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

Let there be peas in Space, and let it begin at the Zoo

I'm rather proud to announce that a detailed History of the Peas is up on the Galaxy Zoo Blog - I hope everybody, especially the pea-pickers, like it. I did a lot of research for that, including checking topics 1 to 157 on the forum to make sure that the possible pea posted by Nightblizzard in topic 158 really was the first!


Thank you very much Kevin and Carie for reading and posting it so speedily. And, of course, to the Peascorps - or Pea Experts - or Pea Pickers - or Pea Maniacs - or whatever name these peaple prefer at any given moment for an incredible amount of hard work!

Update: 2 in 24 hours! Thank you very much to PulseProject for inviting me to write this. We haven't talked much about asteroids publicly yet - there are always more topics at Galaxy Zoo . . . if we ever ran out, the zooites would find something, no problem. In fact, that's what we do anyway.

For a really fabulous piece on asteroids at Galaxy Zoo, check out this Object of the Day by FermatsBrother, who professes to be 359 years old.

PulseProject, Peas, and The Pub where it All Began

Some weeks ago I had an interesting new assignment handed over to me: to find some people to be interviewed for a documentary about Galaxy Zoo. The makers of the documentary are Pulse-Project, who create podcasts and videos of scientific interviews and lectures. I started exchanging e-mails with Colin Murphy, and learnt how much he wants to make lots of good science freely available to the public, and of his determination to keep Pulse-Project going after the idea initially lost funding.

Colin first met Chris a few years ago. "My imagination was fired up," he told me and others a few weeks ago. (I accused Chris of having that effect. Chris declined to reply.) When Pulse-Project did manage to secure some support and interest again, Galaxy Zoo looked like a good way to get that excitement that comes with astronomy across.

Chris had talked Colin through a few of the specific projects on which the Zoo is currently working, especially the Peas and the Irregulars; other suggestions for the documentary to focus on were the Voorwerp, the gravitational lenses, and the asteroids. I sent Colin some notes about each of these: how these came about, how many people were involved, etc. I didn't initially understand why we had to focus on just one thing. Couldn't we do two or more? That would be more representative of the zoo.

Soon afterwards, I got a phone call from Colin. My unexpected piece of work experience was getting very exciting indeed. Listening to him, I felt myself understanding more and more every minute. So many little things I hadn't thought about - for example, if we focussed on both the irregulars and the asteroids, we'd have to go through two different lengthy processes of investigating each and going to interview separate people and so on. The Voorwerp was out: that would involve going to the Netherlands. In any case, I think Colin had made up his mind well before talking to me to focus on the peas.

I have never actually blogged specifically about the peas here, so I'll provide a quick explanation. They're a new class of galaxy discovered at the zoo. Not in one "eureka" moment, but after several months of collecting, never quite sure what we were looking for - and then another year of analysis by the scientists. They are small, round objects of a rich emerald-green, most but not all of which appear to be quasars, and all of which have a high OIII peak in the spectrum, meaning that two electrons have been knocked off the oxygen atoms.

A "pea" from SDSS:

A typical "Pea" spectrum. Note the very narrow, high peaks under specific ions such as OIII. The y axis (up and down) means light intensity, so these ions are emitting high quantities of light in a few particular wavelengths. I explain more about how this works here.

As soon as we thought we'd nailed the reason for their colour - the OIII - another two bunches of objects turned up with similar spectra: nearby starforming galaxies and further-away dark red quasars. In fact, next thing we knew, it seemed that nearly every "energetic" galaxy with any free gas in it had a pea-type spectrum.

When Kevin and Carie Cardamone, a PhD student at Yale, started work on the peas in July last year, the results continued to confound! Peas seem to be neither typical starburst galaxies nor typical active galactic nuclei. What we have found is that they are compact but of low density - so, I presume, of small total mass, yet forming stars at a rate 40 times higher than our own Milky Way Galaxy. But they're part many animals' diet at the zoo nowadays: from Rick's talk at Bristol (on the Markarian peas, a subset) to two recent blog posts by Carie on the paper's submission and process of writing, and more on the peas by Stephen. I'm also preparing the zooite history of the peas, and will post a link to the Galaxy Zoo Blog when that's done.

