A brown dwarf is a celestial object that is larger than the biggest gas giant planets, but smaller than the least massive main-sequence stars. They are sometimes referred to as "failed stars" because they do not have enough mass for their cores to burn nuclear fuel and radiate starlight. Brown dwarfs form like stars do, by the contraction of gas that collapses into a dense core under the force of its own gravity, whereas planets form from the accumulation of leftover debris from these stellar births. Brown dwarfs are usually bigger than gas giant planets but smaller than the lowest mass stars, with masses between 12 to 75 times the mass of Jupiter. Although they lack hydrogen fusion, brown dwarfs do emit light—thermal radiation from the heat within them. They start out relatively hot and over the subsequent billions of years, they cool and dim, and eventually fade away. Brown dwarfs are difficult to detect, but recent studies indicate that they are nearly as common as stars and are everywhere.