What Is a Galaxy?
A galaxy is an enormous family of stars, gas, dust and probably a mysterious invisible substance called dark matter, all held together by gravity. A small galaxy might contain a few hundred million stars. A giant one can hold over a trillion. Almost every big galaxy seems to have a supermassive black hole at its centre. We live in a barred spiral galaxy called the Milky Way, and astronomers think the observable universe holds about 2 trillion other galaxies.
- Galaxies in the universeapprox. 2 trillionIn the observable part alone
- Main types4Spiral, elliptical, irregular, lenticular
- Smallest knownapprox. 1,000 starsTiny dwarf galaxies near the Milky Way
- Largest knownIC 1101Possibly over 6 million light years across
- Our galaxyMilky WayA barred spiral, approx. 100,000 light years across
- Closest big galaxyAndromeda2.5 million light years away
Galaxies, solar systems and clusters
It is easy to mix up the words. A solar system is just one star plus its planets, moons, asteroids and comets. A galaxy is a huge family of stars (and therefore a huge collection of solar systems) bound together by gravity. The Milky Way contains hundreds of billions of stars, each with its own potential solar system.
Galaxies themselves are not loners either. They tend to live in groups (small collections of a few dozen galaxies) or clusters (much bigger collections of hundreds or thousands of galaxies). The Milky Way sits in a small family called the Local Group, alongside Andromeda and about 50 smaller galaxies.
The four main types of galaxy
Astronomers sort galaxies by their shape. The American astronomer Edwin Hubble first set up the basic scheme in the 1920s, and we still use a more refined version today.
- Spiral galaxies have a flat disc with curving arms of stars, gas and dust, plus a bright central bulge. Most also have a straight "bar" running through the centre, including our Milky Way.
- Elliptical galaxies are smooth and egg-shaped, with no spiral arms. They contain mostly older red stars and very little gas to form new ones.
- Irregular galaxies have no clear symmetric shape. They are often the result of two galaxies colliding or being pulled apart by a bigger neighbour.
- Lenticular galaxies are a halfway type: they have a central bulge and a disc like spirals do, but no spiral arms.
How big and how far?
Galaxies are so far apart that astronomers measure the distances in light years: the distance light travels in one year, about 9.5 trillion kilometres. The Milky Way is around 100,000 light years across. The closest big galaxy, Andromeda, is 2.5 million light years away. The most distant galaxies we have spotted with the James Webb Space Telescope are over 13 billion light years away.
The light you see from those distant galaxies left them long before our Solar System even existed. Looking at far-away galaxies is therefore looking back in time: we see them as they were billions of years ago, when the universe itself was much younger.
The first galaxies
Galaxies were not always around. For the first few hundred million years after the Big Bang, the universe was just a thinning soup of hydrogen and helium gas with no stars at all. Gravity slowly pulled the densest patches of gas together, and the first stars switched on around 100 to 200 million years after the Big Bang. Those first stars then drew each other in to form the first proto-galaxies.
The James Webb Space Telescope, launched in 2021, has already seen galaxies whose light set out less than 400 million years after the Big Bang. They are tiny, hot and full of brand-new stars, completely different from the calm spirals around today.
Galaxies, dark matter and the missing mass
One of the strangest discoveries about galaxies came in the 1970s, when American astronomer Vera Rubin measured how stars in spiral galaxies move. She found that the stars on the outside of a galaxy are travelling far faster than they should be, given how much visible mass the galaxy contains. There must be a lot more matter than we can see.
This invisible extra matter is called dark matter. We can detect it only by the way its gravity pulls on stars, galaxies and light itself. Scientists still do not know what dark matter actually is, but it appears to make up roughly five times more mass than all the stars, planets and gas put together.
Deeper dive: how do we measure how far away a galaxy is?
Measuring distances in astronomy is one of the hardest problems in the subject. For nearby galaxies, astronomers use stars called Cepheid variables: pulsating stars whose true brightness can be worked out from the time they take to pulse. By comparing their true brightness to how bright they look, you can work out how far away they are. This is how Edwin Hubble proved Andromeda was an entirely separate galaxy in 1923.
For galaxies further than a few tens of millions of light years, Cepheids are too faint to see and astronomers switch to Type Ia supernovae as standard candles. Type Ia supernovae are exploding white dwarfs, and they always reach approximately the same peak brightness, so a dim one must be further away than a bright one.
For the very furthest galaxies, astronomers use the redshift: as the universe expands, the light from distant galaxies is stretched out to longer (redder) wavelengths. The bigger the redshift, the further away (and further back in time) the galaxy is. The most distant galaxies seen by the James Webb Space Telescope have redshifts of over 13, meaning their light has been stretched to 14 times its original wavelength on the trip across the expanding universe.
Explore particular galaxies and types: spiral galaxies, elliptical galaxies, the Milky Way and the Andromeda Galaxy.