This may all sound a little nitty-gritty, but actually it's a bomb of a story to tell the public! What started off as amateurs playing around led to amateurs getting very interested, nitpicky, and hardworking - asking questions, arguing over definitions, learning SQL searches - until the scientists came along and found out even more. It's also an example of what Nicola Bennert expects will be "Galaxy Zoo's lasting legacy" - the ability of many public eyes to spot strangeness among quantities of data too vast for scientists to check alone. It's cutting-edge science - yet it's something any member of the public can join in.

We have three main "pea experts" on the forum: Rick, Laihro, and Starry Nite. Straight away we ran into the same problem as with the Voorwerp: neither Laihro nor Starry Nite live in the UK. Rick, however, lives in Bristol. "Yes," said Colin, "we could get to Bristol." Waveney was a must for this one, too, as he'd written peas-sorting classification pages to select the ones Carie wanted. Yes, Colin said, he could probably get to Poole too. I walked up and down the flagstones just outside our front door, hoping Rick and Waveney would be happy to be interviewed. It was a beautiful day and my lunatic fluffy cat was alternately rubbing and attacking my feet. I kept preparing to have to excuse myself in order to shut her indoors. But it was such a fascinating chat. "Is there anything else you'd like to tell me about Galaxy Zoo?" Colin asked me.

"Yes," I said. "We've got such an incredible community." And I told him about how much work we do together on the forum and our meet-ups. I wasn't sure if that could be included, but to me it's one of the jewels in our crown. Colin sounded more interested than I'd been expecting. "Some of them have Skype chats," I said. "In fact, I guess you could record a Skype conference to get the people abroad." That wasn't one of my better ideas - the camera people certainly ruled it out - but I also suggested (and we might just still go ahead with this; I don't know yet) that people with Skype point their cameras at themselves from the side or back while they classify. "We could show a montage of people classifying all over the world!" said Colin.

I told him about our get-togethers - and suddenly an idea hit me. "You could come on a get-together and interview us there!"

I heard a tiny little pause - the sort which meant that this was an idea which could work. Even better, if we could have one where zookeepers came along as well, and talked to him and the volunteers . . . If we could have one in Oxford . . . Perhaps he could find us some computers where we could do some classifying . . . I'll talk to Chris, I said, and get back to you. One other thing, Colin said, could we have some JPEGs of galaxies? You bet we could - I'd already done a little of that for "The Sky at Night" for the Galaxy Zoo episode in September! SDSS images have some copyright (we can use them for anything, but have to get permission if it's commercial), but sorting that out would be - and was - a friendly and straightforward process. We hung up and I felt very excited indeed.

A few days and a flurry of e-mails between me, Colin, Chris and our pea heroes later, I invited the zooites to Oxford for 21st June. If you scroll down the first post you'll see the ideas as a list - we haven't done them all yet!

It was a wonderful day. Lots of zooites came to this one, including (as always with our big meet-ups) a few new people. I was especially delighted that Els travelled over from Belgium, Hanny from the Netherlands, and pea genius Laihro from Germany! I think all my organisational skills were given over to the documentary part, because I managed to book myself and two others into the wrong youth hostel and forget to pack my hairbrush - not what you want to do when you're about to be filmed.

During the morning, Colin and cameraman Brendon interviewed Chris while I spent two hours at Oxford Railway Station waiting for arrivals. This was great: invariably surrounded by two or more fellow zooites, and waving a makeshift "Galaxy Zoo" sign around which earned us a fair few funny looks and one question about what we were advertising! It was most interesting to see people introduced for the first time, such as Rick and Laihro; although our forum personalities generally start showing through after a few minutes of conversation, zooites' faces are rarely what many of us picture. Rick had dressed very smartly and Laihro was about 40 years younger than I think most of us had imagined. Hanny and Edd had just got back from Ireland and were dragging a week's luggage around. Waveney turned up in a dark blue shirt with a brilliantly coloured fish pattern and some fancy new business cards he couldn't resist showing us. "Thanks," I said. "I'll know who to ask next time I want to build an aeroplane."

Colin came along to introduce himself and we had a lovely lunch at the Big Bang restaurant . . .

