White Dwarfs

Physical theory predicts white dwarfs. Are they there?

• Mass around 1 solar mass
  • M < 1.4 M_sun because of Chandrasekar limit.
  • M not to small
    • Only stars with M > 1.4 M_sun have had time to die.
      • Lifetime of Sun ~ 10 x 10⁹ years.
      • Age of Milky Way galaxy ~ 14 x 10⁹ years, from turnoff point of HR diagram in oldest globular clusters.
    • Cores could be less massive than original star, but should have a significant fraction of the original mass.
• Size around 0.01 R_sun.

• Temperature likely high because, being so small, they would take a long time to cool off.
• R ~ 0.01 R_sun.
  • This is a line in HR diagram:
  • 

What do we see?

The objects in the lower left of the HR diagram are called ``white dwarfs.'' Dwarfs because they are small. White because they are still hot. Their luminosity is low because, even though they are hot, they are so small. Over billions of years, they will cool off and become black dwarfs.