Galactic Astronomy
Galaxy
Vast gravitationally-bound system of stars, gas, dust, and dark matter — building blocks of the universe
A galaxy is a gravitationally-bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Sizes range from dwarf galaxies (10⁶-10⁸ stars) to giant ellipticals (10¹³). The Milky Way contains ~200-400 billion stars. Galaxy types: spiral, elliptical, irregular, lenticular. Universe contains ~2 trillion galaxies. Dark matter halos contain ~85% of mass. Galaxies cluster — galaxy groups, clusters, superclusters, cosmic web. Origin: ~100 Myr after Big Bang from collapsing gas clouds. Continue evolving via mergers and star formation.
- Mass range10⁶-10¹³ M_sun
- Diameter range100 ly (dwarf) to 500,000 ly (giant)
- Star count (Milky Way)~200-400 billion
- Universe galaxy count~2 trillion (2016 estimate)
- Major typesSpiral, elliptical, irregular, lenticular
- Dark matter fraction~85% of mass
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Why galaxies matter
- Cosmic structure. Building blocks of universe.
- Star formation. Where stars are born.
- Dark matter. Inferred from rotation curves.
- Cosmology. Distance ladder, redshift surveys.
- Habitability. Galactic environment affects planets.
- SMBH growth. Coevolution of BH and galaxy.
- Galaxy evolution. Time-dependent structure.
Common misconceptions
- All galaxies are spiral. Many shapes; spirals are common locally.
- Galaxies are static. Mergers and star formation ongoing.
- Galaxies are stars. Stars + gas + dust + dark matter.
- Universe has billions of galaxies. Trillions (revised by JWST).
- Galaxies cluster around galaxy clusters. Cosmic web — different structure.
- Center is empty. Densely packed; SMBH usually present.
Frequently asked questions
What types of galaxies exist?
Hubble classification: (1) Spiral (Milky Way, M31) — flat disk with arms; central bulge. (2) Elliptical — smooth, ovoid, mostly old stars. (3) Lenticular (S0) — disk without spiral structure. (4) Irregular — no clear shape (Magellanic Clouds). Plus: dwarf (any type, small). Modern classification refines these. Most dwarfs are most numerous.
How are galaxies measured?
(1) Distance — parallax (very nearby), Cepheid stars (10 Mpc), Type Ia SN (further), Hubble redshift (cosmological). (2) Mass — rotation curves (spirals), velocity dispersion (ellipticals), gravitational lensing. (3) Light output — bolometric luminosity in different wavelengths. Multi-wavelength observations key.
How do galaxies form?
~100 Myr after Big Bang. Initial density fluctuations (from inflation) collapsed under gravity. Dark matter collapsed first, providing potential wells. Gas fell in, cooled, fragmented to form stars. Subsequent mergers and accretion built galaxies up. JWST observations: many small galaxies in early universe.
Are galaxies static?
No. (1) Stars orbit galaxy center (Sun: 230 km/s). (2) Galaxies merge — Milky Way is currently consuming Sagittarius dSph. (3) Rotation curves change as DM-baryon interplay. (4) Mergers transform shapes. (5) Star formation depletes gas. (6) Andromeda will merge with Milky Way in ~5 Gyr.
What's at the center of galaxies?
Most have supermassive BH at center. Mass scales with bulge mass (M-sigma relation). Active galaxies — AGN/quasars feeding. Quiescent galaxies (most, including Milky Way) — quiet SMBH. Galactic center is concentrated — billions of stars in ~10-100 pc. Sgr A* in Milky Way: 4 million M_sun.
How do galaxies cluster?
Hierarchical structure. Galaxy groups (a few to ~50 galaxies). Galaxy clusters (~50-1000s, mass 10¹⁵ M_sun). Superclusters (groups of clusters). Cosmic web — filaments, voids. Local Group: Milky Way + Andromeda + ~50 dwarfs. Virgo Cluster: ~1500 galaxies. Mostly bound by gravity + dark matter.
Will galaxies survive forever?
~10⁵ Gyr — mergers decline as galaxies grow apart (universe expanding faster). Within galaxies: stars eventually die. Brown dwarfs and remnants remain. After 10¹⁰⁰ years: most ordinary matter scattered or in BHs. Galaxies as star-forming systems: limited lifetime.