What characterizes a low-mass, optically visible young stellar object known as a T Tauri star?

Answer

It is much more luminous than a main-sequence star of the same mass

T Tauri stars, which are Class II objects, remain significantly more luminous than main-sequence stars of equal mass due to their ongoing energy production via Kelvin-Helmholtz contraction.

What characterizes a low-mass, optically visible young stellar object known as a T Tauri star?
protostarstellar evolutionstar formationastrophysics