Brown dwarf

Resources

A brown dwarf is a pseudostar; a body of gas not massive enough for the gravitational pressure in its core to ignite the hydrogen-fusion reaction that powers true stars. The name brown dwarf is a play on the name of the smallest class of true stars, red dwarf, but while red dwarfs are actually red, brown dwarfs are not brown, but purple or magenta.

Objects ranging in mass between 13 and 75 times the mass of the planet Jupiter—between 1.2% and 7% the mass of the sun—are generally considered brown dwarfs. Clear rules for distinguishing large planets from brown dwarfs, however, are lacking. Some astronomers consider objects down to seven or eight Jupiter masses to be brown dwarfs, while others reserve this term for objects heavy enough to initiate deuterium fusion in their cores, that is, objects of 13 Jupiter masses or more. (Deuterium is a relatively uncommon form of hydrogen that has both a neutron and a proton in its nucleus; deuterium fusion is a minor reaction in true stars and persists for only a few million years even in brown dwarfs.) In 2001, an international committee declared that objects heavier than 13 Jupiter masses should be labeled brown dwarfs regardless of whether they orbit true stars, while objects below this threshold should be labeled as planets if they are orbiting true stars and as sub-brown dwarfs if they are not.

Until recently, astronomers could only theorize that brown dwarfs were common in the universe. They observed that the less massive stars are far more common than the more massive stars, a trend that would suggest that brown dwarfs should be still more numerous. Brown dwarfs are small and faint, however, making them difficult to find.

Hot stars are blue; cool stars are red. Brown dwarfs, cooler than the coolest stars (red dwarfs), should thus be bright in the infrared, that is, they should radiate more heat than light. Therefore, one way to look for brown dwarfs is to search for faint infrared objects. It is difficult, however, to tell if the mass of a solitary infrared object is above or below the dividing line between brown dwarfs and the least massive stars because it is difficult to measure such an object’s mass. Another strategy for finding brown dwarfs is to look for evidence of a low-mass companion orbiting a star. (This strategy is also used to look for planets outside the solar system.) An astronomer discovering evidence of such a companion can estimate its mass from the properties of its orbit and, thus, decide if it is a planet, brown dwarf, or a low-mass star.

In June 1995, three astronomers, Gibor Basri, Geoffrey Marcy, and James Graham, made the first unambiguous discovery of a brown dwarf. They reported observations made with the newly completed Keck 400-inch (10 m) telescope, the largest in the world. Their data confirmed the existence of a previously suspected brown dwarf, which they designated PPL 15, in the cluster of stars known as the Pleiades. The astronomers sidestepped the problem of determining mass by examining the amount of lithium in PPL 15’s spectrum to see if hydrogen fusion reactions are occurring in the core. The reasoning behind the lithium test is as follows: when a star initially forms, it contains some lithium near its surface; for low-mass stars, convection currents mix this surface lithium with the core, where it is destroyed by fusion reactions. In brown dwarfs, lithium also mixes with the core, but the core has no hydrogen fusion reactions to destroy the lithium. Hence, a brown dwarf should have lithium in its spectrum, whereas all the true stars in the Pleiades cluster are old enough to have burnt up all their lithium. Basri’s team found lithium in the spectrum of PPL 15, indicating that it is a true brown dwarf, lacking hydrogen fusion at its core, rather than a low-mass star. Since 1995, hundreds of brown dwarfs have been discovered and it appears that they may be very numerous. Because of their small mass, and despite their large numbers, brown dwarfs are thought to account for only 15% or so of the stellar mass (i.e., the total mass of the stars and star-like objects in the universe).

The Keck telescope also has been used to locate very cool (less than 1,652°F [900°C]) brown dwarfs in the constellations of Ursa Major (the Big Dipper), Leo, Virgo, and Corvus. These brown dwarfs are cool enough that astronomers can detect the presence of methane in their atmospheres—a molecule too fragile to survive the temperatures generated by true stars. In 2002, researchers produced evidence that the atmospheres of some aging brown dwarfs produce forms of gaseous clouds, and rain comprised of liquid iron.

In 2006, a brown dwarf was found orbiting a red dwarf star, SCR 1845-6357. The pair is about 11 light-years away from Earth, within the Corona Australis constellation. The brown dwarf is named SCR 1845-6357 B.

