T Tauri Star

Protostar in the late stages of formation, often exhibiting both periodic and random fluctuations in brightness. T Tauri stars are newly-formed (<10 Ma) low to intermediate mass stars (< 3 Msun) with central temperatures too low for nuclear fusion to have started. For the first ~100 Ma, all emitted radiation comes from gravitational energy released as the star contracts under its own self-gravity. T Tauri stars represent an intermediate stage between true protostars (e.g. YY Orionis stars) and low-mass main sequence (hydrogen burning) stars like the Sun.
The nearest T Tauri stars are in the Taurus and r Ophiuchus molecular clouds, both ~400 light years away. Indications of stellar winds and jets show that at least some T Tauri stars are interacting with their environments. Both the winds and jets of are probably powered by material falling onto the central star from the accretion disk (or protoplanetary disk) observed around many of them. The random variability of T Tauri stars (with time-scales from minutes to years) are probably caused by instabilities in the accretion disk (which also produce the “bullets” of material seen in the jet of HH-30), flares on the stellar surface, or simple obscuration by nearby dust and gas clouds. Periodic (regular) variations (days) are almost certainly associated with huge sunspots on the stellar surface which pass into and out of view as the star rotates.

Image source: http://hubblesite.org/newscenter/archive/releases/2000/32/image/c/.

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