Chapter 15 Stellar Graveyard White Dwarfs Neutron Stars

Chapter 15: Stellar Graveyard White Dwarfs Neutron Stars Black Holes Gamma-Ray Bursts

15. 1 Degenerate Objects • Objects made of degenerate matter (plasma conditions dominated by degeneracy pressure). • White dwarfs are supported by electron degeneracy pressure. • Neutron stars are supported by neutron degeneracy pressure. • When gravity is stronger than neutron degeneracy pressure => black hole.

15. 2 White Dwarfs • WDs have sizes similar to terrestrial planets. • A pair of dice made of degenerate electron matter would weight about 5 tons on Earth. • WD composition reflects the products of the star’s final burning stage. • Intermediate-mass stars yield WDs made of oxygen and heavier elements. • 1 MSun stars leave behind a WD made mostly of carbon. • Very low-mass stars leave behind He WDs.

The White Dwarf Limit • Electron speeds are higher in more massive WDs. • At 1. 4 MSun the e- speeds approach c. Their speed cannot increase and they cannot resist the crush of gravity. This is the Chandrasekhar limit to a WD mass. • All WDs with observed masses comply.

WDs in Close Binary Systems • WDs in close binary systems can gain matter from a companion through an accretion disk. • Particles in the disk obey Kepler’s laws. Inner parts move faster than outer parts creating friction and heating. • Particle orbits become smaller and smaller until they fall onto the WD.

Novae • Dwarf nova: sudden increase in brightness by ~factor 10 due to enhanced accretion (disk instability). • Nova: Thermonuclear flash of H-shell burning. L~105 LSun. • Nova recur each 102104 years.

White Dwarf Supernovae • Despite of novae outbursts, accreting WDs gain net mass until they reach the 1. 4 MSun limit. • When gravity overcomes electron degeneracy, WD collapses until the temperature reaches the threshold of C fusion. • C ignites throughout the WD, making it explode into a WD supernova (Type Ia). • The supernova shines with L~1010 LSun. • No remnant results from a Type Ia supernova.

Type Ia versus Type Ib and II SN • WDs contain little H. • Massive stars have plenty of H. • The spectra and light curves of Type Ia SN are different than those of Type Ib and II SN. • Type Ia SN are one of the primary means to measure large distances in the universe.

Homework 3 • Two stars, p and s, in a binary system are of the same size. “s” has spectral type F, while “p” has spectral type B. Which one is more luminous? • Imagine that you are plunging into the Sun, starting from Earth. Briefly describe what you will see as you descend. • Earth is ~15 x 107 km from the Sun, and the apparent solar brightness in our sky is 1, 300 watts per m 2. Determine the apparent solar brightness if we moved 5 times closer to our star.

Neutron Stars • Basic properties: (1) size ~ 10 km, mass ~ 1. 4 -3 MSun (2) Density similar to atomic nucleus (3) Made of neutrons (4) Gravity bound, vesc~c/2, gravitational redshift (5) origin in massive star supernova.

Little Green Men! • October-November 1967, Jodrell Bank radiotelescope, graduate student Jocelyn Bell and Dr. Anthony Hewish discover a strong source of radio waves in Cygnus pulsating with a period of 1. 337301 s. As precise as atomic clocks. • The mysterious source of radio emission was dubbed LGM for a while.

Pulsars • Pulsars are found at the center of some SN remnants such as the crab. • The period of the Crab’s pulsar is 0. 033 s. • Pulsars are fast spinning neutron stars left behind a supernova explosion.

The Lighthouse Model for Pulsars • The collapse of the iron core makes a fast rotating neutron star with a strong magnetic field. • The magnetic field directs intense beams of radiation along the magnetic poles. • The beams of radiation sweep past the line of sight again and again.

More on Pulsars • Pulsars slow down because of the angular momentum carried away by the beamed particles. • The spinning periods of pulsars are observed to increase. • Only an object as dense and small as a neutron star can spin so fast.

Neutron Stars in Close Binaries • As WDs, neutron stars can accrete mass from a stellar companion. • Infalling matter onto a NS releases a large amount of gravitational energy. • The accretion disks around NSs are very hot. • Her X-1 is an eclipsing Xray binary. • X-ray bursters release ~105 LSun in a few s.

Black Holes • A neutron star’s mass cannot exceed ~3 MSun because gravity overcomes neutron degeneracy pressure. • Laplace & Mitchell speculated about objects so compact that vesc>c. • A BH can be thought of as a region in which spacetime is curved so much that it becomes a bottomless pit.

Black Hole Size • The boundary of a BH is called the event horizon. Information can never reach us from events inside it. • Karl Schwarzschild computed the radius of the event horizon in 1916. RS=2 GM/c 2 • Singularity is the point at which all the mass that created the BH resides.

Gravitational Redshift • Einstein’s general theory of relativity predicts that light coming out of a strong gravitational field should show a redshift. • The gravitational redshift increases for more compact objects.

Time Dilation • Another prediction of the general theory of relativity is that time runs more slowly as the force of gravity becomes stronger. • From the point of view of an outside observer, the time it takes for a particle to cross the event horizon becomes infinite.

Tidal Forces • The larger size of supermassive black holes mean that tidal forces are weaker in the event horizon. • It is safer to approach the event horizon of the BH at the center of the Milky Way than to approach a stellar-mass BH.

Evidence for Black Holes • BHs reveal themselves through their effects on the surroundings. • Cyg X-1 contains an Otype star with a mass of 18 MSun and an unseen companion with a mass of 10 MSun. Fast variations in X-ray radiation indicate a small size of the emitting surface.

Gamma-ray Bursts • Discovered by US military satellites in the 1960 s. • In 1991 the Compton gamma-ray observatory detected about 1 event per day coming from all over the sky. • In 1997 an afterglow was observed at other wavelengths. The explosion came from a very distant galaxy.

Hypernovae • Gamma-ray bursts are now routinely observed. • Some evidence indicates that they are related to particularly powerful supernova events (hypernova). • One popular scenario is that gamma-ray bursts originate from bipolar supernova explosions in which one of the jets is beamed toward us.

The Big Picture • Clear astronomical evidence exists for white dwarfs, neutron stars and black holes. • Close binary systems with stellar corpses produce novae, Type Ia supernovae and X-ray bursters. • Black holes warp spacetime around them into a true hole of information. • Gamma-ray bursts are the most energetic explosions in the universe. They could be related to bipolar supernova explosions.