T Tauri stars (TTS) are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines. T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track, a luminosity-temperature relationship obeyed by infant stars of less than 3 solar masses  in the pre-main-sequence phase of stellar evolution. It ends when a star of develops a radiative zone, or when a larger star commences nuclear fusion on the main sequence.