What is a galaxy? The types of galaxies, explained simply
By the WhyCosmos team · Last updated:
A galaxy is an enormous system of stars, gas, dust and dark matter held together by gravity. Our Sun is one of hundreds of billions of stars in a galaxy called the Milky Way. Galaxies come in three broad shapes, spiral, elliptical and irregular, and the observable universe contains at least hundreds of billions of them.
A galaxy is the idea that most stretches a child's sense of scale. One star is already unimaginably large; a galaxy holds hundreds of billions of them, and the universe holds hundreds of billions of galaxies. This page gives you the shapes, the sizes and a simple way to make that scale feel real at the dinner table.
How to explain it to your child
Explain it to an 8-year-old
If one star were a house, a galaxy would be a giant city with billions of houses, held together by gravity instead of roads. Our star city is called the Milky Way, and our Sun is just one house in it. There are many other star cities out in space, so far away that their light takes millions of years to reach us.
Explain it to a 13-year-old
A galaxy is a gravitationally bound system of stars, gas, dust and dark matter. Galaxies form and change over billions of years through star formation, collisions and mergers. Most large galaxies have a supermassive black hole at the centre, and most of a galaxy's mass is dark matter, which does not give off light and is detected only through its gravity.
Key facts
| What holds it together | Gravity |
|---|---|
| Our galaxy | The Milky Way, a barred spiral |
| Main types | Spiral, elliptical and irregular, plus lenticular (in between spiral and elliptical) |
| Stars in the Milky Way | Roughly 100 to 400 billion |
| Nearest large galaxy | Andromeda, about 2.5 million light-years away |
| Galaxies in the observable universe | At least hundreds of billions |
| At the centre of most large galaxies | A supermassive black hole |
| Can galaxies collide? | Yes, and they often merge into one |
What are the main types of galaxies?
In 1926 the astronomer Edwin Hubble sorted galaxies by their shape, and his scheme is still the starting point today.
| Type | What it looks like | Example |
|---|---|---|
| Spiral | A flat, spinning disc with curved arms of young stars, gas and dust, often with a bar of stars across the centre | The Milky Way, Andromeda |
| Elliptical | A smooth round or oval ball of mostly older stars, with little new star formation | M87, whose central black hole was the first ever photographed |
| Irregular | No clear shape, often pulled out of shape by a neighbouring galaxy's gravity | The Small Magellanic Cloud, a close neighbour of the Milky Way |
A fourth group, lenticular galaxies, have a disc like a spiral but no spiral arms, and sit between spirals and ellipticals.
How big is a galaxy?
Galaxies range from small dwarf galaxies with far fewer stars than ours to giant ellipticals with more than a trillion. Distances inside them are measured in light-years, the distance light travels in a year, about 9.5 trillion kilometres. The Milky Way's disc is roughly 100,000 light-years across, and the Sun, moving at around 230 kilometres per second, takes about 230 million years to go once around the centre.
A simple scale ladder helps children place themselves:
- Earth orbits the Sun at about 150 million kilometres.
- The solar system is the Sun and everything that orbits it.
- The Milky Way is one galaxy of hundreds of billions of stars, about 100,000 light-years across.
- The Local Group is the Milky Way, Andromeda, the Triangulum Galaxy and dozens of smaller galaxies.
- The observable universe holds at least hundreds of billions of galaxies.
Can galaxies collide?
Yes, and it is one of the main ways galaxies grow. When two galaxies pass close, gravity pulls them out of shape, flings out long tails of stars and squeezes clouds of gas, which can set off bursts of new star formation. Over hundreds of millions of years they can merge into a single, larger galaxy.
The stars themselves almost never hit each other. Stars are so far apart compared with their size that two galaxies can pass through one another with almost no direct star-to-star collisions. The Milky Way has swallowed several smaller galaxies in its past, and it may one day merge with Andromeda, billions of years from now.
What is the difference between a galaxy, a solar system and a constellation?
A solar system is one star with the planets, moons, asteroids and comets that orbit it. A galaxy contains hundreds of billions of stars, and many of them have planets of their own, called exoplanets. So Earth belongs to the solar system, and the solar system belongs to the Milky Way galaxy.
A constellation is something else again: a pattern of stars we see in our sky. Almost every star in every constellation lies inside our own galaxy, so a constellation is a small, flat-looking corner of the Milky Way rather than a separate object.
Can you see another galaxy with your own eyes?
Yes. From a dark site away from city lights, the Andromeda Galaxy shows up as a faint, fuzzy oval in the evening sky from around October to December. It is about 2.5 million light-years away, so the light reaching your eye left it long before modern humans existed. That makes it one of the best ways to explain to a child that looking far away in space also means looking back in time.
The structure of our galaxy is lesson 9 of Astronomy 101, the WhyCosmos live course for ages 8 to 16, and lesson 10 climbs the scale ladder from the Sun to the Local Group.
Questions parents and kids ask
A galaxy is a gigantic group of stars, gas and dust held together by gravity. Our Sun is one star in a galaxy called the Milky Way.
The Milky Way, a barred spiral galaxy about 100,000 light-years across. The Sun sits in a minor spiral arm called the Orion Arm, about 26,000 light-years from the centre.
Nobody has counted them all. Estimates from the Hubble Space Telescope and other observatories put the number in the observable universe at hundreds of billions at least, and possibly far more.
The closest large galaxy is Andromeda, about 2.5 million light-years away. Several small dwarf galaxies are much closer, such as the Large Magellanic Cloud at about 160,000 light-years.
Most large galaxies have a supermassive black hole at the centre. The Milky Way's, called Sagittarius A*, has about 4 million times the Sun's mass, and its first image was released in 2022.
Galaxies show how stars form, how matter behaves on the largest scales and how the universe has changed over its 13.8-billion-year history. The way galaxies move is also how astronomers found the first evidence for dark matter.
Your child asks questions like this every week.WhyCosmos runs live online astronomy classes for ages 8–16, taught by real astrophysicists — where questions like these are the curriculum.
Related terms in this glossary
Sources and further reading
NASA: Galaxies · NASA: Types of galaxies · NASA: Black holes