A main-sequence star is hotter and dimmer than it was as a protostar and is the way the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes.
This is defined as any type of star which is characterized by a hot, dense core which undergoes fusion reaction of hydrogen atoms to form helium thereby resulting in the production of energy.
It is hotter and dimmer than a protostar which is a young star with little or no depletion of its surrounding molecular gas.
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