How to identify a galaxy
Hubble’s ‘tuning fork’ sorts a bewildering variety of galaxies into a small number of basic types
Astronomers have studied thousands of galaxies, and they all look slightly different. But certain basic features crop up again and again, and these repeating features can be used to classify galaxies into different types. The first person to do this was Edwin Hubble in 1926, soon after it was first established that galaxies are ‘island universes’ outside our own Milky Way. Hubble did this purely on the basis of appearance, so there are many things we know now that Hubble was unaware of. Radio astronomy, for example, has shown us the distribution of otherwise invisible gas, while high-resolution spectroscopic measurements provide information on stellar motions and chemical composition. Nevertheless, the broad classification system devised by Hubble has survived these new discoveries. As a first cut, galaxies can be divided into two types: elliptical and spiral. Elliptical galaxies are relatively featureless, dominated by old stars that wander round on a random jumble of orbits, with the galaxy as a whole showing little net rotation. Generally ellipticals contain hardly any gas, and as a consequence, very few new stars are formed in them. Spiral galaxies, on the other hand, are full of gas and young stars, with abundant star formation going on in the spiral arms that give them their name. Both the stars and gas tend to move on near-circular orbits in a thin, rapidly rotating disc. About half of all spiral galaxies show a bar-like structure spanning the central region. Hubble classified these ‘barred spirals’ in a parallel sequence to the ordinary spirals, giving a characteristic ‘tuning fork’ shape to his classification system. The same system is still used by astronomers today, often with various tweaks and additions to reflect more recent discoveries.