A contracting cloud of gas and dust that has not yet begun nuclear fusion in its core, representing the earliest recognizable stage of stellar evolution. Protostars are typically embedded in dense molecular clouds and visible primarily in infrared wavelengths due to their low temperatures and dusty surroundings.
From Greek 'protos' (first) and 'star', coined in the mid-20th century as astronomers developed theories of stellar formation. The prefix emphasizes this as the precursor stage before a true star begins nuclear fusion and joins the main sequence.
Protostars are stars in their awkward teenage phase! They can take millions of years to contract enough to start nuclear fusion, and during this time they're powered entirely by gravitational energy - literally getting their warmth from falling inward under their own weight.
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