Brown dwarfs are objects between large planets and small stars, ranging from 13 to 90 Jupiter masses. They are “failed stars” because they lack enough deuterium to form a true star. They generate heat through radioactive elements and compression. They have nearly the same radius, about that of Jupiter, despite varying mass. As mass increases beyond the upper limits of brown dwarfs, volume increases, producing large celestial bodies with radii closer to that of our Sun.
A brown dwarf is a body on the verge of being a very large planet or a very small star. Brown dwarfs range from 13 to about 90 Jupiter masses. The International Astronomical Union puts the boundary between large planets and small brown dwarfs at 13 Jupiter masses, because this is the mass threshold needed for deutrium to merge.
Deutrium is an isotope of hydrogen that includes a neutron in the nucleus, rather than just a proton as in common hydrogen, and is the easiest type of atom to fuse. Because deutrium is quite rare compared to common hydrogen — 6 atoms in 10,000 for Jupiter, for example — there isn’t enough of it to form a true star, and so brown dwarfs are often called “failed stars.”
At about 0.075 solar masses, or 90 Jupiter masses, brown dwarfs become capable of fusing normal hydrogen, albeit at a much slower rate than main-sequence stars like our Sun, making them red dwarfs, stars with about 1/10,000 of solar luminosity. Brown dwarfs in general show little or no luminosity, generating heat mainly through the radioactive elements contained within them, as well as the temperature due to compression. Because brown dwarfs are very faint, they are difficult to observe from a distance and only a few hundred are known. The first brown dwarf was confirmed in 1995. A proposed alternative name for brown dwarfs was “substar”.
An interesting property of brown dwarfs is that they all have nearly the same radius – about that of Jupiter – with a variation of between 10% and 15%, even though their mass varies up to 90 times that of Jupiter. In the low range of the mass scale, the brown dwarf’s volume is determined by the Columb pressure, which also determines the volume of planets and other low-mass objects. At the higher range of the mass scale, the volume is determined by the electron degeneracy pressure, i.e. the atoms are pressed as close together as possible without the electron shells collapsing.
The physics of these two arrangements is such that, as density increases, the radius is approximately maintained. As more mass is added beyond the upper limits of the brown dwarf masses, the volume begins to increase again, producing large celestial bodies with radii closer to that of our Sun.
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