A brown dwarf is a pseudostar; a body of gas not massive enough for the gravitational pressure in its core to ignite the hydrogen-fusion reaction that powers true stars. The name "brown dwarf" is a play on the name of the smallest class of true stars, "red dwarf," but while red dwarfs are actually red, brown dwarfs are not brown, but purple or magenta. Objects ranging in mass between 13 and 75 times the mass of Jupiter—between 1.2% and 7% the mass of the Sun—are generally considered brown dwarfs. Clear rules for distinguishing large planets from brown dwarfs, however, are lacking. Some astronomers consider objects down to seven or eight Jupiter masses to be brown dwarfs, while others reserve this term for objects heavy enough to initiate deuterium fusion in their cores, that is, objects of 13 Jupiter masses or more. (Deuterium is a relatively uncommon form of hydrogen that has both a neutron and a proton in its nucleus; deuterium fusion is a minor reaction in true stars and persists for only a few million years even in brown dwarfs.) In 2001, an international committee declared that objects heavier than 13 Jupiter masses should be labeled brown dwarfs regardless of whether they orbit true stars, while objects below this threshold should be labeled as planets if they are orbiting true stars and as sub-brown dwarfs if they are not.
Until recently, astronomers could only theorize that brown dwarfs were common in the Universe. They observed that the less massive stars are far more common than the more massive stars, a trend that would suggest that brown dwarfs should be still more numerous. Brown dwarfs are small and faint, however, making them difficult to find.
Hot stars are blue; cool stars are red. Brown dwarfs, cooler than the coolest stars (red dwarfs), should thus be bright in the infrared, that is, they should radiate more heat than light. Therefore, one way to look for brown dwarfs is to search for faint infrared objects. It is difficult, however, to tell if the mass of a solitary infrared object is above or below the dividing line between brown dwarfs and the least massive stars, because it is difficult to measure such an object's mass. Another strategy for finding brown dwarfs is to look for evidence of a low-mass companion orbiting a star. (This strategy is also used to look for planets outside the solar system.) An astronomer discovering evidence of such a companion can estimate its mass from the properties of its orbit and thus decide if it is a planet, brown dwarf, or a low-mass star.
In June 1995, three astronomers, Gibor Basri, Geoffrey Marcy, and James Graham, made the first unambiguous discovery of a brown dwarf. They reported observations made with the newly completed Keck 400-in (10 m) telescope, the largest in the world. Their data confirmed the existence of a previously suspected brown dwarf, which they designated PPL 15, in the cluster of stars known as the Pleiades. The astronomers sidestepped the problem of determining mass by examining the amount of lithium in PPL 15's spectrum to see if hydrogen fusion reactions are occurring in the core. The reasoning behind the lithium test is as follows: when a star initially forms it contains some lithium near its surface; for low-mass stars, convection currents mix this surface lithium with the core, where it is destroyed by fusion reactions. In brown dwarfs, lithium also mixes with the core, but the core has no hydrogen fusion reactions to destroy the lithium. Hence, a brown dwarf should have lithium in its spectrum, whereas all the true stars in the Pleiades cluster are old enough to have burnt up all their lithium. Basri's team found lithium in the spectrum of PPL 15, indicating that it is a true brown dwarf, lacking hydrogen fusion at its core, rather than a low-mass star. Since 1995, hundreds of brown dwarfs have been discovered and it appears that they may be very numerous. Despite their large numbers, because of their small mass brown dwarfs are thought to account for only 15% or so of the stellar mass (i.e., the total mass of the stars and star-like objects in the Universe).
The Keck telescope also has been used to locate very cool (less than 1,652°F [900°C]) brown dwarfs in the constellations of Ursa Major (the Big Dipper), Leo, Virgo, and Corvus. These brown dwarfs are cool enough that astronomers can detect the presence of methane in their atmospheres—a molecule too fragile to survive the temperatures generated by true stars. In 2002, researchers produced evidence that the atmospheres of some aging brown dwarfs produce forms of gaseous clouds and rain, comprised of liquid iron.
A major problem in modern astronomy is dark matter. Roughly 90% of the matter in the Universe is unaccounted for; it is called dark matter because it is not illuminated by light, and so cannot, generally, be seen through telescopes. (When some kinds of dark matter—for example, clouds of dust—get between us and a source of light, then they can be observed.) Astronomers know dark matter is there because its mass affects the orbits of objects in galaxies and of galaxies within clusters of galaxies, but astronomers still do not know what most of the dark matter in the Universe consists of. It was long hoped that brown dwarfs might account for much of the dark matter, but there is now wide agreement among astronomers that they do not account for more than a small fraction of it. However, study of brown dwarfs remains important because understanding their formation is essential to understanding that of planets and stars.
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Paul A. Heckert