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Ae and Be stars  Pre–main sequence stars more massive than 3 solarmasses (M_).
association  A group of stars whose gravity is insufficient to hold it together but has not yet had time to disperse.
bipolar outflow  Relatively narrow beams of matter ejected in opposite directions by a protostar.
cloud core  The dense part of a molecular cloud where star formation takes place.
giant molecular cloud  An unusually large molecular cloud that may contain as much as 1 million M_.
Herbig-Haro object  A clump of gas illuminated by a jet of matter streaming away from a young star.
molecular cloud  A relatively dense, cool interstellar cloud in which molecules are common.
planetesimal  A primordial solar system body of intermediate size that accreted with other planetesimals to form planets and satellites.
protostar  A star in the process of formation.
solar nebula  The rotating disk of gas and dust, surrounding the newly formed Sun, from which planets and smaller solar system bodies formed.
T Tauri star  A pre–main sequence star, less massive than about three solar masses, showing intense emission lines.

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