Hi-ho, Hi-ho

Brown dwarfs are the missing link between normal stars and planets.

By Jeffrey Winters|Monday, January 01, 1996

Excitement over the new planet in Pegasus couldn’t obscure another important astronomical discovery made in 1995: the best evidence yet for the existence of brown dwarfs. With a mass less than 8 percent that of our sun, these dim bulbs have been a sort of missing link between normal stars and planets--and perhaps, some astronomers have hoped, part of the missing mass, also known as dark matter, that is needed to make sense of the universe. Astronomers have been hunting brown dwarfs as long as they’ve been hunting extrasolar planets. But 1995 was a watershed.

Compared with normal stars and their steady brightness, brown dwarfs are dim and getting dimmer. Most of their light comes from the glow of hot gases compressed by gravitational contraction, a process that began with the star’s birth in an interstellar cloud. Although nuclear fusion may ignite in the star’s core, it never kicks in at full blast; the mass of the star and thus its internal pressure are just too small.

Because it is so feeble, a brown dwarf tends to retain fragile elements like lithium that are quickly destroyed in normal stars--which makes the presence of lithium a good test of whether a small star is really a brown dwarf. Until 1995, no brown-dwarf candidate had passed the test. But in June, Gibor Basri of the University of California at Berkeley and his colleague Geoff Marcy (who later confirmed the 51 Peg finding) said they had found a brown-dwarf candidate in the Pleiades star cluster--PPL 15--that had lithium in its spectrum. The astronomers calculated the star’s mass to be 7.8 percent that of the sun, just on the boundary between brown dwarfs and proper stars. In August a group of European astronomers nominated another, fainter Pleiades object, Teide 1, as a more solid brown- dwarf candidate, with just 7 percent of the sun’s mass.

Finally, at the same October meeting in Florence at which the 51 Peg discovery was announced, Shri Kulkarni of Caltech reported that he’d found a star just 19 light-years away, GL229, that has a small, cool companion. The companion lies a bit farther from the star than Pluto is from the sun; it has a surface temperature of about 1400 degrees Fahrenheit, and its mass is 2 percent of the sun’s and 20 times Jupiter’s. Is it a very dwarfish dwarf or a very imposing planet? The prevailing opinion seemed to be that, as Basri put it, Kulkarni’s was the most super- duper brown dwarf yet.

All in all, brown dwarfs now seem to be officially here, although not in large enough numbers to account for much of the dark matter. The long-missing brown dwarfs, Basri concludes, have finally made their appearance.

Comment on this article