T Tauri stars
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T Tauri stars 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 the youngest visible 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 burning. Instead, they are powered by gravitational energy released as the stars contract towards the main sequence, which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable. There is evidence of large areas of starspot coverage, and they have intense and variable X-ray and radio emissions (approximately 1000 times that of the Sun). Many have extremely powerful stellar winds. Their spectra show a higher lithium abundance than the Sun and other main sequence stars because lithium is destroyed at temperatures above 2,500,000 K.
Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary disc because they are probably the progenitors of planetary systems like the solar system. Most T Tauri stars are in binary star systems. In various stages of their life, they are called Young Stellar Objects (YSOs).