Chapter 21

Earth Science 11

Constellations

� Constellation: � A group of stars that appear to form patterns in the sky.

� 88 different constellations can be seen from the Northern and Southern hemispheres

� Best known is the Big Dipper:� Part of a larger constellation called Ursa Major, or the Big Bear

� Constellations can be used for: � Navigational aids

� Find other constellations including POLARIS the “North Star”

� Circumpolar constellations: � Never set below the horizon

� In the north appear to rotate around the north star

� How many you see depends on your latitude

� Ex. URSA Major, URSA Minor and Cassiopeia (northern)

� The apparent movement of these circumpolar constellations is due to the earth’s rotation

� Earth rotates from WEST to EAST. ○ Therefore stars, the moon, and the sun all RISE in the EAST and SET in the

WEST.

Seasonal Changes in

Constellations� Circumpolar constellations change in the sky with the change in

seasons� Fall:

○ Big Dipper is near the northern horizon

○ Cassiopeia is nearly straight overhead

� Spring:○ Big Dipper is high overhead

○ Cassiopeia is near the northern horizon

� Each star is moving on its own, sometimes at very high velocities� In time the constellations we are familiar with today will no longer be

there

� Each star will have moved and new constellations will be made

� Some constellations only appear at certain times of the year� The most famous winter constellation is Orion the Hunter

� Orion contains the red supergiant Betelgeuse AND the blue supergiant Rigel.

Distances to Stars

� The closest star to the earth is the SUN

� Distance is measured 2 ways:� 1. Astronomical unit (AU):

○ The average distance between earth and the sun is 150 million km� Sun= 1 AU

� Jupiter = 4 AU (at closest encounter)

� Pluto = 38 AU (at closest encounter)

○ How far is the nearest star?� If the earth was a dot 1 cm from the sun then using that same scale the next star

would be 2.5 km away (Alpha Centauri)

� In normal scale Alpha Centauri is 40 trillion (4.0 x 1013) km away

� Using Kilometers as a measurement is not very practical because it is TOO SMALL

� 2. Light year(LY)○ distance light travels in 1 year (9.5 trillion km in 1 year)

○ Ex. Earth to:� Moon = 2 sec

� Sun = 8 minutes

� Alpha Centauri = 4.3 LY

� Betelgeuse, the red supergiant in Orion = 490 LY

� North Star = 680 LY

Physical Properties of Stars

� Our sun is classified as a Yellow Dwarf

with a diameter of 1,380,000 Km

Sun Some Stars

Density 1.4 x of water air to 1

ton/teaspoon

Diameter 108x Earth’s smaller than Earth

to 2,000 x Sun’s

Mass 300,000 x

Earth’s mass

1/100 to 50x Sun’s

Color yellow red to blue-white

Temperatur

e

5,500 °°°°C

(surface)

3,000 to 30,000 °°°°C

Elements in Stars

� Most stars are 70% Hydrogen and 28% Helium

� 1-2% of a star’s mass may be heavier elements such as oxygen, carbon, nitrogen, calcium, sodium

� The spectrum radiated by a star depends on both its composition and its temperature

� No two stars have exactly the same spectrum. A star’s spectrum is like its fingerprint.

Star Brightness

� 1. Luminosity = true brightness of a star� Depends only upon the size and temperature of a star

� If two stars had the same temperature, the larger star would be more luminous than a cool star

� 2. Apparent magnitude or brightness:� How bright a star appears on earth.

� Dependent on star’s luminosity and distance from us

� The brightest stars are “first-magnitude” stars

� The faintest stars that can be seen with the unaided eye are “sixth magnitude”

� Each star’s magnitude differs from the next by a factor of 2.5○ Ex. first magnitude star is 2.5 brighter than a second magnitude star

� Some stars are even brighter than first magnitude stars (1.0)○ Ex: Sirius the brightest star in our sky, has an apparent magnitude of -1.43.

� Ex. A 100 watt light bulb is much brighter than a flashlight bulb. ○ The 100 watt bulb has GREATER Luminosity\

○ However, if held up close, the flashlight bulb would look brighter.

