Stellar Endpoints: Supernovae, Neutron Stars, and Black Holes

White Dwarf Stellar Explosions

Novae: Recurrent Stellar Flares

The hydrogen build-up on the surface of a white dwarf can ignite into an explosive fusion reaction that blows off a shell of gas. Because so little mass is blown off during a nova, the explosion does not disrupt the binary system. Ignition of the infalling hydrogen can recur again with periods ranging from months to thousands of years.

White Dwarf Supernovae (Type Ia)

Subrahmanyan Chandrasekhar predicted that gravity will overcome the pressure of electron degeneracy if a white dwarf has a mass > 1.4 M_¤. Energetic electrons, which cause this pressure, reach the speed of light. If accretion brings the mass of a white dwarf above the Chandrasekhar limit, electron degeneracy can no longer support the star. The white dwarf collapses, raising the core temperature, and runaway carbon fusion begins, which ultimately leads to an explosion of the star. Such an exploding white dwarf is called a white dwarf supernova (Type Ia).

Brightness Comparison and Distance Indicators

While a nova may reach an absolute brightness of 10 billion suns, white dwarf supernovae are significantly more luminous. They all attain the same peak luminosity, making them excellent distance indicators. They can be used to measure distances out to great cosmic scales.

Types of Supernovae

There are two primary types of supernovae:

• Type Ia Supernova: Results from the explosion of a white dwarf, as described above.
• Type II Supernova: Results from the core collapse and explosion of a single massive star.

Neutron Stars: Remnants of Massive Stars

Neutron stars are the leftover cores from massive star supernova explosions. If the core's remnant mass is less than approximately 3 M_¤, it will stop collapsing and be held up by neutron degeneracy pressure. Neutron stars are incredibly dense and possess extreme properties:

• They rotate very rapidly.
• Their magnetic fields are 10¹³ times stronger than Earth's.
• Gravity is 100 billion times that on Earth.

Pulsars: Beaming Neutron Stars

Pulsars are simply rapidly rotating neutron stars that emit radiation in beams shooting away from their magnetic poles. Whether we observe a pulsar depends on the geometry:

• If the polar beam sweeps by Earth’s direction once each rotation, the neutron star appears to be a pulsar.
• If the polar beam is always pointing toward or always pointing away from Earth, we do not observe a pulsar.

Black Holes: Ultimate Gravitational Collapse

After a massive star supernova, if the core has a mass greater than approximately 3 M_¤, the force of gravity will be too strong for even neutron degeneracy to stop further collapse. The star will collapse into oblivion. This is what we call a black hole.

The star becomes infinitely small, creating a “hole” in the Universe. Since 3 M_¤ or more are compressed into an infinitely small space, the gravity of the star is immense!