The
Hayashi track is a luminosity–temperature relationship obeyed by infant stars of less than in the
pre-main-sequence phase (PMS phase) of stellar evolution. It is named after Japanese astrophysicist
Chushiro Hayashi. On the
Hertzsprung–Russell diagram, which plots luminosity against temperature, the track is a nearly vertical curve. After a
protostar ends its phase of rapid contraction and becomes a
T Tauri star, it is extremely luminous. The star continues to contract, but much more slowly. While slowly contracting, the star follows the Hayashi track downwards, becoming several times less luminous but staying at roughly the same surface temperature, until either a
radiative zone develops, at which point the star starts following the
Henyey track, or nuclear fusion begins, marking the beginning of the
main sequence.