Galaxies

Tiny early density differences grew under gravity. Dark matter collapsed first into halos and filaments, providing potential wells into which ordinary gas could fall and cool.

Galaxy formation is therefore not an isolated collapse. It is a continuing exchange among halos, intergalactic gas, stars, black holes, satellites, and the larger cosmic environment.

The first stars formed from nearly pristine hydrogen and helium. Their radiation, explosions, and chemical enrichment prepared gas for later generations and helped reionize intergalactic space.

Early galaxies were generally smaller, clumpier, and more rapidly changing than mature nearby systems. Webb observations are pushing direct study deep into this formative era.

Spiral galaxies contain rotating disks, central bulges, dark halos, and often stellar bars. Their spiral arms are not fixed material spokes; they are patterns where gas compresses and young stars make the structure conspicuous.

Cold gas allows continued star formation, while bars and interactions can funnel material inward. Thin and thick disk populations preserve different phases of galactic history.

Elliptical galaxies range from small spheroids to giant systems dominating clusters. They usually contain older stars, less cold gas, and weaker current star formation than spirals.

Irregular galaxies may be low-mass systems, interaction-disturbed objects, or galaxies whose growth never produced a stable disk. Morphology is a snapshot of formation history, not a permanent label.

The Milky Way contains a thin disk, thick disk, central bar and bulge, stellar halo, gas, dust, and an extended dark-matter halo. The Sun lies in the Orion Spur, far from the galactic center.

Because we observe from inside the disk, dust obscures visible light. Radio, infrared, stellar motions, chemical abundances, and large surveys reconstruct the galaxy’s three-dimensional structure.

Molecular clouds collapse into stars, but only a small fraction of gas becomes stars each dynamical time. Radiation, winds, jets, and supernovae heat and stir the surrounding medium.

Feedback can disperse clouds, drive galactic winds, and recycle enriched gas. It prevents simple runaway conversion of every available atom into stars.

Most large galaxies host supermassive black holes. When gas accretes rapidly, the nucleus can shine as a quasar and launch jets or winds across intergalactic distances.

Energy from the nucleus can heat or expel star-forming gas. Correlations between black-hole mass and galactic bulges suggest that black holes and their hosts evolve together.

Close passages produce tidal tails, bridges, warped disks, and bursts of star formation. Major mergers can destroy ordered disks and build spheroidal remnants, while minor mergers gradually thicken and enlarge galaxies.

Stars rarely collide directly because they are widely separated, but gas clouds interact strongly. Central black holes may eventually form binaries and merge.

Most galaxies live in groups, while rich clusters contain hundreds or thousands. Hot intracluster gas, tidal forces, and high-speed encounters can strip a galaxy’s fuel and transform its morphology.

Clusters also act as gravitational lenses and cosmic laboratories for dark matter. Their galaxies, gas, and total gravitating mass can be measured separately.

Galaxies acquire gas, form stars, merge, and lose or heat their fuel. A quenched galaxy may appear red because young blue stars are no longer being produced.

Future cosmic acceleration will isolate the Local Group from distant systems. The Milky Way and Andromeda are expected to merge, while star formation gradually fades as usable cold gas declines.