Messier 9 is a roughly spherical swarm of stars that lies around 25 000 light-years from Earth, near the centre of the Milky Way, so close that the gravitational forces from the galactic centre pull it slightly out of shape. Globular clusters are thought to harbour some of the oldest stars in our galaxy, born when the Universe was just a small fraction of its current age.
As well as being far older than the Sun — around twice its age — the stars of Messier 9 also have a markedly different composition, and are enriched with far fewer heavier elements than the Sun.
In particular, the elements crucial to life on Earth, like oxygen and carbon, and the iron that makes up our planet’s core, are very scarce in Messier 9 and clusters like it. This is because the Universe’s heavier elements were gradually formed in the cores of stars, and in supernova explosions. When the stars of Messier 9 formed, there were far smaller quantities of these elements in existence.Messier 9, as its name suggests, was discovered by the great French astronomer Charles Messier in 1764.
Over 250 000 of them are neatly focused on the detector of Hubble’s Advanced Camera for Surveys, in an image which covers an area no bigger than the size of the head of a pin held at arm’s length. As well as showing the individual stars, Hubble’s image clearly shows the different colours of the stars.
A star’s colour is directly related to its temperature — counter-intuitively, perhaps, the redder it is, the cooler it is; and the bluer it is, the hotter. The wide range of stellar temperatures here is clearly displayed by the broad palette of colours visible in Hubble’s image of Messier 9