Stars are often classified by temperature and cold stars describe a variety of stars and protostars with low stellar temperatures. Cold stars have surface temperatures around as low as 1,000K to 3,000K. In contrast, hot stars have temperatures around 40,000K. Cold stars encompass lower temperature brown dwarfs (protostars in which fusion has yet to commence), and also to compact objects such as old neutron stars, in which fusion has ceased.

Brown dwarfs are failed protostars of less than 0.084 solar masses. Below this critical mass, protostars are not massive enough to develop the high core temperatures (approximately 3 million degrees) needed to fuse hydrogen into helium. As a result, the energy emitted from these stars is limited to the heat generated through gravitational contraction. The spectrum of these stars may be identified by the prescience of lithium, which in normal stars is destroyed during the fusion process, and...