What signifies the moment a protostar officially becomes a main-sequence star?

Answer

When stable hydrogen fusion ignites, achieving hydrostatic equilibrium

A star officially forms when the core temperature and pressure are high enough to sustain stable hydrogen fusion (combining hydrogen to form helium). This creates an outward pressure that perfectly counteracts gravity, achieving hydrostatic equilibrium.

What signifies the moment a protostar officially becomes a main-sequence star?

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