. . . after which we went to Oxford Astrophysics and were herded into a lecture theartre - where we met Pamela and Georgia, who had arrived in the UK at 6am! In this picture, our programming genius Arfon is standing in front of the blackboard in a white shirt. "You look better than my undergraduates - a lot more intelligent," said Chris as we found our seats. But when we wouldn't stop yacking, he had to yell, "Quiet. Shut up, you lot. You are as bad as my undergraduates!"

It wasn't a lecture, however, even though they did claim to have hidden the biscuits: it was a sneak preview of two future Zoo projects! The first is about merger simulations. Arfon would put an SDSS image of a merging galaxy up on the screen, and we'd use a program to find a diagram that resembled it most, and then run the simulation to see what happened to it. After that we bombarded Chris and Arfon with suggestions (mine was to tell the program to stop trying too hard to imitate the target image - human eyes are far better at that than computers!). I must confess I spent most of my time getting frustrated with my inability to operate the program, but cleverer people loved it! What was more my line came next: Hubble Space Telescope zoo, and how fuzzy could the galaxies be before they become impossible to classify? Actually, I got interested enough to write an Object of the Day about that one, to get more data for Chris. The main answer was: "It depends what sort of classifications you want." Fair point. I'll be doing another one when Chris's laptop is returned to him (no, I don't know what's happened to it now, but it seems to do this to him). In the following picture, you'll see the frowns of concentration and Geoff and Jules's fabulous Zoo T-shirts:

After all this intellectual business, we went along to the Royal Oak, where Galaxy Zoo was invented. It's a wonderful labyrinth of rooms and stone floors and tables and funny-shaped bits of wood, seeming like a quiet country cottage on the edge of the wide noisy Woodstock Road. The picnic tables perch close together on the flagstones of the beer garden. It was here that Chris and Kevin kindly took me for dinner after a day of beta-testing Zoo 2 last June - and here where Kevin was "whinging" (in Pamela's words) about having 850,000 more galaxies to classify, whereupon he and Chris thought of showing them to the public. Colin had booked us in, and they'd roped us off a room and given us permission to film there. Apparently they hadn't heard of Galaxy Zoo! "This place is always full of astronomers," Chris told us. We Zooites reckoned it ought to have a Galaxy Zoo plaque. Perhaps in our elderly years it will be a place of pilgrimage . . .

The Royal Oak

After we'd settled down and had a few drinks and got started on the serious chattering, Colin and Brendon set up their camera in a table in a corner of the beer garden. There's a sweet little arch leading directly to the pavement; just when it was my turn to be interviewed some very noisy people trotted through it to screech cheerily at the people on our next table, so I hung around on camera for some five minutes without saying a word, waiting for them to be finished! We will all be appearing on camera with a tall hedge behind us, not looking at the camera, but looking at Colin, who asked the questions.

At the edge of the beer garden:

Colin reminded me patiently to please work the question into my answer, because the questions wouldn't appear in the documentary. I kept forgetting to do this. I tried to be expressive, as I'd caught onto when I lived in Spain in 2003-04, and wave my hands around, but I fear I overdid it rather - especially when trying to explain spectra! My mouth was very dry after the interview and I made straight for the bar, but it still took some time to get there because everybody was so friendly and wanted to talk to me. I guess you get used to interviews, as you do to lectures and teaching. Their first question was "What has been your main role in Galaxy Zoo?" I replied that I am the benevolent dictator. The last question was the hardest: Why classify? Why should people bother? I honestly couldn't answer that! I suggested to Chris afterwards that I should have said, "Why breathe?"

The next thing to sort out is the JPEGs and also to see if we're still doing the Skype montage! The documentary won't be out for a while. It was wonderful when Colin and Brendon remarked that without the get-together, the number of interviews they'd recorded today would have taken them 6 months and cost thousands of pounds. I knew, and probably still know, only a small amount about the process of making a documentary - but finding out what I have, and especially being the point of contact and arrangements, has been and is a wonderful experience. You don't need lots of training or bits of paper. You need a bit of imagination and to know who to go to with what; you need to be prepared to send and receive lots of e-mails, and - well, just to know a lot about the people you're working with. Addiction to the forum is a very good thing.

