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  • Writer's pictureSven Piper

Brown Dwarfs

Updated: 6 days ago


Artist's impression of a brown dwarf
Artist's impression of a brown dwarf (Credit: NASA/JPL-Caltech)

Due to the quantum mechanical degeneracy of electrons, brown dwarfs are compact astronomical objects of at least 13 Jupiter masses, with an upper limit of 75 Jupiter masses; they form a special position between a star and a planet.


They are also known as 'failed stars' because their mass is not sufficient to fuse hydrogen into helium in their interior, resulting in low luminosity.


In order for hydrogen fusion to ignite in a star, a core temperature of at least 3 million Kelvin is required. However, in brown dwarfs, the pressures are insufficient to reach this temperature due to their much lower mass compared to stars. Nevertheless, fusion processes occur in brown dwarfs: deuterium fusion in those with at least 13 Jupiter masses, and lithium fusion in those above 65 Jupiter masses. Therefore, high-mass brown dwarfs, containing lithium deposits, are easier to detect, while low-mass stars that undergo hydrogen fusion quickly deplete their lithium supply


Nevertheless, there are special cases that violate the previously mentioned criteria, as objects have been discovered with less than 13 Jupiter masses yet resemble brown dwarfs more than gaseous planets. The decisive factor here seems to be whether a brown dwarf is part of a solar system or stands alone.


According to the current state of research, brown dwarfs appear to form in different ways: They are either formed from a gas cloud by the same mechanisms as stars, with the only difference being that the mass of the resulting body is not sufficient for hydrogen fusion; or, like planets, they form from a protoplanetary disk and are often ejected from the planetary system at a later stage of their development.


Scientists are certain that there is no transport of matter from the core to the surface and that brown dwarfs do not have a shell-like structure like sun-like stars; in fact, it is believed that they harbor an atmosphere similar to that of gas giants.


Other special features of brown dwarfs include having approximately the radius of the planet Jupiter, with lighter brown dwarfs possessing a larger diameter than the heavier ones. Additionally, unlike stars, older, cooler brown dwarfs can harbor larger amounts of methane over a longer period, as demonstrated by Gliese 229B.


History


In 1960, it was first proposed that the star formation process could also produce objects which, due to their low mass, do not reach the temperature required for hydrogen fusion. It was suggested that it should be possible to detect such objects in the infrared range. The name 'brown dwarf' was only proposed later; initially, the term 'black dwarf' was considered, but today that term describes a burnt-out white dwarf.


The name 'brown dwarf' is not technically accurate, as they also appear red; however, the term 'red dwarf' was already reserved for the lightest stars.


Since the 1980s, various attempts have been made to find these hypothetical bodies, but it was not until 1995 that the first brown dwarf, Gliese 229B (GL229B), was detected beyond doubt with the 1.5 m reflecting telescope of the Palomar Observatory and confirmed with the Hubble Space Telescope. Gliese 229, located 19 light years away from Earth, is home to a red dwarf in addition to the brown dwarf.


The first flare of a brown dwarf, comparable to a small solar flare but billions of times stronger than comparable events on Jupiter, was recorded 16 light years away by the Chandra X-ray Observatory in July 2000. The trigger was probably a twisted magnetic field, and this was also the first indication that brown dwarfs have magnetic fields at all. [1]


The brown dwarf discovered in April 2014, designated WISE 0855-0714, has a surface temperature of only 225 to 260 K (-50 to -10 °C or -58 to 14 °F), making it the coolest object in its class.


In recent years, several hundred more brown dwarfs have been detected, and to the surprise of astronomers, the closest brown dwarf to the Sun was discovered in the Epsilon Indi B binary system at a distance of only 11.8 light years.


Historic Developments:


  • November 1995 – Astronomers found with the Hubble Space Telescope the first clear evidence of a brown dwarf in the Gliese 229 system, 19 light-years away.

  • July 2000 – The Chandra X-ray Observatory captured the first flare from a brown dwarf (LP 944-20).

  • April 2003 – Scientists detected X-rays from a low-mass brown dwarf (TWA 5B), 180 light-years away in the southern constellation Hydra, using Chandra.

  • August 2003 – Astronomers detected with the Gemini South telescope in Chile that the closest known brown dwarf in the Epsilon Indi system has a companion.


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