An artist's impression of an extremely low-mass white dwarf (left) and a smaller, more normal-mass white dwarf companion (Illustration: CfA/David A. Aguilar)

Thanks mostly to the Sloan Digital Sky Survey (SDSS), we now know of more than 20,000 spectroscopically identified white dwarf stars. The vast majority, more than 80%, have outer atmospheres composed entirely of hydrogen, as their strong surface gravities have caused the heavier elements to sink down out of visibility (Kleinman et al. 2013).

The SDSS has also given us an opportunity to take census of the range of white dwarfs masses. This mass distribution peaks at about 0.6 Msun (where Msun is the mass of the Sun), and the mean mass for white dwarfs has been determined by multiple methods to fall near this value. It is likely that the progenitor of a typical 0.6 Msun white dwarf was a roughly 2.5 Msun star that sustained an epoch of fusion from hydrogen to helium, exhausted that core hydrogen, then expanded into a red giant, and finally fused that remnant helium to carbon and oxygen. Along the way it lost a majority of its initial mass, eventually leaving behind its remnant, exposed core: a white dwarf.

However, we also observe white dwarfs in nature with considerably higher and lower masses. These low-mass white dwarfs are interesting for a number of reasons:

For one, an isolated low-mass white dwarf stars cannot be formed within the finite age of the Universe, about 13.8 billion years. Since white dwarfs are the endpoints of stellar evolution, a star must go through its other phases, all of which take some amount of time. More massive stars live fast and die young, and shine bright enough to burn through their fuel quickly. But low-mass stars burn through their fuel much more slowly; a main-sequence star with an initial mass of 0.5 Msun (half the mass of our Sun) burns its hydrogen to helium for more than 50 billion years before it evolves its way down to its final white dwarf cooling stage.

Such is the fate for a single star burning on its lonesome, like our Sun. But the Universe has a trick up its sleeve: Binary systems composed of at least two stars orbiting relatively closely to each other. More than half the stars in our Galaxy reside in such binary systems, so they are quite common. The gravitational pull from a binary companion is often responsible for stripping more mass than otherwise would be removed from a single star, leaving behind underweight white dwarfs. Often a companion can strip the now-low-mass white dwarf of enough mass to prevent the ignition of helium in its core, and the lowest-mass white dwarfs likely harbor cores of degenerate helium. These stars were unable to fuse its core helium into any heavier elements.

These low-mass white dwarfs are therefore excellent signposts for close binary systems: low-mass white dwarfs need friends.