White dwarfs are dense stellar remnants that mark the final
evolutionary state of intermediate-mass and low-mass stars. Right after
formation, a white dwarf is very hot and will appear blue-white in colour. Since
material in a white dwarf no longer undergoes fusion reactions like in a normal
star, a white dwarf does not have a source of energy and will gradually cool as
it radiates away its energy. As a white dwarf cools, the radiation emitted by
it will shift from the hot blue-white colour of an O-type star (> 30,000 K)
to the cool red colour of an M-type red dwarf star (< 4,000 K). Given
sufficient time, a white dwarf will cool until it no longer emits significant
heat or light, and become what is known as a black dwarf. Nonetheless, no black
dwarfs are expected to exist in the present Universe because the length of time
required for a white dwarf to cool to this state far exceeds the current age of
the Universe.
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Artist’s impression of a white dwarf shown to scale with the
Earth.
In 1997, a white dwarf identified as WD 0346+246 was serendipitously
discovered during re-examination of images taken for a survey to detect brown
dwarfs in the Pleiades cluster. The images were acquired by the UK Schmidt
Telescope at Siding Spring Observatory in New South Wales, Australia. WD
0346+246 is observed to have an estimated temperature of 3,900 K, making it one
of coolest white dwarfs currently known. A subsequent estimate gives WD
0346+246 a slightly lower temperature of 3,750 K. At that temperature, it will
have a colour temperature similar to a red dwarf star. Parallax measurements
yield an estimated distance of approximately 90 light years for WD 0346+246.
This places WD 0346+246 within the Sun’s neighbourhood.
WD 0346+246 also shows unusually large velocity components
with respect to the disk of the Milky Way galaxy. This indicates WD 0346+246 is
a member of the galactic halo and is currently passing through the Sun’s
neighbourhood. Most stars in the Milky Way galaxy exist within a single plane
known as the galactic plane. However, a population of stars inhabit a spherical
region around the galaxy known as the galactic halo. Stars in the galactic halo
tend to be much older and less enrich in elements heavier than hydrogen and
helium than stars in the galactic disk. Assuming WD 0346+246 is a carbon white
dwarf with a pure hydrogen atmosphere, the length of time required for it to
cool to its current state is at least 10 billion years. The existence of WD
0346+246 suggests that a significant number of such cool white dwarfs reside in
the solar neighbourhood and have yet to be identified.
References:
- Hambly, N. C., Smartt, S. J., & Hodgkin, S. T., “WD
0346+246: A Very Low Luminosity, Cool Degenerate in Taurus”, ApJ 489: L157-L160,
1997 November 10
- N. C. Hambly et al., “On the parallax of WD 03461246: a
halo white dwarf candidate”, MNRAS (1999) 309 (4): L33-L36.
- P. Bergeron (2001), “The Halo White Dwarf WD 0346+246
Revisited”, arXiv:astro-ph/0105333