Brown dwarfs—if they indeed exist—are celestial objects composed of dust and gas that failed to evolve into stars. To be a star, a ball of hydrogen must be large enough so that the pressure and heat at its core produce nuclear fusion, the process that makes stars bright and hot. Brown dwarfs, so named by American astronomer Jill Tarter in 1975, range in mass between the most massive planets and the least massive stars, about 0.002 to 0.08 times the mass of the Sun.
Roughly 90 percent of the material in the universe is unaccounted for. Since it cannot be seen, this substance is called dark matter. The existence of dark matter is confirmed by the fact that its mass affects the orbits of objects near the visible edge of galaxies and of galaxies within clusters of galaxies. If brown dwarfs really are as common as astronomers think, their total mass could account for the mass of dark matter, one of modern astronomy's major mysteries.
Because brown dwarfs are so cool, small, and faint, they cannot be observed through ordinary telescopes. Beginning in the 1930s, astronomers have suggested their existence using various techniques. One method is to look for a bouncing movement in the path of a star across the sky. Astronomers believe this erratic motion is caused by the gravitational pull of a low-mass companion—such as a brown dwarf—orbiting that star. Another method is to search the sky using infrared telescopes. Some astronomers believe brown dwarfs may emit enough infrared energy to be detected.
Cluster of galaxies: A group of galaxies that is bound together by gravity.
Cluster of stars: A group of stars that is bound together by gravity and in which all members formed at essentially the same time.
Dark matter: Unseen matter that has a gravitational effect on the motions of galaxies within clusters of galaxies.
Infrared: Wavelengths slightly longer than visible light, often used in astronomy to study cool objects.
Mass: An object's quantity of matter as shown by its gravitational pull on another object.
Nuclear fusion: Nuclear reactions that fuse two or more smaller atoms into a larger one, releasing huge amounts of energy in the process.
A third method astronomers use to locate a suspected brown dwarf is to observe the amount of the element lithium in its spectrum to see if hydrogen fusion reactions are occurring in its core. Lithium is destroyed in the hydrogen fusion reactions of mature stars, but is still present in infant low-mass stars and brown dwarfs. In June 1995, three astronomers reported they found lithium in the spectrum of a suspected brown dwarf called PPL 15 that is located in the cluster of stars known as the seven sisters of the Pleiades (pronounced PLEE-adees). Since the stars in the Pleiades cluster are old, the astronomers asserted that PPL 15 is a brown dwarf rather than a low-mass star.