Definition · Plain-language
Galaxy
A galaxy is an enormous, gravitationally bound system of stars, gas, dust and dark matter.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
A vast island of stars
A galaxy is a huge collection of stars — anywhere from millions to trillions — together with the gas and dust between them and a large, unseen amount of dark matter, all held together by their mutual gravity. Galaxies are the basic large-scale building blocks of the universe; stars, planets and nebulae are organised within them. Most galaxies host a supermassive black hole at their core. They are not scattered randomly but tend to cluster into groups and larger structures, separated by enormous, comparatively empty stretches of space.
The main types
Astronomers classify galaxies mainly by shape. Spiral galaxies, like the Milky Way, have a flat, rotating disc with curving arms and a central bulge, and are actively forming new stars. Elliptical galaxies are smooth, rounded or egg-shaped collections of mostly older stars, with little gas and dust and slower star formation; the largest galaxies are giant ellipticals. Irregular galaxies have no clear shape, often because they are small or have been distorted by gravitational interactions with neighbours. Galaxies can also collide and merge over billions of years, reshaping one another.
Galaxies versus solar systems
It is easy to confuse a galaxy with a solar system, but they differ vastly in scale. A solar system is a single star with the planets and smaller bodies orbiting it; a galaxy contains hundreds of billions of stars, many with their own planetary systems. In other words, our Solar System is one tiny part of the Milky Way galaxy, which in turn is one of an enormous number of galaxies. The sheer numbers convey the scale of the universe: galaxies hold billions of stars, and the cosmos holds billions of galaxies.
Key facts
At a glance
- Definition: a vast, gravity-bound system of stars, gas, dust and dark matter
- Size range: from dwarfs of millions of stars to giants of trillions
- Three types: spiral, elliptical and irregular
- Usually has: a supermassive black hole at its centre
- Our galaxy: the Milky Way, a barred spiral
- Not the same as: a solar system, which is just one star and its planets
Common misconceptions
What people often get wrong
Often heard: A galaxy and a solar system are roughly the same thing.
Actually: A solar system is a single star with its orbiting planets. A galaxy contains hundreds of billions of stars, many with their own planetary systems — vastly larger in scale.
Often heard: All galaxies are spiral-shaped like the Milky Way.
Actually: Galaxies come in several forms — spiral, elliptical and irregular. Ellipticals are smooth and rounded, and irregulars have no definite shape at all.
Often heard: Galaxies are spread evenly throughout the universe.
Actually: Galaxies cluster into groups, clusters and larger structures separated by vast, near-empty voids. Their distribution forms a web-like pattern across the cosmos.
Going deeper
