The Physics of Stars

Most stars are not alone. Pairs bound by gravity — seen as visual, spectroscopic, or eclipsing binaries — let astronomers weigh stars directly from their orbits. When the stars are close enough, one can spill matter across its Roche lobe onto the other, driving novae, X-ray binaries, and the Type Ia supernovae that light up the distant universe.

A quantum-mechanical pressure that arises because electrons, by the Pauli exclusion principle, resist being packed too closely — independent of temperature. It supports white dwarfs against gravity, but only up to the Chandrasekhar limit of about 1.4 solar masses, beyond which even degeneracy fails and the star collapses further.

A star's spectrum reveals not only its temperature but its size. The Yerkes system adds a luminosity class — from I for supergiants down through III for giants, V for main-sequence dwarfs, to VII for white dwarfs — to the Harvard spectral type. Together they place a star precisely: the Sun, for instance, is a G2V dwarf.

To an astronomer, every element heavier than helium is a "metal." A star's metallicity records the enrichment of the gas from which it formed, since the Big Bang made essentially only hydrogen and helium and successive generations of stars forged the rest. It is usually expressed as [Fe/H], the iron-to-hydrogen ratio relative to the Sun.

Stars fall into broad populations: metal-rich Population I stars in the disc, like the Sun; old, metal-poor Population II stars in the halo and globular clusters; and a hypothesised first generation of metal-free Population III stars, not yet directly observed. Star clusters — young open clusters and ancient globulars — are coeval populations that serve as the great testing grounds of stellar-evolution theory.

A star is a self-regulating sphere of plasma in which the inward pull of gravity is balanced at every depth by the outward push of pressure — hydrostatic equilibrium. Energy generated by fusion in the core is carried outward by radiation and by convection, and a handful of coupled equations capture the whole structure.

The single most important diagram in stellar astrophysics: a plot of stars' luminosities against their surface temperatures. Stars are not scattered at random but fall into distinct regions — the diagonal main sequence, the giant and supergiant branches, and the faint white-dwarf sequence — that trace the arc of stellar evolution.

The distribution of masses with which stars are born. Nature makes far more low-mass stars than high-mass ones — a steep decline first quantified by Salpeter — so faint red dwarfs vastly outnumber brilliant massive stars. The initial mass function shapes how galaxies light up, enrich, and evolve.