Annie Aquino's Notes

Annie Aquino

19 April 2013

Chapter 13

What is a white dwarf?

-White dwarfs are remaining cores of dead stars

-electron degeneracy pressure supports them against gravity

-they cool and grow dimmer with time

What can happen to a white dwarf in a close binary system?

-in a close binary system, gas from a companion star can spill towards a white

dwarf forming a swirling accretion disk around it.

-a nova can be caused by hydrogen fusion on the surface of a white dwarf in a

binary star system.

-If a white dwarf gains enough matter to exceed the 1.4 solar-mass white dwarf

limit, it will explode completely in a white dwarf supernova.

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What is a neutron star?

-a neutron star is a ball of neutrons just a few kilometers in radius but with a

mass like that of the sun.

- the degeneracy pressure of neutrons supports a neutrons star against gravity

- electron degeneracy pressure goes away because electrons combine with

protons making neutrons and neutrinos

-neutrons collapse to the center forming a neutron star

-a neutron star is about the same size as a small city

How were neutrons stars discovered?

-using a radio telescope in 67 jocelyn bell noticed very regular pulses of radio

emission

-A pulsar is a neutron star that beams radiation long a magnetic axis that is not

aligned with the rotation axis

-the radiation means sweep through space like lighthouse beams as the neutron

star rotates.

What can happen to a neutron star in a close binary system?

- the hot accretion disks around a neutron star in close binary systems shine

brightly with X-rays

-accreting matter adds angular momentum to a neutron star increasing its spin

-episodes of fusion on the surface leads to X-ray bursts

What is a black hole?

-an object who's gravity is so powerful that not even light can escape it

-some massive star supernovae can make a black hole

-the 'surface' of a black hole is the radius at which the escape velocity equals the

speed of light

-the spherical surface is known as a event horizon

-the radius of the event horizon is known as the schwarzchild radius

-gravity crushes all the matter into a single point known as a singularity

What would it be like to visit a black hole?

-nothing can escape from the event horizon of a black hole if you fall in it

increases the holes mass and the object loses its identity

-if the sun shrank into a black hole its gravity would be different only near the

event horizon

-black holes aren't a vacuum !

-light waves take extra time to climb out of Deep hole in space and time leading

to a gravitational redshift

-time passes more slowly near an event horizon

Do black holes really exist ?

-its a black hole its its not a star and its mass exceeds the neutron star limit

-some X-ray binaries contain compact objects of mass exceeding the limit so

they must be black holes

What causes gamma ray bursts?

-brief bursts of gamma rays coming from space were first detected in the 60's

-coming from very distant galaxies

-they are one of the most powerful explosions in the universe and could be the

formation of black holes

-observations show that at least some gamma ray bursts are produced by

supernova explosions

-some others may be the result of collisions of neutron stars