Astronomy Chapter 14

Formation of a neutron star or a black hole, depending on the mass

Above the Chandrasekhar limit, the degeneracy pressure in the white dwarf is no longer sufficient to counter gravity.

The Chandrasekhar limit

As a white dwarf's mass approaches 1.4 M¤, its electrons must move at nearly the speed of light. Because nothing can move faster than light (Einstein's Theory of Special Relativity), a white dwarf cannot be more massive than 1.4 M¤.

Nova

As the white dwarf accretes, the surface becomes hot enough to reignite hydrogen fusion, suddenly and explosively. The outer shell is expelled, but the white dwarf is left intact.

The Crab Nebula

Filaments are the outer layers of the dead star that have been blown away (excavated) by the supernova explosion and by the wind from the remnant core (blue glow). Is the remnant of a supernova explosion that was seen on Earth in 1054 AD. It is 6000 light years from Earth. At the center of the bright nebula is a rapidly spinning neutron star, or pulsar that emits pulses of radiation 30 times a second. As time goes on, and the electrons move outward, they lose energy to radiation. The diffuse optical light comes from intermediate energy particles produced by the pulsar. The optical light from the filaments is due to hot gas at temperatures of tens of thousands of degrees. The infrared radiation comes from electrons with energies lower than those producing the optical light. Additional infrared radiation comes from dust grains mixed in with the hot gas in the filaments.

X-ray binary

If one member of a pair of massive stars evolves and explodes as supernova, leaving a neutron star, and assuming the other star is massive enough to survive the cataclysmic event, then the neutron star will begin to accrete matter shed by its companion

Formation of a neutron star

If the mass of the remnant core in a supernova is greater than the Chandrasekhar limit (1.4 M¤), the electron degeneracy pressure is not enough to counter gravity. The core contracts until all electrons and protons fuse to form neutrons (+neutrinos). NSs depend on the degeneracy pressure of neutrons instead of electrons

supernova (type Ia)

If the white dwarf accretes enough matter to exceed the Chandrasekhar limit (1.4 M¤), runaway fusion can occur that completely obliterates the star.

Pulsar

Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. The pulses were coming from a spinning neutron star. Her PhD advisor, who was very skeptical of this signal (believing it was interference or man-made), was the one who actually received the Nobel Prize in 1974 for her discovery...

Accretion disk

Mass falling toward a white dwarf from its close binary companion has some angular momentum.

supernova (type II)

Massive stars never become white dwarfs. Their cores experience an extremely violent collapse that expels the outer layers, leaving behind a neutron star or a black hole.

Tolman-Oppenheimer-Volkoff limit

Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 M¤. Above this mass limit, no known force can resist the crush of gravity, and the NS will collapse: à black hole As far as we know, gravity crushes all the matter into a single point known as a singularity.

Heisenberg Uncertainty Principle

The more precisely you know a particle's position, the less accurately you can know its velocity (and vice versa)

Because of its high surface temperature, it is brighter in _______________ than a normal star.

UV and X-rays

The remnant core of a low-mass star after its nebular phase is a

White dwarf.