TYPES OF STAR STELLAR ASTROPHYSICS A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site brown dwarf Credit: Gemini Observatory/Artwork by Jon Lomberg The dimmest and least massive kind of star; the name "brown dwarf" was coined by Jill Tarter in the 1970s. A brown dwarf, which is not really brown but a very dull red, may be defined in one of two principal ways: by its mass or its origin. The former is normally used for practical purposes since it is easier to measure an object's mass than to establish how it was made. The upper mass for a brown dwarf is that which is just insufficient for normal hydrogen fusion to be triggered in the core. Based on theoretical considerations, this is believed to be about 0.084 solar mass, or about 84 times the mass of Jupiter. The lower mass limit is somewhat arbitrary as there is no obvious point of transition between a high-mass planet and a low-mass brown dwarf, but it is generally taken to be about 0.013 solar mass, or about 13 Jupter masses. Some astronomers argue that a more significant distinction between planets and brown dwarfs is their mode of formation. By this criterion, brown dwarfs are held to form in the same way as stars, as condensations in an interstellar gas cloud (see stars, evolution). Planets, by contrast, accrete from material in a circumstellar disk (see planets, formation). Brown dwarfs give off substantial amounts of infrared radiation as a result of slow gravitational contraction and small-scale deuterium fusion. The more massive a brown dwarf, the higher its surface temperature, though a typical value is about 1000 K. The first brown dwarf to be confirmed, in 1995, on the basis of a combination of mass determination, spectroscopic studies, and direct imaging was Gliese 229B. Since then, many more have been found, including a number in the Pleiades, the Sigma Orionis star cluster, and the Trapezium. While some brown dwarfs, like Gliese 229B, are part of binary systems, others having been found floating around on their own. PPl 15, in the Pleiades, is a binary system in which both components are brown dwarfs. S Ori 47, in the Sigma Orionis cluster, holds the record for the brown dwarf with the smallest known mass – a mere 0.015 solar mass. A number of other strong candidates await confirmation (see the table below), while some objects, such as the companion of HD 114762, lie close to the borderline between massive planets and low-mass brown dwarfs. Determining the precise mass of companion objects, however, is not easy. The most successful technique currently used to search for invisible companions, including both extrasolar planets and brown dwarfs – the radial velocity method – provides only a lower mass limit. To circumvent this problem, Michel Mayor and colleagues used a combination of radial velocity measurements and astrometric observations by the Hipparcos satellite of 10 brown-dwarf candidates. They concluded that most of the candidates orbit in planes that are nearly at right-angles to our line of sight and are therefore much more massive than previously thought. If they are correct, a substantial fraction of suspected brown dwarfs may, in fact, be low-mass stars powered in the conventional way by hydrogen fusion. Mayor's work suggests there may be relatively few objects in the range 5 to 50 solar masses and thus a fairly clear distinction between brown dwarfs and planets. A similar claim was made in July 1998 by Tsevi Mazeh and Dorit Goldberg of Tel Aviv University and David Latham of the Harvard-Smithsonian Center for Astrophysics. Other astronomers, however, are not convinced by these analyses and argue that there is more likely to be an unbroken continuum in mass between heavyweight planets and lightweight brown dwarfs. At the high-mass, high-temperature end of the brown dwarf scale are the ultra-cool dwarfs, with dusty atmospheres and a spectral type of M7 or later. Late M-types, with surface temperatures as low as 2,200 K, have water and strong oxide features in their spectra and may be red dwarfs or brown dwarfs, depending on their mass. For brown dwarfs smaller and cooler than type M, a new spectral category has been allocated – type L. With temperatures of about 1,500 to 2,200 K, L dwarfs have spectra characterized by strong metal hydride bands and even prominent water bands. Cooler still are the so-called T-types, or methane dwarfs, with surface temperatures ranging from about 1,500 K to 1,000 K or even 800 K, and spectra that show strong absorption by methane and water. Brown dwarfs do not glow, even dully for very long. As soon as they have used up their meager supply of deuterium, which takes about 10 million years, they fade from dim dark red to black. However, there are stars that start out as ordinary hydrogen-fusing red dwarfs and then get whittled away to brown dwarf size. The binary systems LL Andromedae and EF Eridani both contain white dwarf primaries that have looted material from their partners and reduced them to 40-Jupiter-mass objects, with surface temperatures of about 1,300 K and 1,650 K, respectively. A surprisingly high proportion of brown dwarfs have been found as companions to low-mass (red dwarf or other brown dwarf) stars, and, within these systems, the separation between the two components is typically very small, averaging about 4 AU. This goes against the prediction by some theorists that most very low-mass stars and brown dwarfs are solo objects, wandering though space alone after being ejected out of their stellar nurseries during the star formation process. Very few brown dwarf companions of larger, Sun-like have been found inside 5 AU, a deficiency that has been dubbed the "brown dwarf desert;" however, there is no such desert associated with low-mass stars. The observations to date strongly support the idea that low-mass binaries form in a process similar to that of more massive binaries, and that the percentage of binary systems is similar for bodies spanning the range from one solar mass to as little as 0.05 solar mass. Yet, there are also many lone brown dwarfs, such as KELU-1, discovered in 1997. At a distance of only 33 light-years from the Sun, it was one of the closest brown dwarfs known at that time. Such solitary dwarfs could be ejected stellar embryos – small infant stars that were still accreting material when they were kicked out of the nest by more massive siblings in multiple stellar systems. On the other hand, observations of some brown dwarfs in the Orion Nebula, which show an excess of near-infrared radiation, point to the presence of dusty disks around these objects. Not only does this suggest a normal stellar formation process but also the possibility that brown dwarfs might develop planetary systems. Whether life could ever emerge on such worlds whose sun is so cool and dim, is a question for the future. The possibility has been discussed as to whether there could be life on brown dwarf stars. Some brown dwarf candidates Host star Spectral type Distance (ly) Mass (Jup=1) Semimajor axis (AU) Period Eccentricity HD 110833 K3 V ~56 17 ~0.8 270.04 d 0.69 BD-04 782 K5 V - 21 ~0.7 240.92 d 0.28 HD 112758 K0 V ~54- 35 ~0.35 103.22 d 0.16 HD 98230 F8.5 V - 37 ~0.06 3.8 d 0.00 HD 18445 K2 V - 39 ~0.9 554.17 d 0.54 HD 29587 G2 V ~148 40 ~2.5 3.17 y 0.37 Gliese 229 M1 V 19 ~40 ~40 ~200 y - HD 140913 G0 V - 46 ~0.54 147.94 d 0.61 HD 283750 K2 V ~54 50 ~0.04 1.79 d 0.02 HD 89707 G1 V ~82 54 - 198.25 d 0.95 HD 217580 K4 V ~59 60 ~1 454.66 d 0.52 Related categories    • TYPES OF STAR    • STELLAR ASTROPHYSICS Archived news Out of the shadows: a brown dwarf revealed (Jan 9, 2002) Hubble survey sheds new light on brown dwarfs (Aug 24, 2000) Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP