brown dwarf 2MASSWJ 1207334−393254The brown dwarf 2MASSWJ 1207334−393254 (centre) as seen in a photo taken by the Very Large Telescope at the European Southern Observatory, Cerro Paranal, Chile. The brown dwarf has a mass 25 times that of Jupiter and a surface temperature of 2,400 K. Orbiting the brown dwarf at a distance of 8.3 billion km (5.2 billion miles) is a planet (lower left) that has a mass five times that of Jupiter and a surface temperature of 1,250 K.
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 electrondegeneracy pressure. (Those brown dwarfs above 60 Jupiter masses begin to fuse hydrogen, but they then stabilize, and the fusion stops.)
Learn about the different types of stars categorized according to their mass and temperature - red dwarfs, red giants, supergiants, white, and brown dwarf starsOverview of several types of stars, notably the red dwarf, red giant, supergiant, white dwarf, and brown dwarf.
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.
A star is any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources. Of the tens of billions of trillions of stars in the observable universe, only a very small percentage are visible to the naked eye.
Why do stars twinkle?
As the light emitted from a star passes through the different layers of Earth’s atmosphere, turbulence causes the starlight to bend. To an observer on Earth, this distortion of the starlight makes the star appear to be “twinkling.”
How is a star’s brightness measured?
Astronomers define stellar brightness in terms of magnitudes: the apparent magnitude (the perceived and measured brightness of a star) and the absolute magnitude of the brightness of the star, which is the brightness of a star seen from a standard distance of 32.6 light-years, or 10 parsecs.
Why do stars tend to form in groups?
Stars tend to form in groups because of where star formation occurs. Stars form within a molecular cloud, where protostars begin to take shape in areas rich in molecular gases and dust. If they accumulate enough mass in these star-forming regions, some stars are pulled toward each other by gravity, forming pairs, multiple systems, or star clusters.
Why do stars evolve?
Stellar evolution occurs when a star loses its energy from continuous nuclear fusion reactions, causing instability due to decreasing gas pressure. In order to maintain stability, the star burns fuel in its core until it is depleted, causing the core to collapse into, depending on whether the star is low- or high-mass, either a dense white dwarf, a neutron star, or a black hole.
star, any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources. Of the tens of billions of trillions of stars composing the observable universe, only a very small percentage are visible to the naked eye. Many stars occur in pairs, multiple systems, or star clusters. The members of such stellar groups are physically related through common origin and are bound by mutual gravitational attraction. Somewhat related to star clusters are stellar associations, which consist of loose groups of physically similar stars that have insufficient mass as a group to remain together as an organization.
This article describes the properties and evolution of individual stars. Included in the discussion are the sizes, energetics, temperatures, masses, and chemical compositions of stars, as well as their distances and motions. The myriad other stars are compared with the Sun, strongly implying that “our” star is in no way special.
imaging using ultraviolet lightThe Sun as imaged in extreme ultraviolet light by the Earth-orbiting Solar and Heliospheric Observatory (SOHO) satellite. A massive loop-shaped eruptive prominence is visible at the lower left. Nearly white areas are the hottest; deeper reds indicate cooler temperatures.
With regard to mass, size, and intrinsicbrightness, the Sun is a typical star. Its approximate mass is 2 × 1030 kg (about 330,000 Earth masses), its approximate radius 700,000 km (430,000 miles), and its approximate luminosity 4 × 1033 ergs per second (or equivalently 4 × 1023 kilowatts of power). Other stars often have their respective quantities measured in terms of those of the Sun.
Learn about the different types of stars categorized according to their mass and temperature - red dwarfs, red giants, supergiants, white, and brown dwarf starsOverview of several types of stars, notably the red dwarf, red giant, supergiant, white dwarf, and brown dwarf.
Many stars vary in the amount of light they radiate. Stars such as Altair, Alpha Centauri A and B, and Procyon A are called dwarf stars; their dimensions are roughly comparable to those of the Sun. Sirius A and Vega, though much brighter, also are dwarf stars; their higher temperatures yield a larger rate of emission per unit area. Aldebaran A, Arcturus, and Capella A are examples of giant stars, whose dimensions are much larger than those of the Sun. Observations with an interferometer (an instrument that measures the angle subtended by the diameter of a star at the observer’s position), combined with parallax measurements (which yield a star’s distance; see belowDetermining stellar distances), give sizes of 12 and 22 solar radii for Arcturus and Aldebaran A. Betelgeuse and Antares A are examples of supergiant stars. The latter has a radius some 300 times that of the Sun, whereas the variable star Betelgeuse oscillates between roughly 300 and 600 solar radii. Several of the stellar class of white dwarf stars, which have low luminosities and high densities, also are among the brightest stars. Sirius B is a prime example, having a radius one-thousandth that of the Sun, which is comparable to the size of Earth. Also among the brightest stars are Rigel A, a young supergiant in the constellationOrion, and Canopus, a bright beacon in the Southern Hemisphere often used for spacecraft navigation.
solar flareOne of the strongest solar flares ever detected, in an extreme ultraviolet (false-colour) image of the Sun taken by the Solar and Heliospheric Observatory (SOHO) satellite, November 4, 2003. Such powerful flares, called X-class flares, release intense radiation that can temporarily cause blackouts in radio communications all over Earth.
X-ray observations that were made during the early 1980s yielded some rather unexpected findings. They revealed that nearly all types of stars are surrounded by coronas having temperatures of one million kelvins (K) or more. Furthermore, all stars seemingly display active regions, including spots, flares, and prominences much like those of the Sun (seesunspot; solar flare; solar prominence). Some stars exhibit starspots so large that an entire face of the star is relatively dark, while others display flare activity thousands of times more intense than that on the Sun.
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The highly luminous hot, blue stars have by far the strongest stellar winds. Observations of their ultraviolet spectra with telescopes on sounding rockets and spacecraft have shown that their wind speeds often reach 3,000 km (roughly 2,000 miles) per second, while losing mass at rates up to a billion times that of the solar wind. The corresponding mass-loss rates approach and sometimes exceed one hundred-thousandth of a solar mass per year, which means that one entire solar mass (perhaps a tenth of the total mass of the star) is carried away into space in a relatively short span of 100,000 years. Accordingly, the most luminous stars are thought to lose substantial fractions of their mass during their lifetimes, which are calculated to be only a few million years.
Ultraviolet observations have proved that to produce such great winds the pressure of hot gases in a corona, which drives the solar wind, is not enough. Instead, the winds of the hot stars must be driven directly by the pressure of the energetic ultraviolet radiation emitted by these stars. Aside from the simple realization that copious quantities of ultraviolet radiation flow from such hot stars, the details of the process are not well understood. Whatever is going on, it is surely complex, for the ultraviolet spectra of the stars tend to vary with time, implying that the wind is not steady. In an effort to understand better the variations in the rate of flow, theorists are investigating possible kinds of instabilities that might be peculiar to luminous hot stars.
Observations made with radio and infrared telescopes as well as with optical instruments prove that luminous cool stars also have winds whose total mass-flow rates are comparable to those of the luminous hot stars, though their velocities are much lower—about 30 km (20 miles) per second. Because luminous red stars are inherently cool objects (having a surface temperature of about 3,000 K, or half that of the Sun), they emit very little detectable ultraviolet or X-ray radiation; thus, the mechanism driving the winds must differ from that in luminous hot stars. Winds from luminous cool stars, unlike those from hot stars, are rich in dust grains and molecules. Since nearly all stars more massive than the Sun eventually evolve into such cool stars, their winds, pouring into space from vast numbers of stars, provide a major source of new gas and dust in interstellar space, thereby furnishing a vital link in the cycle of star formation and galactic evolution. As in the case of the hot stars, the specific mechanism that drives the winds of the cool stars is not understood; at this time, investigators can only surmise that gas turbulence, magnetic fields, or both in the atmospheres of these stars are somehow responsible.
Strong winds also are found to be associated with objects called protostars, which are huge gas balls that have not yet become full-fledged stars in which energy is provided by nuclear reactions (see belowStar formation and evolution). Radio and infrared observations of deuterium (heavy hydrogen) and carbon monoxide (CO) molecules in the Orion Nebula have revealed clouds of gas expanding outward at velocities approaching 100 km (60 miles) per second. Furthermore, high-resolution, very-long-baseline interferometry observations have disclosed expanding knots of natural maser (coherent microwave) emission of water vapour near the star-forming regions in Orion, thus linking the strong winds to the protostars themselves. The specific causes of these winds remain unknown, but if they generally accompany star formation, astronomers will have to consider the implications for the early solar system. After all, the Sun was presumably once a protostar too.
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Chaisson, Eric J., Aller, Lawrence Hugh, Brecher, Kenneth, Fernie, John Donald. "star". Encyclopedia Britannica, 21 Mar. 2025, https://www.britannica.com/science/star-astronomy. Accessed 29 May 2025.
