Chapter 18 ASTR-1020-90
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This 3 page Class Notes was uploaded by Hannah Moore on Wednesday March 2, 2016. The Class Notes belongs to ASTR-1020-90 at Clayton State University taught by Dr. Campbell in Winter 2016. Since its upload, it has received 21 views. For similar materials see Stellar and Galactic Astronomy in Science at Clayton State University.
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Date Created: 03/02/16
Chapter 18: The Bizarre Stellar Graveyard White Dwarfs o What is a white dwarf? A white dwarf is the core of a low or intermediate mass star after it runs out of fuel, and fusion stops. It begins hot and colds slowly over time. Electron degeneracy pressure keeps White dwarfs from collapsing No white dwarf has reached the “black dwarf” coolness. White dwarfs are high density, the more mass, the smaller it is. Mass of the sun, but the size of the earth White dwarfs cannot be bigger than 1.4M sun o What can happen to a white dwarf in a close binary system? Mass exchange happens when two stars are close the each other. \ One star dumps mass on the other Mass falling toward a white dwarf has some angular momentum and this matter orbits the white dwarf in an accretion disk Friction causes the accretion disk to heat up, and matter falls on the surface of the star The temperate of the surface becomes hot enough for hydrogen fusion. Fusion begins suddenly and causes a nova. Nova star system becomes much brighter and causes an explosion that drives the accreted matter out into space. Two types of Supernova Massive star supernova: Iron core of star reaches the white dwarf limit and collapses into a neutron star or black hole o Shows hydrogen absorption lines o Expanding debris cloud and neutron star or black hole is left over. White dwarf supernova: Carbon fusion begins suddenly as white dwarf in a close binary system and the mass reaches white dwarf limit. This causes an explosion that leaves nothing behind. o Each one looks the same. Very high luminosity o Expanding debris cloud is left over. Supernova is much brighter than a nova. Nova: Hydrogen to Helium fusion of a layer of matter, white dwarf is left intact. White Dwarf supernova: Nothing is left of the white dwarf. Neutron Stars o What is a neutron star? A neutron star is a ball of neutrons left over from a massive star supernova, and it is held up by neutron decency pressure. It is about the size of a small city. The density is extremely high. Teaspoon of neutron star is about the weight of a mountain o How were neutron stars discovered? Jocelyn Bell found regular pulses of radio emission coming from a singular part of the sky The pulses were coming from a spinning neutron star (a pulser) Extremely precise, Pulser at the center of Crab Nebula pulses 30 times per second A pulser is a neutron star that beams radiation along it’s magnetic axis, but the magnetic axis is not aligned with its rotation axis. The radiation beams sweep through space like a lighthouse beam Pulsers spin extremely fast. o What can happen to a neutron star in a close binary system? Similar to white dwarfs, but the accretion disk is hotter and faster. Their accretion disks are strong x-ray emitters Accreting matter speeds up the rotation of the neutron star if it is going in the same direction. Slows it down of it is in the opposite direction As the neutron becomes hot enough for helium fusion, it causes an x-ray burst that blows mass off. Black Holes o What is a black hole? A black hole is an object with gravity so strong that not even light can escape. It has an escape velocity equal to the speed of light. Large objects have a smaller escape velocity, small objects have larger escape velocities The “surface” of a black hole is the radius where the escape velocity equals the speed of light. This surface is known as the event horizon, or as the Schwarzschild radius. The bigger the mass, the bigger the black hole Nothing can escape from a black hole’s event horizon Some massive star supernova can make a black hole if enough mass falls into it’s core. It is beyond the neutron star limit. When all the matter is crushed in to one spot, it is called a singularity. o What would it be like to visit a black hole? Near an event horizon there is a redshift and time slows down. It is extremely difficult to fall into a black hole A black hole is about the size of a city. As a person goes toward an event horizon they would be stretched and squeezed by it’s tidal forces. o Do black holes really exist? Need to measure mass to verify a black holes existents It is a black hole if it exceeds neutron star limit (3sun) Gravitational Wave Discovery Announced Feb 11, 2016 Final Step to proving Einstein’s theory of Relativity Pulses of gravitational waves go through space, and stretch and squeeze space in a miniscule way. Caused when two black holes collide. o The Origin of Gamma Ray Bursts Where do gamma-ray bursts come from? Most gamma-ray bursts come from distant galaxies. They must be among the most powerful explosions in the universe, probably signifying the formation of black holes • What causes gamma-ray bursts? At least some gamma-ray bursts come from supernova explosions.
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