Brown dwarfs

What is a star?

This is Jill Tarter, an American astronomer who in 1975 coined the term "brown dwarf" to define the stars of insufficient mass to sustain a hydrogen fusion. A star is a object similar to our sun, which shines through nuclear reactions that occur in the center. With the exception of the Sun, the stars appear to the naked eye as a bright, glittering due to atmospheric turbulence, without immediate apparent motion relative to other fixed objects in the sky. All the stars are considerably more distant from Earth than the Sun. The nearest star, Proxima Centauri, is located about 4 light years of the solar system, nearly 250 000 times farther than the Sun. The mass of a star is the order of 10³⁰ kg and its radius of about a few million miles. The power radiated by a star like the Sun is about 10²⁶ watts. Stars form due to the contraction of a nebula of gas and dust under the influence of gravity. If the heating of the material is sufficient, it will trigger the cycle of nuclear reactions in the heart of the nebula to form a star. The energy released by these reactions is then sufficient to stop its contraction due to the radiation pressure generated.

The number of stars in the universe is estimated between 10²² and 10²³. Apart from the Sun, the stars are too faint to be observable in daylight.

Image: Birth of a star: image made from data of X-ray telescope Chandra (blue) and data from the Spitzer infrared telescope (red and orange). At about 4000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active where the presence of clusters of young blue stars in the picture.
That we see born, yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen.

Brown dwarfs

Brown dwarfs are not stars, or rather what are stars "failed". Their mass is between those of small stars and the large planets. Indeed, it is 0.08 solar masses for a protostar begins thermonuclear reactions and become a real star. Brown dwarfs are not massive enough, but they radiate a little heat, the residue of their training.
It is possible that early in their training they have started a fusion but they were eventually extinguished. Brown dwarfs have never reached the critical mass (13 times the mass of Jupiter, or 0.08 times the mass of the Sun) to ignite and maintain a sustainable state.
It refers to a cold brown dwarf to 1 000 ° C and hot from 2000 ° C.
Brown dwarfs are difficult to observe because they emit a weak radiation in the infrared.
In addition, U.S. researchers have detected in the Orion Nebula, two very young brown dwarfs, young as 1 million years.
The researchers were able to determine their size and mass respectively. Surprisingly, they are almost as big as our Sun, but their masses are only 5.5 and 3.5% of the mass of the Sun.

This discovery confirms the theory that at birth, young brown dwarfs have a size similar to that of the Sun, but once they turned their deuterium into helium, they shrink slowly, reaching a size too small that of Jupiter.

Image: Brown dwarfs, these stars "failed" too little mass to light, have been two discoveries by the telescope VLT (Very Large Telescope) in Chile. An international team has identified one of our neighbors, a brown dwarf, located only 12.7 light years from Earth.