MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Brown dwarf. Computer artwork of a brown dwarf "star". A brown dwarf is a hypothetical planet that began to accumulate material like a star, but which never collected enough for nuclear reactions to begin in its core. Because they do not shine, brown dwarfs are invisible, and hence there is no direct evidence that they exist. In spite of this, many astronomers believe that there are millions of brown dwarfs throughout space, and that this accounts for some of the missing dark matter that is thought to make up 90% of the Universe.
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