1. The Evolution of Stars

2. Binary Star Evolution About half the stars in the sky are binaries. These stars may begin life as separate entities, but often times this does not last.

3. If 2 stars in a binary system are close enough to each other, one star can engulf the other when it expands to become red giant.

4. If one star doesn’t engulf the other, it may still expand enough so that material from its surface is pulled onto the other star. The matter spiraling onto the second star is called an accretion disk. Accretion Disks

5. X-rays from Accretion Disks According to Kepler’s laws, matter closer to a star orbits faster than matter farther away. Because the atoms in the inner part of an accretion disk are orbiting very fast, there’s a lot of friction among those atoms which leads to a very high temperature. The disk around a white dwarf, neutron star, or black hole can be hot enough to produce lots of X-rays.

6. Looking for Accretion Disks in X-rays Because accretion disks can be much hotter than stars, they can be detected by X-ray telescopes. Optical PictureX-ray Picture

7. Novae If a white dwarf is in orbit with another star, and the two are close enough, matter can be pulled from the 2nd star onto the white dwarf. When enough matter accumulates on the white dwarf, it undergoes a hydrogen fusion explosion, like a hydrogen bomb. This explosion appears as a sudden burst of light (500,000 L  ) and is called a nova. Only the layer of hydrogen on the white dwarf’s surface explodes; the star itself is unharmed. Many novae can occur over millions of years as layers of hydrogen repeatedly accumulate and detonate.

8. Type Ia Supernovae Recall that white dwarfs are held up by electron degeneracy. If accretion of matter from another star increases a white dwarf’s mass above 1.4 M , it has enough gravity to overcome electron degeneracy, and it begins to collapse. The collapse increases the temperature of the white dwarf enough so that fusion of C and O is ignited, and it explodes. This is a Type Ia supernova. A Type Ia supernova is just as bright as Type II supernova, but it doesn’t leave behind a neutron star or black hole. The white dwarf is completely destroyed.

9. Two Types of Supernovae

10. Accretion onto Neutron Stars: Millisecond Pulsars When a star explodes as a supernova, the neutron star that is left behind rotates about once a second. However, if a star accretes onto this neutron star, it can cause it to spin 1000 times faster!

11. Evaporated Stars Accretion disks around neutron stars (or black holes) emit large numbers of very energetic x-ray photons. These x-rays can strike the companion star’s atmosphere, and heat it up so much that the star literally evaporates. All that remains may be some rubble around a bare millisecond pulsar.

12. Time Mass