A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and — crucially — dark matter. The Milky Way contains somewhere between 200 and 400 billion stars, and it is just one of an estimated two trillion galaxies in the observable universe.
Galaxies come in three main morphological types. Spiral galaxies (like the Milky Way and Andromeda) feature a central bulge with flat, rotating disc arms of stars, gas, and dust. Elliptical galaxies are smooth, featureless, and range from spherical to highly elongated — they tend to be older and star-formation-sparse. Irregular galaxies lack a defined shape, often due to gravitational interactions or mergers.
We live in a barred spiral galaxy — meaning the Milky Way has a bar-shaped central structure from which its spiral arms extend. Our Sun sits about 26,000 light-years from the galactic centre, roughly two-thirds of the way out, on a minor spur off the Orion Arm.
At the very centre of the Milky Way lies Sagittarius A* (Sgr A*), a supermassive black hole with a mass of about 4 million Suns. In 2022, the Event Horizon Telescope published the first direct image of Sgr A*. The entire galactic disc rotates, but not like a solid wheel — stars at different radii orbit at different speeds, following the dynamics set by gravity and dark matter.
The Milky Way is surrounded by a Galactic Halo — a diffuse spherical region containing older globular star clusters, some individual stars, and a large quantity of invisible dark matter that extends far beyond the visible disc.
Galaxies are not evenly spread through space. They cluster together under gravity. Our Milky Way belongs to a small grouping called the Local Group, which contains about 80 galaxies, dominated by two large spirals: the Milky Way and Andromeda (M31).
On larger scales, clusters of hundreds to thousands of galaxies form. The Local Group itself is falling towards the Virgo Cluster, the centre of the Virgo Supercluster (also called Laniakea). These superclusters are nodes in the vast Cosmic Web we first encountered in Chapter 1.
In about 4.5 billion years, the Milky Way and Andromeda will collide and merge, forming a large elliptical galaxy. Despite the drama, individual stars are so far apart that almost none will directly collide — but the shapes of both galaxies will be permanently transformed.
Test what you've just learned.
1.What type of galaxy is the Milky Way?
2.What is Sagittarius A*?
3.What is the Local Group?
4.What will happen when the Milky Way and Andromeda collide?