Astrophysics and Cosmology

Physics at the Largest Scales

Astrophysics and cosmology apply the tools of physics — mechanics, thermodynamics, electromagnetism, special and general relativity, quantum mechanics — to the largest structures in the universe. This is physics without walls, labs, or instruments you can hold: it is physics done by observing the sky.

Stars

Stars are spheres of plasma held together by gravity and supported against gravitational collapse by the pressure produced by nuclear fusion. The Sun fuses 600 million tons of hydrogen into helium every second, converting mass to energy via E = mc².

The life cycle of a star is determined by its mass. Low-mass stars like the Sun become red giants and then white dwarfs. High-mass stars become supernovae and leave behind neutron stars or black holes.

The Hertzsprung-Russell Diagram

The H-R diagram plots stars by luminosity versus surface temperature. Most stars lie on the main sequence — a diagonal band corresponding to hydrogen-burning stars. The H-R diagram is one of the most powerful tools in stellar astrophysics.

Black Holes

A black hole forms when a massive object is compressed to within its Schwarzschild radius — the event horizon. Inside the event horizon, not even light can escape. Black holes are predicted by general relativity and confirmed by direct observation (the Event Horizon Telescope images of M87 and Sgr A*).

Galaxies

Stars form galaxies — gravitationally bound systems containing billions to trillions of stars. The Milky Way is a barred spiral galaxy about 100,000 light-years across. Galaxies group into clusters and superclusters, forming the large-scale structure of the universe.

The Expanding Universe and the Big Bang

Edwin Hubble discovered in 1929 that distant galaxies are receding from us with velocities proportional to their distances. The universe is expanding. Running the expansion backward in time leads to the Big Bang — a hot, dense initial state approximately 13.8 billion years ago.

The cosmic microwave background (CMB) is the afterglow of the Big Bang — thermal radiation from when the universe cooled enough for atoms to form. The CMB is the most precisely measured blackbody spectrum in physics and contains a wealth of information about the early universe.

Dark Matter and Dark Energy

Galaxies rotate too fast for the gravity of visible matter to hold them together. Something else provides the gravity — dark matter. It makes up about 27% of the universe's energy content. What dark matter is remains one of the great open questions in physics.

The expansion of the universe is accelerating. The energy driving this acceleration is called dark energy. It makes up about 68% of the universe's energy content. Its nature is perhaps the deepest mystery in contemporary physics.