Galaxies
Galaxies are huge collections of stars, gas and dark matter.
About this topic
A galaxy is a vast system of stars, gas, dust and dark matter bound by gravity, typically containing millions to trillions of stars. Types: spiral (with arms, like the Milky Way and Andromeda), elliptical and irregular. Most have a supermassive black hole at the centre.
The Milky Way is about 100,000 light-years across, with our Sun in one of its arms. There are hundreds of billions of galaxies in the observable universe, clustered into groups and superclusters. Galaxies collide and merge over time.
The Milky Way is a barred spiral, and our Solar System sits about 26,000 light-years from the centre, orbiting once every roughly 230 million years, so the last time we were here dinosaurs were just appearing. From inside it we see the disc as a faint band of light across the night sky. Andromeda, our nearest large neighbour at about 2.5 million light-years, is approaching us and will collide with the Milky Way in about 4 to 5 billion years, although stars are so far apart that few if any would actually hit.
Dark matter is deduced because stars at the edges of galaxies orbit much faster than the visible mass can explain, so extra unseen mass must be present. Galaxies matter because they are the building blocks of the universe, the places where star formation takes place and where heavy elements are recycled. Images such as the Hubble Deep Field revealed thousands of galaxies in a tiny patch of sky. A common misconception is that all the stars we see are in different galaxies; every individual star visible to the naked eye is in the Milky Way.
Key ideas
- Spiral, elliptical, irregular
- Milky Way is a barred spiral
- Contains about 100 billion stars
Quick check
Use cases: where Galaxies helps
- Homework: review the key idea behind Galaxies before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Galaxies without notes.
- Visual learning: play with the interactive lab to see how Galaxies changes when you adjust its inputs.
- Teaching a friend: explain Galaxies in your own words, then compare your version with the key ideas.
Earth science helps with weather forecasts, natural hazards, energy and climate planning.
Frequently asked questions
What is Galaxies?
Galaxies are huge collections of stars, gas and dark matter.
What will I learn in Galaxies?
Spiral, elliptical, irregular. Milky Way is a barred spiral. Contains about 100 billion stars.
Is Galaxies part of Earth & Space?
Yes. It is one of 60 Earth & Space topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Galaxies lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Galaxies?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.