Hunting the Great White Dwarf

In the early 1900s astronomers were stunned to discover that some stars had masses comparable to the Sun's but packaged into earth-sized volumes. The first such stars discovered were spectral type A (white) and thus became known as White Dwarfs. The most famous white dwarf is Sirius B, the companion to Sirius. Most white dwarfs don’t make good targets for washed-out astronomers because they are faint, inconspicuous stars or are hard to see companions to bright stars such as Sirius. But there is one white dwarf that is easy: 40 Eridani B.

40 Eridani A, B, & C by David Darling40 Eridani (also known as ο2 Eridani or Keid) is a relatively bright 4th magnitude star. Even in urban areas, it is easy to find by star-hopping from Rigel going via β Eri, μ Eri, and ν Eri. I recently did it from my light polluted front yard using a 50mm finder. Even looking directly over my neighbor’s very bright holiday lights it was an easy hop. 40 Eridani is a triple star system. The primary has magnitude 4.4. Component B—the white dwarf—has magnitude 9.5 and is widely separated from A (83”, PA 105°) making it easy to see in even small telescopes. (Image courtesy of David Darling)

As a special prize, the third component C is a red dwarf flare star of magnitude 11.2. The B-C separation is much tighter at about 8” but it isn’t hard to split except for the faintness of the C component. You’ll probably need at least a 4 inch (100mm) aperature to see C in washed-out skies, but it’s rather easy with 6 inches (150mm) or more. In my 10 inch (250mm) Dob, it looks pretty much like the photo above.

So don’t miss the chance to “bag” a white dwarf. If you want to learn more about white dwarfs, read the excellent Wikipedia article. Now 40 Eridani is interesting for several other reasons, including a connection to Star Trek....

40 Eridani A is considered a likely candidate to have Earth-like planets and is one of the 100 top targets proposed for NASA’s Terrestrial Planet Finder Mission. Component A has a reasonably sized habitable zone around it that could potentially sustain life. Neither components B nor C are candidates: any habitable planets around B would have been sterilized as B evolved into a white dwarf and C’s flares (which include a signifcant X-ray component) would not be healthy for life forms on any of its planets.

40 Eridani A also has a Star Trek connection. Although never directly mentioned in any of the TV shows or films, there are various clues that 40 Eridani is the home star of the planet Vulcan. This was confirmed in the authorized Star Trek book Star Trek: Star Charts. And most importantly, it was also confirmed by Gene Roddenberry in a Sky & Telescope article in July 1991.