4.3 A zoo of galaxies

We began our discussion of galaxies by talking about the spiral nebulae, but these account for only about a third of the entire population. For more than a century, astronomers have learnt to classify galaxies according to their shape (or, more formally, their ‘morphology’); the best-known scheme was popularised by Edwin Hubble who arranged different systems along a two-pronged ‘tuning fork’ diagram.

Individual activity: Classification

Look at a set of galaxies imaged with the Hubble Space Telescope (you will find plenty of images on the Hubble site). Decide what classification scheme makes sense, and arrange the galaxies within it. Write a short paragraph justifying your choice.

The tuning fork essentially divides galaxies into two categories: ellipticals, which range from the perfectly round to those squashed flat into a cigar shape, and spirals. It then subdivides spirals into those which have a central bar, and those which don’t, creating the two prongs of the tuning fork. Along each tine systems are further divided according to how tightly wound their arms are. While these are often regarded as discrete categories – Sa galaxies have tighter arms than Sb, which are themselves tighter than Sc, for example – there is undoubtedly a true continuum. The tightness of the arms is linked often, if not always, to the size of the spiral’s central bulge, a feature comprising mostly older stars compared to the bluer disk. A catch-all category of ‘irregular’ accounts for disrupted or disturbed galaxies, along with a large number of small dwarf galaxies which so no distinct morphology at all.

This classification was primarily driven by the galaxies’ appearance in the telescope. It persists today because it has turned out to be extremely useful in an astrophysical sense; many galaxy properties, including the star formation rate, the ratio of gas mass to stellar mass and the distribution of matter, have turned out to be closely correlated with morphology. The stereotypical elliptical galaxy, for example, is old, dead and red – composed of old stars, devoid of ongoing star formation and therefore red due to an absence of bright blue short-lived massive stars. These ellipticals include the most massive galaxies in the local Universe, and are most common in dense environments such as the Virgo cluster.

Spirals, by contrast, contain most of the star formation in the local Universe. While they almost always have an elliptical-like bulge at their centre, their thin disk is shot through with star formation, typically concentrated in the spiral arms themselves. This is evident from the presence within them of many bright, blue stars. As we shall learn, blue stars are massive and hence short-lived, running through their stock of available ‘fuel’ much faster than smaller stars such as the Sun. Their presence in a region thus indicates that star formation has taken place within the last few hundreds of millions of years.

The spiral arms themselves are produced by density waves, the galactic equivalent of a traffic jam on the motorway. Think about how a traffic jam behaves; it is essentially a group of slow moving cars, which join the jam at the back and progress (slowly!) through the jam until they reach the front, at which point they can accelerate. The jam will thus slowly migrate backwards along the road. In an exactly analogous fashion so the spiral arms proceed around the disk of the galaxy, and each star will spend sometime within arms and sometime outside. (There are some speculative attempts to link the passage of our Solar System through an arm to mass extinctions and other events on Earth, but the evidence is far from clear.)

For a better idea of how this process occurs, see the animations at the Density wave theory page on Wikipedia (and don’t be put off by the equations).