Galactic astronomy

The Milky Way, galaxy types, dark matter evidence, and supermassive black holes

What is galactic astronomy?

Galactic astronomy studies galaxies — enormous, gravitationally bound collections of stars, gas, dust, and other matter, each containing anywhere from millions to hundreds of billions of stars. Our own solar system belongs to one such galaxy, the Milky Way.

The Milky Way, our home galaxy

The Milky Way is a large spiral galaxy containing several hundred billion stars, including our Sun, arranged in a flattened, rotating disk with spiral arms winding outward from a dense central region. From Earth, we see the Milky Way's disk edge-on as a faint band of light stretching across the night sky, made up of the combined light of countless distant stars too faint to distinguish individually.

Types of galaxies

Galaxies come in a few broad shapes. Spiral galaxies, like the Milky Way, have a flattened disk with winding spiral arms where new stars actively form. Elliptical galaxies are rounder, smoother, and generally contain older stars with comparatively little ongoing star formation. Irregular galaxies don't fit either pattern, often having a chaotic shape, sometimes as the result of a past collision or close encounter with another galaxy.

Galaxies rarely exist alone

Galaxies are commonly grouped together by gravity into clusters, which can themselves contain anywhere from a handful to thousands of individual galaxies. Our own Milky Way belongs to a relatively small group called the Local Group, alongside the similarly sized Andromeda galaxy and a number of smaller companion galaxies.

Why galactic astronomy matters

Galactic astronomy places our own solar system in its proper cosmic context, reveals how stars and galaxies form and evolve over billions of years, and provides essential evidence (including the galaxy rotation measurements covered in the Astrophysics article) for the existence of dark matter, one of the most important open questions in modern astronomy.