A star is a self-gravitating astronomical body whose luminous life is governed by the competition between gravity, internal pressure, energy loss and nuclear fusion. A main-sequence star sustains core hydrogen burning; a pre-main-sequence star is still contracting toward that stage, while evolved stars can burn different fuels or in shells. The name therefore includes more than the currently hydrogen-burning phase.
Hydrostatic equilibrium relates the inward pull of gravity to an outward pressure gradient. Radiative transfer and convection carry energy through the interior, and the photosphere supplies much of the observed spectrum. Mass, composition, age and interactions determine the resulting stellar radius and luminosity. The Sun offers a nearby example in which resolved observations and solar neutrino detection test different parts of the same physical picture.
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