Cosmology: the study of the universe

Cosmology is the branch of astronomy concerned with the universe as a whole — its contents, its large-scale structure, and its origin and fate. It builds on the study of stars and galaxies but asks broader questions: how the matter we see is arranged, how it came to be, and how the universe has changed over its history. Modern cosmology combines observation with physics to test models against measurements.

Understanding the universe starts with understanding stars. Astronomers describe stars by properties such as brightness, expressed as magnitude, and spectral type, which is related to temperature and colour. A star forms from a collapsing cloud of gas, spends most of its life fusing hydrogen in its core, and eventually changes as that fuel runs low. The path it takes — this stellar evolution — depends mainly on its mass. Lower-mass stars swell into red giants and shed their outer layers, while the most massive end in violent supernova explosions, leaving behind dense remnants such as neutron stars or black holes.

Stars are grouped into galaxies, vast systems held together by gravity; our own is the Milky Way, with its disc, central bulge and spiral arms. Beyond it lies extra-galactic astronomy, the study of other galaxies and their distribution across space. A key discovery is that the universe is expanding: distant galaxies recede from us at speeds proportional to their distance, a relationship known as Hubble's law, with the rate set by the Hubble constant. This expansion underpins the Big Bang model of an evolving universe. Cosmology also considers open questions such as the possibility of extra-terrestrial life.

Frequently asked questions

What is stellar evolution?

It is the sequence of changes a star goes through during its life, from its formation in a gas cloud to its final state, governed largely by the star's mass.

What is Hubble's law?

It is the observation that distant galaxies recede from us at speeds proportional to their distance, evidence that the universe is expanding.

What can stars leave behind when they die?

Depending on mass, a dying star may leave a white dwarf, a neutron star or a black hole, and massive stars do so after a supernova explosion.