The Anatomy of Galaxies
Following on from my post about the Hubble diagram, I thought I’d mention a bit about the main types of galaxies that are out there. Galaxies come in three basic types: spirals, ellipticals and irregulars. Each of these three broad morphologies of galaxy tells us a little about what is going on inside the galaxy itself. They are all structured differently.
The spiral arms of a galaxy contain most of the interstellar medium – dust and other material between stars – within a galaxy. It is in the spiral arms that new stars are forming, hence their usually bright, blueish or white colour. Spirals are made of about 10-20% dust and gas. This is the source material for the stars that are forming within the spiral arms. It is the dust that obscures background light to create the dark lanes you see in spiral galaxies. You can the arms and the dust lanes very well in this artistic impression of our own galaxy, the Milky Way from Nick Risinger / NASA.
The central bulge of spiral galaxy contains older, redder stars and often also contains a invisible, massive black hole. Some, but by no means all, central bulges have the appearance of a mini elliptical galaxy.
The central bulge and spiral arms vary greatly in appearance from galaxy-to-galaxy. But of course, you know this from working on Galaxy Zoo!
Spiral galaxies are also made up of a third component: the galactic halo. This is an almost spherical fuzz of stars and globular clusters surrounding the galaxy, trapped by gravity. You can see the halo quite well in the above image of the Sombrero Galaxy, which is a spiral seen almost edge-on. This image is from Hubble Heritage
Elliptical galaxies are essentially all bulge and nothing else! In an elliptical galaxy the stars tend to be older and there is less gas and dust around. The stars orbit around the centre of mass of the galaxy in a more random way – their orbits are not constrained to a disk shape. There is very little star formation going on in elliptical galaxies and so they usually appear reddish in colour: dominated by older, cooler stars.
There is obviously little to say about the structure of irregular galaxies because they are irregular. They make up about a quarter of all galaxies. It is thought that many irregulars were once ellipticals or spirals and have been distorted by interactions or collisions with other galaxies. Irregular galaxies can have very high star formation rates and can contain a lot of dust and gas – often more than spiral galaxies.
Galaxy Zoo: Hubble has a whole new branch of questions to try and help classify these clumpy galaxies.
You could add this fourth category to the list of galaxy types. Dwarf galaxies might appear to be just smaller versions of the above types, but they are the most common type of galaxy. There are more dwarfs than any of the others, if you just count them up.
The Large and Small Magellanic Clouds – the LMC and SMC, which are visible in the Southern Hemisphere – are actually two small galaxies, orbiting around our own larger Milky Way. The image below, from Mr. Eclipse, shows both of these objects. The LMC is an irregular galaxy and the SMC is a dwarf.
We’ll continue talking about the different types of galaxies – and how they all fit together – in the next post in this series. In the meantime might I suggest yet another type of galaxy, perhaps with a coffee and a bit of classification?