Lecture 11 62507 Astro 1001 White Dwarfs A

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. 4 x 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 XRay sources • Strong evidence for supermassive black holes at the center of galaxies

Gamma Ray Bursts • In the 60 s, we began to detect intense bursts of Gamma Rays • In the 90 s 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 = H 0 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