White Dwarfs

Stellar Corpses

Nearly 98% of all stars in the universe will one day become white dwarfs. “Living” stars are held up against gravitational collapse by the energy generated in their cores from nuclear fusion (conversion of one element into another). A star like the Sun, for instance, actively converts hydrogen to helium at a rate of about 600 million metric tons per second! Once a star has completed all possible stages of fusion, however, its core starts to collapse due to gravity, and if its mass is less than ~1.4 solar masses, it becomes a white dwarf. Electron degeneracy pressure (think quantum mechanics!) holds up this type of star, and since it isn’t actively fusing one element into another, you can think of it as a stellar ‘corpse,‘ slowly cooling off in the vastness of space. As you might imagine, white dwarfs have extremely high densities; one teaspoon of white dwarf material would weigh as much as as 1 elephant (if the scale were on Earth)! White dwarfs in our Galaxy are great tools for studying how the laws of physics operate at temperatures and pressures not attainable in conventional Earth-based laboratories.

Why study them?

As stated above, nearly all stars will eventually become white dwarfs; thus, to have a complete handle on stellar evolution, we must thoroughly understand how they are formed, what their structure is like, and how they will evolve in time as they cool (WDs will crystallize once the Coulomb energy between ions exceeds the thermal energy of ions in the core!).

More info. coming later!

Quick Facts

General Properties of White Dwarfs:
  • "dead" stars that no longer undergo nuclear fusion processes
  • the final evolutionary state for 98% of all stars
  • supported against gravitational collapse by electron degeneracy pressure (quantum mechanics!)
  • will continue to cool slowly for all of time
  • most are about the size of Earth
  • masses range from 0.3Msun to 1.4Msun
  • have an upper mass limit; if this mass is exceeded (via mass transfer, binary mergers), will explode as a Type 1a supernova
  • excellent laboratories for studying extreme physics

FIGURE 1: Earth, a typical white dwarf, and a neutron star, all to scale.         Credit: NASA