Brown Dwarf

Brown dwarves are stars with mass greater than about 0.01 solar mass but less than 0.08 solar mass; its core temperature does not increase high enough to start thermonuclear reactions. It is luminous, however, because it slowly shrinks in size and radiates away its gravitational energy. As its surface temperature is below the 2500 K lower limit for Red Dwarfs, it is known as a brown dwarf.

It is a sub-stellar (almost a star) celestial object. It has star like material composition (i.e., mostly hydrogen with some helium) but contains too small a mass (generally between 1 percent to 8 percent of a solar mass) to allow its core to initiate thermonuclear fusion. Without such thermonuclear fusion reactions, the brown dwarf has a very low luminosity and is very difficult to detect. Today astronomers use advanced infrared (IR) imaging techniques to find these unusual degenerate stellar objects or failed stars. Some astronomers believe brown dwarfs make a significant contribution to the missing mass (or dark matter) of the universe.

The first unambiguous detection and image of a brown dwarf, Gliese 229B (GL229B), was made in 1995. GL229B is a tiny companion of the cool red star Gliese 229, which is located 19 light years from Earth in the constellation of Lepus. It is estimated to be 20 to 50 times the mass of Jupiter. Infrared spectroscopic observations, made with the 200-inch (5-m) Hale telescope at Palomar, show that the brown dwarf has an abundance of methane, making it similar in composition to Jupiter and the other gas giant planets in the Solar System. More than 100 brown dwarf candidates have since been discovered in infrared surveys, many of which are warmer than GL229B but cooler than M dwarf stars. A new class of L dwarf has thus been added to the spectral classification system.

The Hubble Space Telescope (HST) has carried out a survey of brown dwarfs, showing that there are more low- mass brown dwarfs than high-mass ones. Brown dwarfs are far more abundant than previously thought, but they are probably not abundant enough to contribute significantly to the missing mass. The HST survey also showed that many brown dwarfs are ‘free-floating’ isolated bodies rather than members of close binary systems, suggesting that they form in a manner similar to stars, not planets. However, some of the more massive extra-solar planets may well be brown dwarfs.

Questions to Ponder

• What is the difference between a white dwarf and a brown dwarf?
• What is the mass required for a Star to become a Brown Dwarf?