GALACTIC EVOLUTION

How their different sizes can affect how galaxies form

1 A lonely cloud of gas

In order for what astronomers call a small galaxy to be made, a relatively large and isolated gas cloud is needed.

2 The making of stars

Under gravity, the cloud will collapse because there’s not enough pressure from the gas itself to fight against it. Baby stars are made in the fight between gravity and pressure.

3 Forming a disc

The matter spins quickly, causing a flattened disc-like structure. At the centre is a bulge where the older firstgeneration stars can be found. The rest of the disc is teeming with younger stars.

4 A galaxy with arms

Internal processes make the arms and bars found in spiral galaxies. However, if conditions are more favourable, a lenticular galaxy – an intermediate between an elliptical and a spiral – is made instead.

5 A team of gas clouds

Small clouds of gas collapse early on to form the galaxy’s very first stars.

6 A party of stars

These gas clouds, with their newly formed stars, clump together to make a larger cloud with a party of stellar populations.

7 Gaseous add-ons

There isn’t much spinning going on during the making of a large galaxy. Instead the merging of nearby gas clouds stops any chances of a disc-like structure forming.

8 A gigantic galaxy

Since most of the gas needed to make a new generation of baby stars was mopped up, no more can be made. What’s left is a gigantic elliptical galaxy that’s dominated by old stars.