Spiral, elliptical, lenticular and irregular galaxies, and what happens when galaxies collide.
In the 1920s Edwin Hubble sorted galaxies by their shape. His classification, extended since then, is still used today.
- Spiral galaxies
- Flat, rotating discs with spiral arms full of young stars, gas and dust. Many of them, including the Milky Way, have a central bar.
- Elliptical galaxies
- Smooth, round or oval galaxies with mostly old stars and little gas. The largest galaxies in the universe are ellipticals.
- Lenticular galaxies
- A shape between spiral and elliptical: a disc without spiral arms.
- Irregular galaxies
- Galaxies without a regular shape, often shaped by interactions with other galaxies.
Colliding galaxies
Galaxies often interact and merge. The image above shows Stephan's Quintet, a group of five galaxies. Four of them are about 290 million light-years away and are interacting, while the fifth is much closer to us and only appears to be part of the group.