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.
T Tauri stars are the youngest visible F, G, K, M spectral type stars. Their surface temperatures are similar to those of main-sequence stars of the same mass, but they are significantly more luminous because their radii are larger.
Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years.
Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems like the Solar System. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years.
Most T Tauri stars are in binary star systems. A T Tauri stage for the Solar System would be one means by which the angular momentum of the contracting Sun was transferred to the protoplanetary disc and hence, eventually to the planets.
Analogs of T Tauri stars in the higher mass range (2-8 solar masses) – A and B spectral type pre-main-sequence stars, are called Herbig Ae/Be-type stars. More massive (> 8 Solar mass) stars in pre-main sequence stage are not observed, because they evolve very quickly.
When they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main sequence objects.
History of the Future
There are no habitables planets possible at T-Tauri Stars.