A Brief Guide to Galaxy Classification: Part 1 - Spirals and Ellipticals

The other night, whilst contemplating my Open University assignment, I tweeted that at least there was one part I'd have no trouble with - the galaxy classification section. Someone else said they wished they could do that, and it occurred to me that I've never blogged about the first thing about Galaxy Zoo: how to classify galaxies.

Galaxies come in a few major types, with miscellaneous variations within these types. Each type has some distinct characteristics of its own, which allow for recognition. Galaxy Zoo, you may or may not be surprised to hear, has found that some galaxies contain characteristics more typical of other types. And some galaxies seem to defy classification altogether. Which makes it less simple and more interesting. But I'll get onto that later.

However, the majority of galaxies do fall into a few basic types, and we'll start with them.

Here are some spiral galaxies, of the kind we live in. They come in many shapes, but all of them are flat, with arms spinning around a nucleus.

NGC 5364, credit SDSS

Messier 66, credit SDSS

NGC 3521, credit SDSS

Messier 77, credit SDSS

Messier 101, also known as "The Pinwheel Galaxy", credit SDSS

"Messier Objects", incidentally, are a useful collection of well-known objects visible with small telescopes, and their names are often shortened to M101, for example. Amusingly, they were actually a dump of pesky bright objects that had the ill manners not to be comets, which Charles Messier was looking for back in 1771. Now, having been studied with better telescopes, they are a gorgeous collection! The "NGC" numbers refers to the New General Catalogue, a much larger collection of deep sky objects. There are several more of these catalogues - don't worry about them until I've got you fully drowned in geekhood.

You'll notice some characteristics in common with these galaxies. For instance, their arms tend to be blue or bluish, while their nucleuses (I believe it's nucleuses for galaxies and nuclei for atoms . . .) are more yellow. This is because star formation is taking place in the arms.

Whenever star formation takes place, some of these stars will be huge blue supergiants. These monsters only live a few million years, outshining all the normal stars around them and casting their hot blue glow over the whole area. (Blue light is given off by hotter sources than red light.) But they go supernova pretty soon. Once they've done that, the area will be a more normal red or yellow colour. So you can safely bet that if an area is red or yellow, the star formation has stopped. This has happened in the majority of spiral galaxy nucleuses: there is no free gas left to fuel the star formation.

Why does star formation take place around the edge? Two main reasons. Firstly, there is movement. Looking at those galaxies, you can picture them twizzling round - in over 90% of cases in the opposite direction from the one shown in this video here, by the way (that always gives me the horrors, just like seeing signs for "tomatoe's" in the shops!). It's surprising to learn that the arms themselves don't move, or not much as far as I know - the stars are the objects that do! The arms are like traffic jams. Let's say that at Exit X on the M25 there's a traffic jam. Cars enter it, and eventually wriggle to the front, and get out again. The jam itself doesn't move, though cars enter and leave it.

But all this makes for a changing environment. Space isn't just full of stars, but also full of free gas and a tiny bit of dust. The clouds are extraordinarily thin - much thinner than the best laboratory vaccuum on Earth! - so they won't shrink to form stars all on their own. When something does, such as a shock wave from an explosion or the pull of gravity from some passing stars, that may jerk enough of the atoms into the same area for their gravity to start pulling on their surroundings. And then . . . Well, I'll write a post on star formation if you want me to. You get the point. Spiral arms are starforming because things are happening there.
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Also, spiral arms are starforming for a simpler reason: that's where the free gas is. It's thought - or so I read in this book - that spiral galaxies build up gradually over time. Space, as I mentioned, is full of free gas; this gas is drawn into the galaxy bit by bit, so new stars can keep forming at the edges. In the middle, of course, it is all used up.

But spirals only account for about a third of large galaxies (which are certainly not the majority!). What about the other major type, the ellipticals?

Ellipticals are, on the surface, much simpler than spirals: a sphere or rugby ball shape of golden stars, a bright nucleus, and an orb that slowly "fades out" into space (rather than an abrupt start, as spirals are). They are far more three-dimensional than spirals; their stars do not all travel round in an orderly fashion along the same plane, but pretty much randomly. (A star's orbit does not have to be strictly circular or elliptical, as long as it is stable. That's something I'm not going to go into here, not least because I haven't researched it lately.)

They look peaceful, at least at first glance.

NGC 4261, credit SDSS

You will usually find ellipticals living in giant clusters:

NGC 5892, credit SDSS

You will usually find ellipticals living in giant clusters:

Cluster CGCG 087-040, credit SDSS

Abell Cluster, credit SDSS

The Coma Cluster, credit SDSS (the bright pink object is a star in our own galaxy).

Look at the difference between the colours of spirals and ellipticals. Ellipticals tend to be pretty much the same colour throughout - and to have no blue, so no star formation. For one reason or another, elliptical galaxies have no fuel available for making it.

This could be for various reasons. It could be that the galaxies are so old, all the gas has run out. It could be that the wacky and varied orbits of the stars thoroughly mixes the gas and effectively scoops it all up. It could be that the gas is too hot to condense - yes, for star formation you do need cold gas. Isn't that contradictory? But if it's too hot, the particles will just bounce off each other in such low pressures.

More galaxies are ellipticals than spirals. Ellipticals are also more variable in size - they can be very small or very large, while spirals seem to be more middling. There are a great many theories about how ellipticals form.

In the early days, when Edwin Hubble had just found out that galaxies existed - that the Milky Way was not the confines of the Universe - elliptical galaxies were called "early-types", meaning they were presumed to turn into spirals. Today it seems very unlikely that this could be so. To suddenly alter all these random star orbits and organise them into a flat spinning disk - oh, and insert a lot of free gas - and this is on a scale of perhaps 100,000 light years across, remember - well, if you have a mechanism for doing that, I'd like to hear about it.

It seems marginally more likely that it is vice versa; but not much likelier. Once a system of bodies are spinning in a circle, why should it stop? A possible mechanism for elliptical formation is a galaxy merger - when two galaxies collide. I'll show you some mergers in a subsequent post, and it's certainly an incident which guzzles gas and generates massive disorder. An alternative theory is that ellipticals are grown very rapidly (again unlike spirals) in an extremely fast infall of gas towards the centre of the elliptical. This would explain why ellipticals often have especially giant supermassive black holes in their centres.

There! Spiral and elliptical galaxies: the two most famous types. Have a galaxy party (well, actually Copeland's Septet) to celebrate, and tell me which are which!

Credit: SDSS