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13.9 Brown Dwarfs

Gravity tends to form more low-mass stars than high-mass stars. Therefore, collapsing gas clouds are expected to produce many objects near the bottom of the mass range of stars. They are astronomically important — there are many more of these objects than there are stars like the Sun, for example. Objects with less than 0.08 solar mass (about 80 times Jupiter’s mass) but bigger than planets are called brown dwarfs. They are known as sub-stellar objects because, lacking nuclear reactions, they are not true stars. The name brown dwarf comes from an early theory that they glow with a dull red light. In fact, they emit nearly all of their radiation at invisible infrared wavelengths.

How do brown dwarfs relate to lower mass planets and to higher mass stars? Suppose we could do this experiment: start with a planet with the mass of Jupiter, and steadily add gas with the same chemical composition as the Sun, 3/4 hydrogen and 1/4 helium. As we add mass, we progress from planets through the brown dwarfs to true stars. As we add mass to a Jupiter-like planet, it gets only a little bigger until it reaches roughly 2 MJupiter, where it levels off. At 13 MJupiter, an object begins a short lived period of deuterium burning, and at 65 MJupiter lithium burning takes place for a short period. As we add even more mass, the object actually gets a little smaller, because gravity compresses the gas to a denser state. Then, as we approach 80 MJupiter, where nuclear reactions begin, it "turns on" as a star and again gets much bigger as we add more hydrogen fuel. The size increases because energy from nuclear reactions creates gas pressure which "puffs up" the star. Conceptually, there is natural physical division between planets and brown dwarfs and between brown dwarfs and stars. We will use the word planet for anything smaller than 13 MJupiter and the term brown dwarf, or sub-stellar object, for objects from 13 to 80 MJupiter.

Brown dwarf Gliese 229b next to its dwarf companion star. Click here for original source URL.

Astronomers have been searching for good examples of brown dwarfs, which would help us understand the relations between planets and stars. Brown dwarfs, of course, are hard to detect because of their faintness. It is also tricky to distinguish between brown dwarfs and low mass stars; the key feature is litium in the spectrum since litium is depleted in normal stars on their way to making helium. A number of brown dwarf candidates have been reported, including one with luminosity only 0.0004 that of the Sun — the lowest luminosity object yet found outside the solar system. There have also been many false alarms, and it is hard to prove these objects are in the 13 to 80 MJupiter mass range needed to call them true brown dwarfs. The first dwarf dwarf discovered was Tiede 1 in the Pleiades open cluster in 1995. Perhaps the best example is an object called Gliese 229B, the spectrum of which shows methane and water vapor, as well as the signature lithium line. The atmosphere of Gliese 229B has a temperature of 1000 K, too hot to be a planet but too cool to be a star. Recently, using infrared detectors sensitive to cool objects, astronomers have begun to discover brown dwarfs in increasing numbers. Almost 2000 are known. Despite their dim appearance, brown dwarfs are important in the census of stars — about 10% of the mass in the solar neighborhood is in the form of objects too cool to be fusing hydrogen into helium.

", conceptually, there is natural physical division between planets and brown dwarfs and between brown dwarfs and stars. We will use the word planet for anything smaller than 13 MJupiter and the term brown dwarf, or sub-stellar object, for objects from 13 to 80 MJupiter.

Astronomers have been searching for good examples of brown dwarfs, which would help us understand the relations between planets and stars. Brown dwarfs, of course, are hard to detect because of their faintness. A number of brown dwarf candidates have been reported, including one with luminosity only 0.0004 that of the Sun — the lowest luminosity object yet found outside the solar system. There have also been many false alarms, and it is hard to prove these objects are in the 13 to 80 MJupiter mass range needed to call them true brown dwarfs. The best example is an object called Gliese 229B, the spectrum of which shows methane and water vapor. The atmosphere of Gliese 229B has a temperature of 1000 K, too hot to be a planet but too cool to be a star. Recently, using infrared detectors sensitive to cool objects, astronomers have begun to discover brown dwarfs in increasing numbers. Despite their dim appearance," brown dwarfs are important in the census of stars — about 10% of the mass in the solar neighborhood is in the form of objects too cool to be fusing hydrogen into helium.