Astrophysics & Cosmology

Astrophysics is physics done at a distance. Everything we know about a star or a galaxy arrives as light, so the subject begins by learning to read that light — its brightness, its colour, its spectrum — and ends by reconstructing the history of the whole universe from it.

The founding observation is that distant galaxies are almost all receding, faster the farther away they lie. That linear velocity–distance law, v = H₀ d, is what an expanding space looks like from the inside, and running it backwards points to a hot, dense beginning.

Between the light we collect and the cosmos we infer sits the physics of stars. A star is a self-gravitating ball of plasma held up by the pressure of its own nuclear fire, and its whole life is the slow contest between gravity pulling in and fusion pushing out.

Plot luminosity against temperature and stars fall into a pattern — the Hertzsprung–Russell diagram. Most sit on the main sequence burning hydrogen; when the core is spent they swell into giants and then shed their envelopes, ending as white dwarfs, neutron stars, or black holes.

The densest of those remnants make the most extreme laboratories in nature. When two of them orbit closely, general relativity bleeds the orbit of energy as gravitational waves, and the pair spirals together toward a merger that detectors on Earth can hear.