○ Flash light has a greater apparent magnitude

� 3. Absolute magnitude: � How bright the star would be at 32.6 light-years

� Used to express the luminosity of stars as if all stars were the same distance from Earth

� Ex. Sun = 4.8 (average); Rigel = -6.4 (very bright)

Giants, Supergiants, and Dwarfs

� Giant:� Larger in diameter than the Sun

� Luminosity: 10 – 1000 times the Sun

� Stars more luminous than giants are called supergiants

� Ex. Aldebaran and Arcturus (Red Giants) – HUGE and luminous

� Supergiant:� Mass: 8-12 times the Sun

� Luminosity: 10,000 – 1,000,000 times the Sun

� Red supergiants are the largest of all stars

� Ex. Rigel (blue-white); Canopus (white-yellow), Antares and Betelgeuse (red)

� Dwarf:� Less luminous

� Absolute magnitude (brightness) no more than 1

� Most are red, orange, yellow or white

� White dwarfs are very faint, small, and dense (same size as earth but 100,000 time more dense)

Variable Stars

� Variable stars:� Most stars shine with a steady brightness; however, some stars vary in

brightness over regular periods or cycles

� These cycles can vary from 1 to 50 days○ Most have periods of about 5 days.

� 1. Pulsating stars:� Variable stars that change diameter (size) as they change brightness

� Contraction = star becomes hotter and more bright

� Expansion = star become cooler and less bright

� Ex. Cepheids in the constellation Cepheus (a.k.a. Cepheid Variables)

� 2. Eclipsing Binary:� When TWO stars are orbiting each other and one star is dimmer than

the other

� When the dim star moves in front, the apparent magnitude (brightness) drops

� The effect is like a pulsating star

Pulsars

� Pulsar:� an object that gives off powerful bursts of radio

waves and light waves in a regular period of time

� Originally found using a Radio Telescope (picks up low luminosity)

� Ex. In the middle of the Crab Nebula is a pulsar that produces these radio/light wave bursts every second or less

� Hundreds of Pulsars are now known and believed to be neutron stars formed in the aftermath of a supernova explosion

� The fastest pulsar found pulsates 642 times per second.

Origin of a Star

� According to the Proto-star Theory, stars form wherever dense clouds of gas and dust exist. � These are called stellar nurseries and have an average diameter of 25 light years

� HUGE clouds of gas and dust occur (nebulae) in parts of space between the stars� Contain as much material as the stars themselves

� These clouds are about 99% gas (Hydrogen)

� Remaining 1% is a mixture of very fine particles of silicon carbide, graphite, diamonds, nitrogen and other elements

� It is believed this gas and dust comes from the remains of exploding stars and supernovas

� Sometimes these great clouds of dust and gas start to come together under their own gravity○ Other times it takes an explosion of a nearby star to send a shockwave through the nebulae and

kick-start the process

� Nebulae:� Areas in space where such cloud formations can be found

� Most are invisible

� When these clouds are lit up by a star we can see them and they are immense

� Nebulae that are not near stars may show up as a dark patch in space○ Known as a dark nebula like the Horsehead Nebula also in Orion

� The brightest nebula is the Great Nebula in Orion

Formation of a Red Giant

� When a star has used up its “stable” fuel, the force of fusion no longer balances with the force of gravity and the star loses its stability� 1. When this occurs the core contracts in upon itself

and becomes very hot causing the outer layers of the star to expand away from the core○ This expansion enlarges the star’s surface area

○ The star again radiates more light and appears brighter

� 2. Now this radiation and heat starts fusion in the star’s outer layers causing even greater expansion○ The core is composed mostly of helium formed from the

original hydrogen fusion process

� 3. As the expansion continues the star becomes a red-giant or SUPERGIANT

Formation of White Dwarfs

� Finally we come to a stage in the stars life where most of the fuel for fusion is used up� 1. The temperature and pressure of the core can no

longer support the weight of its outer layers

� 2. The Giant then collapses○ The nuclei of its atoms are squeezed tightly together and this

can form what we call a White Dwarf (no larger than our Earth)

� 3. With most of its fuel gone the white dwarf cannot maintain its high temperature and in a billion years it will eventually glow fainter until it becomes cold and dark○ Occasionally a white dwarf can flare up again due to

bombardment of material from another star. This is called a NOVA.

○ Our Sun is expected to have this fate. It will eventually collapse in upon itself, after flaring into a red giant, and become a white dwarf

Supernovas

� When fusion has stopped it leaves an iron core� As the star cools this core collapses in upon itself

� With this collapse, the pressure and temperature within the core rises dramatically

� The iron core starts to fuse into even heavier elements

� Now the core wants to collapse even further� In this rush to collapse the star EXPLODES so violently that half its mass is blown out into

space

� This explosion has a very intense flare and bright light we call a SUPERnova

� For just a few weeks or months this one star can outshine an entire galaxy

� Ex. The best recorded supernova was recorded by the Chinese in the year 1054. � This brilliant star faded after a year and its outer shell was changed into a great expanding

cloud of gas we now know as the Crab Nebula

� The Crab Nebula can be found in the constellation Taurus the Bull.

