A T Tauri star is a young object that has emerged from its opaque dusty envelope and has become visible at optical wavelengths. T Tauri stars are divided into two categories: Classical and Weak-lined.
Classical T Tauri stars were first discovered by the presence of strong chromospheric spectroscopic lines, especially the Hydrogen alpha line. This line is believed to be produced by interaction between the disk and the stellar surface. This disk may be the precursor to planetary systems like our own solar system and many have been detected by their millimeter and infrared emission. Recently there has also been some direct imaging of these disks by the Hubble Space Telescope.
Classical T Tauri stars are about 1-10 million years old, and are Class II infrared objects. This means that most of the infrared emission from the object is from the disk, since the envelope has mostly dissipated. Classical T Tauri stars are strong X-ray emitters and can also produce powerful winds.
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