Ages and masses of young stars are often estimated by comparing their luminosities and effective temperatures to pre-main sequence stellar evolution tracks, but magnetic fields and starspots complicate both the observations and evolution. The atmospheric structure of the T Tauri stars is investigated in a summary of observational evidence. Figure 1.5 is the HR diagram of T Tauri stars in Taurus-Auriga region (Kenyon & Hartman 1995). Ages and masses of young stars are often estimated by comparing their luminosities and effective temperatures to pre-main sequence stellar evolution tracks, but magnetic fields and starspots complicate both the observations and evolution. T Tauri star,, any of a class of very young stars having a mass of the same order as that of the Sun.So called after a prototype identified in a bright region of gas and dust known as the Hind’s variable nebula, the T Tauri stars are characterized by erratic changes in brightness. A number of theoretical evolutional tracks for pre-main-sequence stars with are shown in a solid line, while the isochorones for ages of , , and are plotted in a dashed line. WTTSs distribute near the main-sequence and CTTSs are found even far from the main-sequence. The defining characteristics and general properties of the class are reviewed, the position of the T Tauri stars on the H-R diagram is discussed, and rotation, emission line profiles, IUE and Einstein-Observatory data, radio and IR observations, and polarization measurements are examined. Ages and masses of young stars are often estimated by comparing their luminosities and effective temperatures to pre-main-sequence stellar evolution tracks, but magnetic fields and starspots complicate both the observations and evolution. During the T Tauri phase of pre-stellar evolution, the protostar will actually fluctuate in brightness; however, on average, T Tauri stars are cooler and fainter than their final location in the HR diagram (0.7 Solar luminosities, 4,500 K).