Any of a class of very young star s with masses less than about twice the Sun's.
Characterized by unpredictable changes in brightness, they represent an early stage in stellar evolution, having only recently been formed by the gravitational condensation of interstellar gas and dust. The energy by which they shine derives from the gravitational collapse itself. These young stars, though now contracting more slowly, are still relatively unstable and will remain so until their interior temperatures become high enough to support nuclear fusion for energy generation. More than 500 T Tauri stars have been observed.