A starlike body whose mass is too small to sustain nuclear fusion reactions in its core. All stars, including the Sun, shine because they engage in nuclear fusion in their hot and dense cores. In the early 1960s, S. S. Kumar noted that, if they existed, stars with mass less than 8% that of the Sun would not have the high temperatures in their cores necessary to sustain nuclear fusion reactions.

These objects, called brown dwarfs, would not truly be normal stars because their lack of nuclear fusion would inhibit their ability to shine. Indeed, brown dwarfs would grow dimmer as they aged. At even lower masses are the planets, such as Earth and Jupiter (which is approximately 0.1% the mass of the Sun). The distinction between planets and brown dwarfs has been debated; at the end of 2003, no consensus on the scientific definition of the word “planet” had been reached. Some scientists maintained that planets ought to be distinguished from brown dwarfs by the way they form. Planets form in disks of dust and gas swirling about nascent stars, while brown dwarfs are thought to form like stars, out of the gravitational collapse of a huge cloud of gas in space. However, a basic physical distinction is that brown dwarfs are hot enough when they are young to host evanescent nuclear reactions while planets never host any fusion reactions.