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.