Brown dwarf
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What is a brown dwarf in astronomy?
A brown dwarf is an astronomical object between a planet and a star, with a mass usually less than 0.075 that of the Sun. Unlike stars, brown dwarfs don't sustain stable luminosities from thermonuclear fusion of normal hydrogen. They produce energy by fusing deuterium in their early years but can't maintain hydrogen fusion due to electron degeneracy pressure in their cores.
Despite their name, brown dwarfs aren't actually brown, but appear deep red to magenta, depending on their temperature. Their surface temperatures depend on mass and age, ranging from as high as 2,800 K to as cool as 300 K. They radiate energy, especially at infrared wavelengths.
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How are brown dwarfs different from regular stars?
A brown dwarf is an astronomical object that falls between a planet and a star. Unlike stars, brown dwarfs do not sustain stable hydrogen fusion. They produce energy through the fusion of deuterium in their early years, but they lack the mass to maintain sustained nuclear fusion of normal hydrogen.
Brown dwarfs are typically less than 0.075 solar mass, or 75 times the mass of Jupiter. Their surface temperatures depend on age and mass, ranging from 300 K to 2,800 K. They appear deep red to magenta, and the coolest have mineral grains in their atmospheres. Stars, however, are much hotter, with the Sun having a surface temperature of 5,800 K.
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How do brown dwarfs form in space?
Brown dwarfs form in a similar way to stars. They originate from the densest regions of molecular clouds, called cloud cores. These cores become unstable and collapse due to gravity, resulting in a condensed protostar that grows by accretion.
Unlike stars, brown dwarfs do not sustain stable hydrogen fusion because they are less massive. However, they can fuse deuterium during their first few million years. Brown dwarfs prevent further contraction because their cores are dense enough to hold themselves up with electron degeneracy pressure. They may constitute 1–10 percent of the number of stars.
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Why don’t brown dwarfs shine as brightly as stars like the Sun?
Brown dwarfs don't shine as brightly as stars like the Sun because they do not sustain stable hydrogen fusion in their cores. Unlike stars, brown dwarfs' cores are not hot and dense enough to fuse normal hydrogen.
While both stars and brown dwarfs produce energy by fusing deuterium during their first few million years, stars continue to contract and get hotter until they fuse hydrogen. Brown dwarfs, however, prevent further contraction because their cores are dense enough to hold themselves up with electron degeneracy pressure.
The Sun, a main-sequence star, generates energy through nuclear fusion, primarily by converting hydrogen into helium. This process requires extremely high temperatures and pressures, conditions not met in brown dwarfs.
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How do astronomers detect or find brown dwarfs?
Astronomers detect brown dwarfs using several methods. One way is to search their spectra for lithium, which stars destroy when hydrogen fusion begins. Another method involves looking for faint objects below the minimum stellar temperature.
Infrared sky surveys have also uncovered hundreds of brown dwarfs. Infrared astronomy studies celestial objects by observing the infrared radiation they emit, allowing investigators to examine objects that cannot be seen from Earth due to dust particles blocking optical wavelengths. The Infrared Space Observatory discovered a large number of brown dwarfs by using a camera sensitive to wavelengths in the range of 2.5–17 micrometres, as well as a photometer and a pair of spectrometers that extended the range to 200 micrometres.
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What is the importance of studying brown dwarfs in understanding our galaxy?
Brown dwarfs, objects between planets and stars, are vital to understanding our galaxy because they help astronomers bridge the gap between the smallest stars and the largest planets. Studying brown dwarfs helps to refine star formation theories, as they form similarly to stars but lack the mass to sustain stable hydrogen fusion. They also help in understanding the mass distribution in the galaxy, offering insights into the prevalence of these substellar objects compared to stars. Furthermore, the study of brown dwarfs enhances our knowledge of exoplanet atmospheres, as the coolest brown dwarfs share similar atmospheric properties with giant planets.
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Brown dwarf, astronomical object that is intermediate between a planet and a star. Brown dwarfs usually have a mass less than 0.075 that of the Sun, or roughly 75 times that of Jupiter. (This maximum mass is a little higher for objects with fewer heavy elements than the Sun.) Many astronomers draw the line between brown dwarfs and planets at the lower fusion boundary of about 13 Jupiter masses. The difference between brown dwarfs and stars is that, unlike stars, brown dwarfs do not reach stable luminosities by thermonuclear fusion of normal hydrogen. Both stars and brown dwarfs produce energy by fusion of deuterium (a rare isotope of hydrogen) in their first few million years. The cores of stars then continue to contract and get hotter until they fuse hydrogen. However, brown dwarfs prevent further contraction because their cores are dense enough to hold themselves up with electron degeneracy pressure. (Those brown dwarfs above 60 Jupiter masses begin to fuse hydrogen, but they then stabilize, and the fusion stops.)
Brown dwarfs are not actually brown but appear from deep red to magenta depending on their temperature. Objects below about 2,200 K, however, do actually have mineral grains in their atmospheres. The surface temperatures of brown dwarfs depend on both their mass and their age. The most massive and youngest brown dwarfs have temperatures as high as 2,800 K, which overlaps with the temperatures of very low-mass stars, or red dwarfs. (By comparison, the Sun has a surface temperature of 5,800 K.) All brown dwarfs eventually cool below the minimum main-sequence stellar temperature of about 1,800 K. The oldest and smallest can be as cool as about 300 K.
Brown dwarfs were first hypothesized in 1963 by American astronomer Shiv Kumar, who called them “black” dwarfs. American astronomer Jill Tarter proposed the name “brown dwarf” in 1975; although brown dwarfs are not brown, the name stuck because these objects were thought to have dust, and the more accurate “red dwarf” already described a different type of star. Searches for brown dwarfs in the 1980s and 1990s found several candidates; however, none was confirmed as a brown dwarf. In order to distinguish brown dwarfs from stars of the same temperature, one can search their spectra for evidence of lithium (which stars destroy when hydrogen fusion begins). Alternatively, one can look for (fainter) objects below the minimum stellar temperature. In 1995 both methods paid off. Astronomers at the University of California, Berkeley, observed lithium in an object in the Pleiades, but this result was not immediately and widely embraced. This object, however, was later accepted as the first binary brown dwarf. Astronomers at Palomar Observatory and Johns Hopkins University found a companion to a low-mass star called Gliese 229 B. The detection of methane in its spectrum showed that it has a surface temperature less than 1,200 K. Its extremely low luminosity, coupled with the age of its stellar companion, implies that it is about 50 Jupiter masses. Hence, Gliese 229 B was the first object widely accepted as a brown dwarf. Infrared sky surveys and other techniques have now uncovered hundreds of brown dwarfs. Some of them are companions to stars; others are binary brown dwarfs; and many of them are isolated objects. They seem to form in much the same way as stars, and there may be 1–10 percent as many brown dwarfs as stars.
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