The Double Cluster is formed, as the name suggests, from two separate star clusters. The one on the left of my image is called NGC 869, while the one on the right is NGC 884. They’re fairly similar, and are useful examples of what happens after a burst of star formation.

The stars in these clusters contain a lot of “metals”, which is what astronomers call everything except hydrogen and helium. This is a clue that the stars themselves formed from the remnants of earlier stars that shed their material or exploded as supernovae (as those events are necessary to form the elements present within the Double Cluster’s stars). In other words, these stars are made from recycled material!

The Double Cluster contains a few red supergiant stars, such as AD Persi and RS Persei, which I’ve labelled. These have a cooler outer temperature that most of the other stars in the cluster (around 3500K, compared to up to 25,000K for the others). This is why they appear red; as far as stars are concerned, red is a cooler temperature colour than yellow, white, or blue.

These red supergiants are racing through their material at a furious pace, and will likely explode as supernovae in just a few million years.