Brown dwarf

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.

Words to Know

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.

[See also Infrared astronomy; Star ]

brown dwarf in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do. Also called "failed stars," brown dwarfs form in the same way as true stars (by the contraction of a swirling cloud of interstellar matter ). True stars have enough mass (greater than 0.084 times that of the sun) to compress their core until the increasing temperature and pressure ignite the hydrogen fusion reaction, but brown dwarfs have only a relatively short period of deuterium (heavy hydrogen) burning before they cool and fade. Their coolness gives brown dwarfs two distinguishing characteristics: One is that most of the radiation they emit is in the infrared part of the spectrum; the other is that brown dwarfs can be distinguished by traces of lithium in their spectrum because, unlike true stars, brown dwarfs never get hot enough to burn the lithium that was in the interstellar cloud as it condensed.

Although they should exist in large numbers, brown dwarfs are difficult to find using conventional astronomical techniques because they are dim compared with true stars. A number of brown dwarfs have been identified, the first in the Pleiades star cluster in 1995. The first X-ray-emitting brown dwarf was detected in Chamaeleon dark cloud number I in 1998. A year later, several so-called methane dwarfs were discovered; these are thought to be older brown dwarfs that have cooled sufficiently over billions of years so that large amounts of methane could form in their atmospheres. The closest brown dwarf to Earth, Epsilon Indi B, less than 12 light-years from the Sun, was discovered in 2003.

Brown dwarfs belong to the "T dwarf" category of objects straddling the domain between stars and giant planets. Because brown dwarfs are typically 10-80 times the mass of Jupiter, some of the large extrasolar bodies discovered orbiting stars may be brown dwarfs rather than giant Jupiterlike planets. Observations of 100 young brown dwarfs in the Orion Nebula in 2001 strongly supported the theory that they originate as failed stars; many of the brown dwarfs were surrounded by disks of dust and gas that conceivably could condense and conglomerate to create planets orbiting them. Brown dwarfs are believed to play an important role in the process of stellar evolution . They are a component of the dark matter that along with dark energy may account for more than 90% of the mass of the universe.

brown dwarf An object that, because of its low mass (less than 0.08 solar masses), never becomes hot enough to begin hydrogen fusion in its core although it can produce some nuclear energy for a few million years from the fusion of deuterium, a heavy isotope of hydrogen, which requires lower temperatures than normal hydrogen fusion; hence it is considered to be not a star, but a substellar object. Brown dwarfs have surface temperatures of about 2000 K and cooler. They thus have a very low luminosity and are difficult to detect. Brown dwarfs may be at least as numerous as visible stars but because of their low masses are not thought to be a significant component of the galactic dark matter. The first brown dwarf to be identified with certainty was a companion to the nearby red dwarf Gliese 229, photographed by the Hubble Space Telescope in 1995. Brown dwarfs are now assigned the spectral types L and T (see L Dwarf; T Dwarf). An object below about 0.01 solar masses (about 10 Jupiter masses) is regarded as a planet.