The life of a star
A star is a huge ball of hot gas, mainly hydrogen and helium. Our Sun is an average, stable main-sequence star. Energy is released in its core by nuclear fusion, in which hydrogen nuclei join to form helium. A main-sequence star is stable because the inward pull of gravity is balanced by the outward forces due to the high temperature and pressure of the hot gas and radiation.
Key idea
Stable star: gravity (inward) = outward forces from radiation and hot-gas pressure. Fusion of hydrogen into helium releases the energy that keeps the star hot and shining.
How stars end
Stars form when gravity pulls a cloud of gas and dust (a nebula) into a protostar. A star like the Sun later swells into a red giant, throws off its outer layers as a planetary nebula and leaves a small hot white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova and leaves a neutron star or a black hole.
Galaxies and the Universe
Billions of stars form a galaxy; the Sun lies in the Milky Way. The Universe contains billions of galaxies separated by empty space. Light from distant galaxies shows redshift — shifted to longer wavelengths — and more distant galaxies are redshifted more. This shows the Universe is expanding, supporting the Big Bang theory that it began from a hot, dense point.
Worked example
A light-year is the distance light travels in one year. With speed 3.0×10⁸ m/s and one year ≈ 3.15×10⁷ s: distance = speed × time = 3.0×10⁸ × 3.15×10⁷ ≈ 9.5×10¹⁵ m.
Remember
- Sun-like star: main sequence → red giant → planetary nebula → white dwarf.
- Massive star: red supergiant → supernova → neutron star or black hole.
- Greater redshift → galaxy moving away faster → the Universe is expanding.