A major problem in modern astronomy is dark matter. Roughly 90% of the matter in the universe is unaccounted for; it is called dark matter because it is not illuminated by light, and so cannot, generally, be seen through telescopes. (When some kinds of dark matter—for example, clouds of dust—get between the Earth and a source of light, then they can be observed.) Astronomers know dark matter is there because its mass affects the orbits of objects in galaxies and of galaxies within clusters of galaxies, but astronomers still do not know what makes up most of the dark matter in the universe. It was long hoped that brown

KEY TERMS

Brown dwarf— An object intermediate in mass between planets and stars.

Cluster of galaxies— A group of galaxies that is gravitationally bound.

Cluster of stars— A group of stars that is gravitationally bound; all the members of the cluster were formed at essentially the same time.

Dark matter— The unseen matter in the universe, detected by its gravitational effect on the motions and structures of galaxies.

Infrared light— Light with wavelengths longer than those of visible light, often used in astronomy to study dim objects.

Nuclear fusion— Any nuclear reaction that fuses two or more smaller atoms into a larger one. Hydrogen fusion reactions provide the energy for the sun and other stars.

dwarfs might account for much of the dark matter, but there is now wide agreement among astronomers that they do not account for more than a small fraction of it. However, study of brown dwarfs remains important because understanding their formation is essential to understanding that of planets and stars.

See also Infrared astronomy; Stellar evolution.

Resources

BOOKS

Jones, Hugh R.A., and Iain A. Steele, eds. Ultracool Dwarfs: New Spectral Types L and T. New York: Springer, 2001.

Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell’s Exploration of the Universe. 7th ed. Philadelphia: Saunders College Publishing, 1995.

Reid, I. Neill. New Light on Dark Stars: Red Dwarfs, Low-mass Stars, Brown Dwarfs. New York: Springer, 2005.

Zelik, Michael. Astronomy: The Evolving Universe. Cambridge and New York: Cambridge University Press, 2002.

PERIODICALS

Biller, B.A., et al. “Discovery of a Very Nearly Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357.” Astrophysical Journal Letters. 2006.

Gizis, John E. “Enhanced: Brown Dwarfs.” Science. (October 26, 2001): 501–507.

Irion, Robert. “The Runts of the Cosmic Litter.” Science. (January 4, 2002): 64–65.

Morales, Ayana. “Forecast for Brown Dwarf ‘Stars’: Iron Rain, Heavy at Times.” New York Times August 6, 2002.

Reid, I. Neill. “Failed Stars or Overachieving Planets?” Science. (June 21, 2002): 2154–2155.

Paul A. Heckert

Larry Gilman

Brown dwarf

A brown dwarf is a pseudostar; a body of gas not massive enough for the gravitational pressure in its core to ignite the hydrogen-fusion reaction that powers true stars. The name "brown dwarf" is a play on the name of the smallest class of true stars, "red dwarf," but while red dwarfs are actually red, brown dwarfs are not brown, but purple or magenta. Objects ranging in mass between 13 and 75 times the mass of Jupiter—between 1.2% and 7% the mass of the Sun—are generally considered brown dwarfs. Clear rules for distinguishing large planets from brown dwarfs, however, are lacking. Some astronomers consider objects down to seven or eight Jupiter masses to be brown dwarfs, while others reserve this term for objects heavy enough to initiate deuterium fusion in their cores, that is, objects of 13 Jupiter masses or more. (Deuterium is a relatively uncommon form of hydrogen that has both a neutron and a proton in its nucleus; deuterium fusion is a minor reaction in true stars and persists for only a few million years even in brown dwarfs.) In 2001, an international committee declared that objects heavier than 13 Jupiter masses should be labeled brown dwarfs regardless of whether they orbit true stars, while objects below this threshold should be labeled as planets if they are orbiting true stars and as sub-brown dwarfs if they are not.

Until recently, astronomers could only theorize that brown dwarfs were common in the Universe. They observed that the less massive stars are far more common than the more massive stars, a trend that would suggest that brown dwarfs should be still more numerous. Brown dwarfs are small and faint, however, making them difficult to find.

Hot stars are blue; cool stars are red. Brown dwarfs, cooler than the coolest stars (red dwarfs), should thus be bright in the infrared, that is, they should radiate more heat than light . Therefore, one way to look for brown dwarfs is to search for faint infrared objects. It is difficult, however, to tell if the mass of a solitary infrared object is above or below the dividing line between brown dwarfs and the least massive stars, because it is difficult to measure such an object's mass. Another strategy for finding brown dwarfs is to look for evidence of a low-mass companion orbiting a star . (This strategy is also used to look for planets outside the solar system.) An astronomer discovering evidence of such a companion can estimate its mass from the properties of its orbit and thus decide if it is a planet , brown dwarf, or a low-mass star.