� Ex. The most famous SuperNova occurred in the Large Magellanic Cloud and was visible by the naked eye in 1987.� This explosion was used to, and is still used, to define the study of supernovas and help predict

when and where they may occur.

� Scientist predicted that supernovas produce a particle called neutrinos� Hours before light became visible from this explosion instruments detected the neutrinos here

on earth

Neutron Stars and Black Holes

� Supernovas eject half of their mass during the explosion. So what happens to the other half?� The mass that remains is what astronomers call a neutron star

� In the core of a supernova the forces are so great that every atom’s electrons are crushed into its nucleus� The collapsed electrons combine with the protons to form neutrons

� A neutron star is only about ten kilometers in diameter and trillions of times more dense than the sun

� What would happen if an even more massive star would explode into a supernova leaving behind a core that is even more dense than a neutron star?� Such gravitational forces would be so great that not even light could escape

� We call these Black Holes

� We cannot see these Black Holes� Must determine their location by the effect they have on other objects nearby

� By the energy (X-Rays) given off by the matter that is falling into them

� Ex. Cygnus is a constellation that contains a star called Cygnus X-1. � This star is orbiting something we cannot see

� Very powerful X-Rays are being emitted from this star (not normal)

� Astronomers now feel that this is the first tangible proof of a Black Hole

What are Galaxies?

� Astronomers used to look at the night sky and see many stars and fuzzy images of what they thought were nebulae

� With the Hubble Space Telescope we now recognize that space has BILLIONS of Galaxies and each galaxy has BILLIONS of Stars

� The galaxy to which our Sun belongs is the Milky Way galaxy� Our Sun is one of 100 Billion stars in the Milky Way

� Every star seen with the naked eye belongs in the Milky Way

� The diameter of the Milky Way is about 140,000 light years

� At its thickest point, in the middle, it is about 20,000 light years thick

� Our Sun, and us, are approximately 23,000 light years from the galaxy’s center

� The Milky Way belongs to a small cluster of 17 galaxies called the Local Group� The nearest neighbors in the Local Group, the two Magellanic Clouds, are in the

Southern Hemisphere. These two galaxies can be seen without a telescope

� Another neighbor, Andromeda Galaxy, is faintly visible to the unaided eye in the Northern Hemisphere.

� The Andromeda Galaxy is larger than the Milky Way and is about two million light years away.

Types of Galaxies

� 1. Spiral Galaxies� These have a central lens shape, bright nucleus made of millions of

stars

� Around the nucleus is a flat disk of stars arranged in spiral arms

� Each Spiral Arm usually come out from opposite sides of the nucleus

� The arms trail behind the galaxy as it rotates

� Each arm contains millions of stars

� Although clouds of dust and gas can be found within the spiral arms almost NO dust or gas occur between the arms

� Ex. The Milky Way and Andromeda Galaxies

� 2. Elliptical Galaxies� These range from nearly spherical to lens shaped

� Most stars are close to the center

� They have no arms and almost no gas and dust clouds

� 3. Irregular Galaxies� These are smaller and fainter

� Their stars are spread unevenly

� Ex. The two Magellanic Clouds are in this class

Quasars

� Short for “Quasi-stellar radio sources”

� Quasars appear as very faint objects because they are very far away

� Most luminous objects in the universe� Steadily and continuously produce both light and radio waves at

very high rates

� Larger than any known star

� Scientist think that a quasar may actually be an entire galaxy in an early stage of development.

� Example:� One quasar called PKS 2000-330 is about 12 BILLION light-

years away○ At this distance no known star can be seen.

� Therefore, to be seen at that range, it must be as bright as 100 trillion suns and be billions of times more massive

Origin of the Universe

� “Big-Bang Hypothesis”:� The whole universe was originally packed into one dense sphere of

hydrogen○ Not much bigger than the sun is today

� About 15 Billion Years ago this mass of Hydrogen exploded forming a gigantic expanding cloud

� Some parts of the cloud moved faster than others, but all parts moved outward, away from the center and are still doing so today

� Eventually the clouds cooled and condensed into galaxies. ○ Billions of galaxies were formed (all moving outward)

� What is the support for this theory?� 1. DOPPLER Shift

○ Red Shift (moving away) / Blue Shift (moving closer)

� 2. Background Radiation○ In 1964 two physicists (A. Penzias and R. Wilson) discovered microwave

radiation coming from all directions in space

○ This background radiation is thought to be the echo of the Big Bang