Galaxies

Edwin Hubble classified galaxies into types still used today. Spirals (S) have a disk, bulge, and arms; subclasses Sa/Sb/Sc reflect decreasing bulge size and progressively looser arms. Barred spirals (SB) are spirals with a central bar (the Milky Way is one). Ellipticals (E0–E7) are smooth, dust-poor, old-star systems ranging from spherical (E0) to highly elongated (E7), spanning from giant ellipticals (trillions of stars) to dwarf ellipticals (< a million). Lenticulars (S0, SB0) have a disk and bulge but no arms or gas. Irregulars lack symmetry and are often gas-rich.

To reach beyond ~25 Mpc, two new distance methods extend the cosmic ladder. The Tully-Fisher relation links a spiral galaxy's rotation speed (from Doppler-broadened spectral lines) to its luminosity (good to ~200 Mpc). Type Ia supernovae as standard candles reach ~1 Gpc.

Hubble's law is the foundation of modern cosmology: galaxies recede from us with velocity proportional to distance — v = H₀ × d, with H₀ ≈ 70 km/s/Mpc. Measure a galaxy's redshift, compute v, divide by H₀ → distance. The universe is expanding, and there is no center of expansion: every observer sees the same Hubble law.

Galaxies cluster gravitationally. The Local Group (~45 galaxies, including the Milky Way, Andromeda, M33) is part of the larger Virgo Supercluster.

About 20–25 % of galaxies are active galaxies with extraordinarily luminous cores emitting "nonstellar" (nonthermal) radiation. Three main classes: Seyfert galaxies (luminous variable cores in spirals), radio galaxies (huge double radio lobes), and quasars (point-like, enormously redshifted, the most luminous AGN). The unifying engine is a supermassive black hole with an accretion disk, often with relativistic jets. Variability over hours implies emitting regions smaller than the solar system.