Brown Dwarf (Encyclopedia of Science)
Brown dwarfsf they indeed existre celestial objects composed of dust and gas that failed to evolve into stars. To be a star, a ball of hydrogen must be large enough so that the pressure and heat at its core produce nuclear fusion, the process that makes stars bright and hot. Brown dwarfs, so named by American astronomer Jill Tarter in 1975, range in mass between the most massive planets and the least massive stars, about 0.002 to 0.08 times the mass of the Sun.
Roughly 90 percent of the material in the universe is unaccounted for. Since it cannot be seen, this substance is called dark matter. The existence of dark matter is confirmed by the fact that its mass affects the orbits of objects near the visible edge of galaxies and of galaxies within clusters of galaxies. If brown dwarfs really are as common as astronomers think, their total mass could account for the mass of dark matter, one of modern astronomy's major mysteries.
Because brown dwarfs are so cool, small, and faint, they cannot be observed through ordinary telescopes. Beginning in the 1930s, astronomers have suggested their existence using various techniques. One method is to look for a bouncing movement in the path of a star across the sky. Astronomers believe this erratic motion is caused by the gravitational pull of a low-mass companionuch as a brown dwarfrbiting that star. Another method is to search the sky...
(The entire section is 482 words.)
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