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Chapter 14

AY 101 Chapter Notes - Chapter 14: White Dwarf, Accretion Disk, Observable Universe

Course Code
AY 101
Raymond White

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The Big Picture
We have now seen the mind-bending consequences of stellar death. As you think about the bizarre objects
of the stellar graveyard described in this chapter, try to keep in mind these “big picture” ideas:
Despite the strange nature of stellar corpses, clear evidence exists for white dwarfs and neutron stars,
and the case for black hoes is very strong.
White dwarfs, neutron stars, and black holes can all have close stellar companions from which they
acrete matter. These binary systems produce some of the most spectacular events in the universe,
including ovae, white dwarf supernovae, and X-ray bursters.
Black holes are holes in the observable universe that strongly warp space and time around them. The
nature of black hole singularities remains beyond the frontier of current scientific understanding.
Summary of Key Concepts
What is a white dwarf?
A white dwarf is the core left over from a low-mass star, supported against the crush of gravity by
electron degeneracy pressure. A white dwarf typically has the mass of the Sun compressed into a size
no larger than Earth.
What can happen to a white dwarf in a close binary system?
A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion
disk that swirls toward the white dwarfs surface. As hydrogen builds up on the white dwarfs surface,
it may begin nuclear fusion and cause a nova that, for a few weeks, may shine as brightly as 100,000
Suns. In extreme cases, accretion may continue until the white dwarfs mass exceeds the white dwarf
limit of 1.4MSun, at which point it will explode as a white dwarf supernova.
What is a neutron star?
A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star
supernova. It resembles a giant atomic nucleus 10 kilometers in radius and with more mass than the
How were neutron stars discovered?
Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of
radiation that sweep through space as the neutron star spins. We detect such neutron stars as pulsars,
and these pulsars provided the first direct evidence for the existence of neutron stars.
What can happen to a neutron star in a close binary system?
Neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot
accretion disks. The hot gas emits strongly in X rays, so we see these systems as X-ray binaries. In
some of these systems, frequent bursts of helium fusion occur on the neutron stars surface, causing X-
ray bursts.
What is a black hole?
A black hole is a place where gravity has crushed matter into oblivion, creating a hole in the universe
from which nothing can escape, not even light. The event horizon marks the boundary between our
observable universe and the inside of the black hole; the size of a black hole is characterized by its
Schwarzschild radius.
What would it be like to visit a black hole?
You could orbit a black hole just like you could any other object of the same mass. However, you’d see
some strange effects if you watched an object fall toward the black hole: Time would appear to run
more slowly for the object, and light from it would appear increasingly redshifted as the object fell
closer to the black hole. The object would never appear to pass through the event horizon but instead
would disappear from view as its light became too redshifted for any instrument to detect it.
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