Stars — OCR A-Level Physics

Stars form when a region of interstellar dust and gas, mainly hydrogen, becomes dense enough for gravity to overcome internal pressure. Gravitational collapse pulls matter inward, gravitational potential energy converts to thermal energy, and the core temperature rises. When the core reaches roughly 10⁷ K, hydrogen nuclei fuse into helium, releasing energy. Fusion products include gamma-ray photons, which are absorbed and re-emitted at lower energies, and the resulting radiation pressure plus gas pressure acts outward. A stable main-sequence star exists when inward gravitational force balances outward pressure, a condition called hydrostatic equilibrium. The mass of the collapsing cloud determines the star's eventual luminosity, surface temperature and lifetime.

A low-mass star such as the Sun spends most of its life on the main sequence fusing hydrogen into helium in its core. When core hydrogen is exhausted, the core contracts and heats while hydrogen fusion continues in a shell around it. The outer layers expand and cool, so the star becomes a red giant with a larger radius and lower surface temperature. Later, the outer layers are ejected as a planetary nebula, an expanding shell of gas ionised by the exposed hot core. The remaining core is a white dwarf, supported by electron degeneracy pressure rather than fusion. It gradually cools and fades over billions of years. The planetary nebula has nothing to do with planets; the name is historical.

A white dwarf is the dense, hot remnant left after a low-mass star ejects its outer layers. It is roughly the size of a planet but contains about the mass of the Sun, so its density is extremely high, around 10⁹ kg m⁻³. It no longer fuses nuclei; instead, it is supported against gravitational collapse by electron degeneracy pressure, which arises because electrons cannot occupy the same quantum state. A white dwarf gradually cools and dims. The Chandrasekhar limit is the maximum mass a white dwarf can have, approximately 1.4 solar masses; above this limit, degeneracy pressure cannot support the star. In this specification, the limit is used to explain why more massive remnants collapse further.

A massive star, meaning one with much more mass than the Sun, leaves the main sequence and expands into a red super giant. Its core continues fusing heavier nuclei until iron builds up, and fusion can no longer release net energy. The core then collapses rapidly, and the outer layers are blown away in a supernova explosion. What remains depends on the mass of the collapsed core: a neutron star forms if the remnant mass is below roughly three solar masses, while a black hole forms if it is greater. So the route is massive star, red super giant, supernova, then either neutron star or black hole.

A neutron star is the collapsed core left after a supernova. It is extremely dense, with a mass similar to the Sun packed into a radius of about 10 km, and it is supported against further collapse by neutron degeneracy pressure. Many neutron stars spin rapidly and emit beams of radiation, detected as pulses when the beam sweeps past Earth. A black hole is a region where gravity is so strong that the escape velocity exceeds the speed of light, so nothing, not even light, can escape from inside the event horizon. Its observable effects come from its gravitational pull on nearby matter and light.

An HR diagram plots luminosity on the vertical axis against temperature on the horizontal axis, with temperature increasing towards the left. Stars do not scatter randomly but group into regions. Most stars, including the Sun, lie on the main sequence, a diagonal band from hot, luminous stars to cool, dim ones. Red giants are cool but luminous, so they sit above the main sequence on the right. Super red giants are even more luminous and occupy the top right. White dwarfs are hot but dim, so they lie below the main sequence on the left. The position of a star on the diagram therefore tells you about its stage of evolution.