Galaxies

Galaxies are gravitationally bound systems consisting of stars, stellar remnants, interstellar gas and dust, and dark matter. They range from dwarfs with millions of stars to giants with trillions, spanning tens of thousands of light-years across. The observable content is embedded in a dark matter halo.

The main classifications are elliptical (smooth, ellipsoidal), spiral (flattened disks with spiral arms, often with a central bulge), and irregular (no distinct shape). Lenticular (S0) sits between spirals and ellipticals. Many spirals host central bar structures. The Hubble sequence organizes these types.

Galaxies contain stars of various ages, giant molecular clouds, ionized gas, dust, and dark matter. Spiral galaxies show star-forming regions in arms; ellipticals host older populations. Central regions often harbor supermassive black holes whose masses correlate with bulge properties. Most galaxies reside in dark matter halos; rotation curves reveal dark matter presence.

They form from fluctuations in the early universe, growing via accretion of gas and mergers with other galaxies. Interactions can trigger star formation, distort shapes, or lead to mergers that build larger systems. Galaxy evolution is influenced by environment, including group and cluster membership, where interactions are frequent.

The Milky Way is a barred spiral in the Local Group, which also includes Andromeda and Triangulum. Observationally, distances and motions are measured with redshift, standard candles, and spectroscopy across radio, optical, and infrared wavelengths. The study of galaxies informs cosmology, star formation, and dark matter properties.