a star is related to temperature

How astronomers classify stars according to their temperature and spectrum, and the

basic types of stars which are, from hottest to coldest, O-B-A-F-G-K-M.

The H-R diagram, the most important tool for stellar astronomers to understand and

classify stars

Observing Techniques

A detailed look at the design and the pros and cons of the major types of telescopes

available to amateur astronomers: refractors (achromatic and apochromatic); Newtonian

and Dobsonian reflectors; Schmidt-Cassegrain reflectors; and Maksutov-Cassegrain

reflectors

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Deep-Sky Objects

More galaxies in and around the Virgo Galaxy Cluster, including M87, M104, NGC

4216, and Markarian’s Chain; the very fine globular clusters in Hercules, M13 and M92;

the double stars Porrima and Rasalgethi; the planetary nebula NGC 6543 (the “Cat’s

Eye’)

Solar System Tour

A detailed look at the planet Saturn: its structure and composition, its ring systems, and

its moons

How to observe Saturn with a small telescope

Notes: The Art of Stargazing Month 4: May - June 2013 · -6-The Art of Stargazing – Month 4...

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Transcript of Notes: The Art of Stargazing Month 4: May - June 2013 · -6-The Art of Stargazing – Month 4...