The Starry End:
White Dwarf, Neutron, and Black Hole
Kathryn Simons

#1: White Dwarf

White Dwarf

• This occurs as the fusion in the star decreases as the star is contracting and heating up.
• When this occurs from very low mass stars, it takes them more than 20 billion years to completely burn through their hydrogen.
• White dwarfs are the cores of red giants.

In Our Sky

• Astronomers estimate that about 10% of all stars are white dwarfs.
• Sirius B was the first white dwarf to be identified in our sky.
• Their typical size is 0.8 solar masses.

How to Become One

• A low mass star starts as a protostar, becomes a main sequence star, turns into a red giant, becomes a planetary nebula, and then becomes a white dwarf.
• Its mass is limited by the Chandrasekhar limit, which states that about 1.4 solar masses is the limit of the size that electron degeneracy can support.

#2: Neutron Star

Neutron Star

• This occurs after the mass of a star's core has increased beyond the Chandrasekhar limit, which causes it to collapse even more and causes its electrons and protons to combine to form neutrons.
• It has collapsed to the point where it can only be supported by neutron degeneracy.

Size and Density

• A neutron star only has a diameter 0.2% that of a white dwarf, but it is a billion times denser because although it is between 1.4 and 3 solar masses, it must be denser to fit in all of its mass.

How to Become One

• A moderately massive star starts as a protostar, becomes a main sequence star, turns into a red giant, becomes a supernova, and then becomes a nebula (or pulsar).

#3: Black Hole

Black Hole

• This occurs after the mass of a star's core has become so dense that electron degeneracy can no longer support it.
• It has collapsed to the point where even neutron degeneracy cannot support it because it has compressed to greater than 3 solar masses.

Quick Facts

• A black hole has an escape velocity that is greater than the speed of light and a radius smaller than the Schwarzchild radius, which is the radius of the sphere around the black hole; no light can escape.
• The event horizon is the sphere around a black hole. Nothing can escape from this.
• The single point of infinite density is called the singularity.

In Our Sky

• The first black hole to be detected was Cygnus X-1.
• X-Ray emissions from the accretion of either a black hole or a neutron star will differentiate the two of them.
• A black hole can be described by its mass, electric charge, and angular momentum.
• Whatever information exists in a black hole is gone forever from our universe.