Our Galaxy and star clusters

Star clusters are groups of stars bound by gravity that formed together from one cloud. Open clusters contain hundreds or thousands of stars, are irregular, lie in the disc, are young (millions to a billion years) and often contain blue stars: the Pleiades (M45, ≈ 440 light-years away, ≈ 100 million years old), the Hyades. Globular clusters contain hundreds of thousands to millions of stars, are spherical, lie in the halo and are very old (≈ 11–13 billion years): M13 in Hercules (≈ 25,000 light-years); about 150 are known in the Galaxy. Stars of one cluster have the same age and distance, so on the HR diagram they lie along one line; massive stars evolve quickly and leave the main sequence – the lower the “turn-off point”, the older the cluster. Charles Messier’s catalogue (first published in 1771, now 110 objects with later additions) includes many clusters.