White dwarfs within 10 parsecs

By early 2005, astronomers had found at least 22 white dwarfs within 10 parsecs (32.6 light-years) of Sol, although these objects are small and dim compared to OBAFGK stars. In 2002, a team of astronomers claimed that their census of white dwarfs within 13 parsecs (pc) of Sol appeared to be nearly complete, although their analysis of 109 white dwarfs within 20 parsecs (pc) indicated that around 50 additional white dwarfs may yet lie undiscovered within that distance (Holberg et al, 2002). Unfortunately, none are bright enough to observe with the unaided Human eye in Earth's night sky.

NASA, HST, WFPC 2,
H. Richer (UBC)


Larger image.


Dim, small white dwarfs
are common at the center
of old globular clusters
such as M4 (more).

Although some white dwarfs may have had their gaseous outer envelopes removed by companion objects, most are the naked cores of stars that have evolved off the main sequence and thrown off much of their outer layers of hydrogen and/or helium (some passing through the relatively brief planetary nebula stage). They become such stellar remnants after their cores have consumed enough of their fusionable elements that they have undergone gravitational collapse into planet-sized objects, but not violently enough (through supernovae) to end up as small as neutron stars or even smaller black holes. In theory, all white dwarfs must mass less than 1.4 Solar-masses (the Chandrasekhar limit), so that electron degeneracy pressure acts to prevent them from further gravitational collapse.

NASA Observatorium





See a discussion of
"stellar remnants"
as part of stellar
evolution and death.

Controlling for age, the more massive white dwarfs are usually less luminous because greater gravitational contraction has reduced their radiative surface area. At least two (Sirus B and Procyon B) of the nearby white dwarfs that are now much dimmer and smaller than Sol were once also brighter, larger, and more massive. While stars with more than about eight Solar-masses become neutron stars and black holes through supernovae, those with less mass become white dwarfs. Over the 14 billion or so years since the birth of the known universe, many massive stars of BAF and early G spectral types have already evolved into ancient, cool white dwarfs. While some of these cool, dim objects were born in the Milky Way's disk like Sol, the oldest ones may be a major constituent of the galaxy's halo. Indeed, some astronomers believe that around 10 percent of the local dark matter halo may be in the form of very old, cool, white dwarfs (Ibata et al, 2000).

Modified Sion-type (1983) Classification System for White Dwarfs

In general, the spectral appearance (and therefore classification) of white dwarfs are affected by their evolution and final state as isolated objects and by those objects occurring in binary or multiple star systems. The strong gravity of white dwarfs causes a rapid settling of heavier elements so that hydrogen rises to the surface (called type DA), which can mask other elements, as is found in as many as three-quarters of all white dwarfs known. Sometimes, however, the outer hydrogen envelope is stripped away by companion objects or lost by some other evolutionary process. Without the heat generated by core fusion, however, newly born white dwarfs slowly cool over time and may accrete trace elements from the interstellar medium, comets, and other sources (Professor Shri Kulkarni's class notes on "White Dwarfs," in pdf). Although their outer surface may spectroscopically exhibit a hydrogen-rich "DA" surface when hot, white dwarfs change spectral class to non-DA species as they cool off (Holberg et al, 2002). (For more discussion about the classification of white dwarfs, see: Liebert and Sion, 1994.)