What is a brown dwarf?
And why do I need to know any of that? Brown dwarfs are sub-stellar objects. They a mass below what’s necessary to maintain hydrogen-burning nuclear fusion reactions in their cores as do stars. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars. Currently there is some debate as to what criterion to use to define the separation between a brown dwarf from a giant planet at very low brown dwarf masses. Because they can’t maintain the hydrogen-burning nuclear fusion, they appear like a charcoal briquette: dark but hot! Thus they are hard to find out there in the universe. America and many other nations are very, very interested in attempting to discover how many are out there and if we’re about to see a visitation for one in particular. Mythology tells us that there was one that visited us about 3,600 years ago or so and the results were interpreted in the Bible and called the plagues of Egypt. This particular brown dwarf goes by the name
A brown dwarf is a body on the edge 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 line between large planets and small brown dwarfs at 13 Jupiter masses, because this is the mass threshold necessary for the fusion of deutrium. Deutrium is an isotope of hydrogen that includes a neutron in the nucleus, rather than solely a proton as in common hydrogen, and is the easiest type of atom to fuse. As deutrium is quite rare compared to common hydrogen — 6 atoms in 10,000 for Jupiter, for instance — not enough is present for the formation of a true star, and thus brown dwarfs are often called “failed stars.” At around 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 solar luminosity. Brown dwarfs in general display very littl
A brown dwarf is a “failed star”. Star form when clouds of gas collapse and thermonuclear fusion stars in the centre. If the mass of the cloud is 13-75 Jupiter masses, the thermonuclear fusion stops when dueterium runs out. Dueterium is rare so this phase lasts less than 100 million years. Stars like the sun are hotter and denser in the core so they can burn hydrogen for billions of years. Planets never get dense or hot enough to start thermonuclear fusion.
It is not easy to tell a star from a planet when you look up at the night sky with your eyes. However, the two kinds of objects look very different to an astronomer using a telescope or spectroscope. Planets shine by reflected light; stars shine by producing their own light. So what makes some objects shine by themselves and other objects only reflect the light of some other body? That is the important difference to understand — and it will allow us to understand brown dwarfs as well. As a star forms from a cloud of contracting gas, the temperature in its center becomes so large that hydrogen begins to fuse into helium — releasing an enormous amount of energy which causes the star to begin shining under its own power. A planet forms from small particles of dust left over from the formation of a star. These particles collide and stick together. There is never enough temperature to cause particles to fuse and release energy. In other words, a planet is not hot enough or heavy enough to
