White Dwarfs, Novae, and Type 1a Supernovae – The Vampire Stars

What’s a White Dwarf Like? Same size as the earth to slightly larger. Very hot – surface temperatures of up to 100,000 Kelvin. Very dense – 200,000 times the density of the earth. Made of crystalline C and O.

What’s a White Dwarf Like? Contains about ½ the mass of the original star: from 0.4 to 1.4 solar masses. This upper limit (1.4 solar masses) is the Chandrasekhar Limit. If the core of the star is heavier than 1.4 solar masses, it will turn into a neutron star instead of a white dwarf.

What’s a White Dwarf Like? Nuclear fusion has completely shut down – the star shines only from residual heat. If there’s no nuclear fusion to provide outward pressure…why doesn’t the white dwarf instead collapse into a black hole?

Why No Black Hole? At the enormous density of a white dwarf, nearly all the empty space between the atoms is squeezed out. The electrons of the atoms repel the electrons of the other atoms, providing an outward pressure which the star’s gravity isn’t strong enough to overcome.

This electron pressure is called degenerate electron pressure. The atoms are so close together that the electrons can’t really figure out which atom they belong to. They simply flow around all the atoms (electrons are degenerate.)

Sometimes white dwarfs explode! Since at least half of the stars occur in binary systems, we ought to find many white dwarfs in binary systems with other stars. If a white dwarf is in a close binary system with a red giant or supergiant, sometimes its gravity will pull hydrogen gas from the larger star.

The gas from the red giant spirals into the white dwarf – forming an accretion disk.

Ready to go BOOM! When enough hydrogen gas from the red giant accumulates on the surface of the white dwarf, the high temperatures cause the gas to fuse into Helium. The star briefly flares hundreds of times brighter than a normal star.

A nova in Hercules. novas/novas.htm novas/novas.htm

Although the explosion is violent, most of the energy and mass comes from the “stolen” hydrogen gas. The star itself isn’t destroyed. Therefore, the star can go through the process dozens, maybe hundreds, of times!

Nova Light Curves The brightness of a nova explosion increases rapidly from the “normal” brightness of the star. It typically takes a few weeks to fade back to the star’s normal brightness. Here’s a light curve for a typical nova explosion:

Nova light curve

cosmos/images/nova.jpg

Sometimes it goes too far… A Nova happens when the white dwarf is far below the Chandrasekhar Limit of 1.4 M sun. What happens if the white dwarf is just below or right at the Chandrasekhar limit?

Type 1 a Supernova As hydrogen from the red giant piles onto the surface of the white dwarf, it may cause the total mass of the white dwarf to surpass the Chandrasekhar Limit. The electron repulsion can no longer support the star, so the entire star collapses and explodes.

Type 1 a Supernova Since a Type 1a Supernova always occurs at the same mass, (1.4 solar masses), these supernovas always explode with the same brightness. This makes them perfect “standard candles” or distance markers.

Supernova 1994, a Type 1 supernova in a distant galaxy