Stars — Describe

Describe the life cycle of a star, from formation to its final stage.

(a) A star forms from an interstellar cloud (nebula) of gas and dust that contains hydrogen. (b) Gravitational attraction causes the cloud to collapse inwards; as it collapses its temperature increases, and it is now called a protostar. (c) The protostar becomes a stable star when nuclear fusion of hydrogen begins at its very hot centre, and the resulting outward force balances the inward force of gravitational attraction, keeping the star a stable size. (d) Eventually every star uses up the hydrogen fuel in its core. (e) Once most of the core hydrogen has fused into helium, the star expands: stars like the Sun expand into red giants, while much more massive stars expand into red supergiants. (f) A red giant (from a less massive star) sheds its outer layers to form an expanding shell of gas called a planetary nebula, leaving behind a small, dense core called a white dwarf. (g) A red supergiant explodes as a supernova; this scatters a nebula containing hydrogen and heavier elements (formed during the star's life and the explosion) into space, leaving behind either a neutron star or, if the original star was massive enough, a black hole. (h) The nebula thrown out by a supernova can later collapse under gravity to form new stars, possibly with their own orbiting planets.