In June 1995, three astronomers, Gibor Basri, Geoffrey Marcy, and James Graham, made the first unambiguous discovery of a brown dwarf. They reported observations made with the newly completed Keck 400-in (10 m) telescope , the largest in the world. Their data confirmed the existence of a previously suspected brown dwarf, which they designated PPL 15, in the cluster of stars known as the Pleiades. The astronomers sidestepped the problem of determining mass by examining the amount of lithium in PPL 15's spectrum to see if hydrogen fusion reactions are occurring in the core. The reasoning behind the lithium test is as follows: when a star initially forms it contains some lithium near its surface; for low-mass stars, convection currents mix this surface lithium with the core, where it is destroyed by fusion reactions. In brown dwarfs, lithium also mixes with the core, but the core has no hydrogen fusion reactions to destroy the lithium. Hence, a brown dwarf should have lithium in its spectrum, whereas all the true stars in the Pleiades cluster are old enough to have burnt up all their lithium. Basri's team found lithium in the spectrum of PPL 15, indicating that it is a true brown dwarf, lacking hydrogen fusion at its core, rather than a low-mass star. Since 1995, hundreds of brown dwarfs have been discovered and it appears that they may be very numerous. Despite their large numbers, because of their small mass brown dwarfs are thought to account for only 15% or so of the stellar mass (i.e., the total mass of the stars and star-like objects in the Universe).

The Keck telescope also has been used to locate very cool (less than 1,652°F [900°C]) brown dwarfs in the constellations of Ursa Major (the Big Dipper), Leo, Virgo, and Corvus. These brown dwarfs are cool enough that astronomers can detect the presence of methane in their atmospheres—a molecule too fragile to survive the temperatures generated by true stars. In 2002, researchers produced evidence that the atmospheres of some aging brown dwarfs produce forms of gaseous clouds and rain, comprised of liquid iron .

A major problem in modern astronomy is dark matter . Roughly 90% of the matter in the Universe is unaccounted for; it is called dark matter because it is not illuminated by light, and so cannot, generally, be seen through telescopes. (When some kinds of dark matter—for example, clouds of dust—get between us and a source of light, then they can be observed.) Astronomers know dark matter is there because its mass affects the orbits of objects in galaxies and of galaxies within clusters of galaxies, but astronomers still do not know what most of the dark matter in the Universe consists of. It was long hoped that brown dwarfs might account for much of the dark matter, but there is now wide agreement among astronomers that they do not account for more than a small fraction of it. However, study of brown dwarfs remains important because understanding their formation is essential to understanding that of planets and stars.

See also Infrared astronomy; Stellar evolution.

Resources

books

Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell'sExploration of the Universe. 7th ed. Philadelphia: Saunders College Publishing, 1995.

Zeilik, Michael. Astronomy: The Evolving Universe. 7th ed. New York: Wiley, 1994.

periodicals

Gizis, John E., "Enhanced: Brown Dwarfs." Science. (October 26, 2001): 501–507.

Irion, Robert. "The Runts of the Cosmic Litter." Science. (January 4, 2002): 64–65.
"Making Sense of The Smallest Stars." Sky & Telescope 89 (May 1995): 12–13.

Morales, Ayana. "Forecast for Brown Dwarf 'Stars': Iron Rain, Heavy at Times." New York Times August 6, 2002.

Reid, I. Neill. "Failed Stars or Overachieving Planets?" Science. (June 21, 2002): 2154–2155.

Wilford, John N. "Big Telescope is First to Find Brown Dwarf, Team Reports." New York Times June 14, 1995.

Paul A. Heckert
Larry Gilman

KEY TERMS

Brown dwarf

—An object intermediate in mass between planets and stars.

Cluster of galaxies

—A group of galaxies that is gravitationally bound.

Cluster of stars

—A group of stars that is gravitationally bound; all the members of the cluster were formed at essentially the same time.

Dark matter

—The unseen matter in the Universe, detected by its gravitational effect on the motions and structures of galaxies.

Infrared light

—Light with wavelengths longer than those of visible light, often used in astronomy to study dim objects.

Nuclear fusion

—Any nuclear reaction that fuses two or more smaller atoms into a larger one. Hydrogen fusion reactions provide the energy for the Sun and other stars.

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 ]