A dwarf star is a small, dim star that emits low levels of energy. There are several synonyms for the term, including red dwarf, brown dwarf, and M-type star. Red dwarfs are the most common dwarf stars, with masses less than half that of the Sun. Brown dwarfs, on the other hand, are sub-stellar objects that are not massive enough to sustain nuclear fusion in their cores. M-type stars, also known as red M dwarfs, are small, cool stars that are less than half the mass of the Sun. These synonyms all refer to stars that are much smaller and dimmer than the Sun, yet play a crucial role in the Universe's evolution.