Lecture 11

6/25/07

Astro 1001

White Dwarfs• A White Dwarf is the exposed

core of a star that has died and shed its outer layers

• Very small radius, but lots of mass– Large amounts of degeneracy

pressure needed to counter gravity

• The most massive White Dwarfs are the smallest

The Chandrasekhar Limit

• At 1.4x the mass of the Sun, gravity overpowers electron degeneracy pressure

• A type Ia Supernova occurs if too much mass is added after the White Dwarf forms

• A nova may occur as material is dumped onto the White Dwarf

Neutron Stars

• Remains of a massive star supernova

• Supported by neutron degeneracy pressure– About 10 kilometers in

diameter

• The structure of a neutron star is somewhat uncertain– Probably contains a crust and

then a sea of neutrons

Pulsars• Pulsars are neutron stars that

emit a very regular signal• As the massive star went

supernova, it contracted and strengthened its magnetic fields

• The first extrasolar planets were discovered around a pulsar

Neutron Star Binaries

• Immense gravitational field means that lots of potential energy is released by anything falling onto the neutron star

• X-Ray Binaries occur when matter is regularly accreted onto the neutron star

Black Holes• Sometimes no pressure can

stop gravity from collapsing a star

• The event horizon is the point of no return

• Black Holes appear to make information be irretrievable– Can only measure the BH’s

mass, charge, and angular momentum

Group Work

• The Sun is not massive enough to form a Black Hole. However, lets say that by some mysterious process it suddenly collapses to form a Black Hole of exactly 1 solar mass. What would happen to Earth’s orbit after the Sun became a Black Hole?

Visiting a Black Hole

• As you approach the black hole, time slows down and you experience a gravitational redshift

• Whether or not you fall into the black hole depends on who is telling the story

• Tidal forces are 1 trillion times as strong as the one that causes the tides

Do Black Holes Really Exist?

• Theoretically, Black Holes must form at 2-3 solar masses

• You can detect Black Holes by looking for X-Ray sources

• Strong evidence for supermassive black holes at the center of galaxies

Gamma Ray Bursts

• In the 60s, we began to detect intense bursts of Gamma Rays

• In the 90s it was discovered that the sources were evenly distributed across the sky

• Since then, the Bursts have been traced to massive explosions in distant galaxies

What Causes GRBs?• If the energy was emitted in

all directions, the energy would be millions of times that of an entire galaxy– Energy is probably beamed

• At least some GRBs are associated with supernovae

• Two types of GRBs: short and long– Short bursts do NOT appear to

come from supernovae

The Milky Way

• A faint band goes across the sky– Galileo showed that

the band was made of individual stars

• We are inside of the galaxy, so its hard to see what the overall structure of it is

Structure Basics

• We live in a spiral galaxy• Has spiral arms in a flat

disk• In the center is a bulge of

stars• The outskirts of the

galaxy are called the Halo• Also a series of nearby

galaxies

Orbits of Stars• Disk stars go around the center of the galaxy

– Also oscillate above and below the disk

• Halo and bulge stars move around randomly– Can be very far away from the disk

• We can look at the orbits of stars to figure out the mass of the galaxy

Galactic Recycling

• Stars dump their processed material into the ISM as they die– Also create cosmic rays

• Gravity drags the gas together and cools it

• Eventually large gas clouds are formed, from which stars can form

Where Do Stars Form?

• Stars don’t form uniformly in the galaxy

• Stars like to form in the spiral arms of galaxies– We say that the arms appear

“blue” while other parts appear “red”

• Spiral density waves are probably responsible for this

Galaxy Formation

• The galaxy formed from a Protogalactic Cloud in a way similar to how stars form

• There may have been multiple clouds

• Or, many Milky Way stars originally formed in other cannibalized galaxies

The Galactic Center

• The galactic center lies in the constellation Sagittarius

• Probably a black hole 3 million times the mass of the Sun– Sgr A*

• Not much matter appears to be accreted by the black hole

Other Galaxies

• There are lots of other galaxies out there– Over 100 billion in the

observable universe

• Galaxies come in many different shapes and sizes

• All galaxies appear to have formed at the same time

Types of Galaxies

• Spiral Galaxies– Like our own galaxy– Relatively rare– Might be Lenticular (no

spiral arms)

• Elliptical Galaxies– Red and round– Often football shaped

• Irregular Galaxies– Strange shapes

Elliptical Galaxies

• Small ellipticals are the most common type of galaxy

• Usually contain very little gas or dust

• Large ellipticals are probably the result of smaller galaxies being absorbed– Contain lots of hot gas and

dust

The Hubble Tuning Fork

• Hubble came up with a system to classify galaxies

Distances to Galaxies

• Standard Candles– If we know how bright something is and how bright it

appears, we can figure out how far away it is

• Main Sequence fitting– Done using the Hyades as an example

• Cepheid Variables– Historically important– Period-Luminosity relationship

• Type Ia Supernovae

Cepheid Variables• Pulsating stars that vary in brightness

• How long they take to repeat their pattern announces how bright they are

• Used by Hubble to determine how far away galaxies are

Type Ia Supernovae

• The exact same conditions occur for every Type Ia Supernova– A star of exactly 1.4 solar masses goes through

the exact same process

• Supernovae are very luminous, so you can use this method to determine the distance to very distant galaxies

Group Work

• A typical Type Ia supernova has a luminosity of about 1 x 1045 watts. Lets say that we observe a supernova that appears to be 5 x 10-15 watts. How far away is it? Express your answer in meters and in lightyears (example on page 623).

Hubble

• Shapley-Curtis debate was unable to resolve whether or not galaxies were island universes or part of our own galaxy

• Hubble used a new 100 inch telescope to resolve individual stars in Andromeda– Noticed Cepheid variables– Was a bit off, but close enough

Hubble’s Law

• Hubble realized that the further away a galaxy was, the more redshifted it was– V = H0 x d

• A few caveats:– Galaxies do not obey the

law exactly since they might have speeds not associated with the expansion of the universe