A star that has not yet evolved to the main sequence on the Hertzsprung–Russell diagram. This means that it is not yet burning hydrogen in its centre. A typical pre-main-sequence star is less than 10 million years old. Its main energy source derives from the contraction of the star, causing it to heat up and emit radiation. There are different types of pre-main-sequence star, depending on the star’s mass. A solar-mass pre-main-sequence star is called a T Tauri star. A high-mass pre-main-sequence star is called a Herbig Ae/Be star.