Thanks to everybody who came along - as ever, it was you folks who made it such a special day. Thanks for all your photos, several of which I have stolen, and for all your enthusiasm and great messages on the thread. The icing on the cake was when Chris handed out lots of the first pea paper to everyone who appeared on the acknowledgements. People without astronomy qualifications, without PhDs, some of whom knew little astronomy before starting Galaxy Zoo, were finding their names on a scientific paper. And we've discovered a new class of galaxy, and there's still a lot more to find out about them. We really have given peas a chance!

Sailing Among the Blues

The first name thought up for Galaxy Zoo was "Galaxy Safari". Astronomy is like a journey sometimes - through space and time, stopping at astonishing tourist sites or, further on, the wilderness. It's definitely true that Google Sky tours have suddenly become the height of fashion on the forum - now Half65, Ben Hoyle and Fermats Brother are all making them, and I expect more will follow!

I often also get the feeling that we're a little like biologists - or rather, that we've changed the whole of galaxy science from specialist zoology (sorry about that) to worldwide ecology. Traditionally, an astronomer might spend a lifetime studying up to 30 galaxies, and we've got used to the usual types - ellipticals are red, spirals are blue, galaxies are pretty, in science and view (and sorry about that too). Our discovery of blue ellipticals and red spirals was like discovering a parrot in a penguin colony, or a polar bear spotted like a leopard. We now know that, as polar creatures are white, the environment dictates galaxy colour. Being in a cluster turns galaxies red. And being alone turns galaxies blue.

That isn't due to natural selection, of course - it's due to the available gas in the local environment. And there's one type of galaxy that always seems to be blue, and that's the irregular galaxy. Irregular galaxies might be past mergers, or they might be little splodges or wisps of star formation, far smaller than your average spiral or elliptical, and with no defined shape we can so far describe scientifically.

(Credit: SDSS.)

Our brilliant Waveney wrote an Object of the Day on our irregular galaxies project yesterday. I won't try to cover all our aims or findings in this post, but one result is definite: they are nearly all blue. We've found a few that aren't, but none that are red, like ellipticals in clusters. Generally speaking, they are much, much bluer than your average Galaxy Zoo galaxy.

Irregular galaxies seem very common, although we can only see the nearest few because they're so small. As sepos stated in another Object of the Day, 90% of galaxies are the low surface brightness variety - again, not the sort we're studying. Could we be focussing on the magnificent tigers and elephants and whales of the universe, and missing out on the beetles and plankton? Do large bright galaxies depend on the existence of these dim little puffs as large organisms depend on the existence of bacteria? Or are these lowly little galaxies simply a by-product of far-flung gas, and have no effect on their larger neighbours?

Going back a moment - why are they mostly blue? It's because they're forming stars. As blue light is more energetic than red, that means the hottest stars give off blue light. That doesn't mean stars start off blue and go red later. It means that whenever star formation takes place, a few massive stars invariably form. These hydrogen-guzzling monsters outshine all the red stars in the galaxy, giving the whole galaxy a blue appearance - but they don't live very long. After a few million years, only the more sedate yellow and red ones are left.

Star formation is the subject of yet another game on the zoo (do you think I need to create a "games" label?) - not to mention Chris's current research interest at Oxford, or so the websites say. Intriguingly, his research apparently "focusses on the use of sulphur compounds". Which seems contradictory to me, since such compounds would not survive the heat and fury of stars. Perhaps they're a feature in gas clouds, or a product of something or other. This is the most informative paper Google could provide. If he ever has time, I'll ask him to talk me through it.

(Starforming region Corona Australis. Credit: NASA.)

So are all irregular galaxies young? Or are they simply like lonely blue spirals and ellipticals - continuing to drift through space and always encountering more gas? Is the Universe still too young for any area to be fully empty of gas, and will we see red irregulars in another few billion years? What about irregulars in giant superclusters, assuming they wouldn't all merge with the giant ellipticals already living there?

Some questions we already seem to have answered, at least to the best of our ability, on Galaxy Zoo so far. For example, we've established that galaxy rotation is generally random, though spirals near each other have a greater likelihood of rotating in the same direction - another indication that environment is very important in galaxy formation. (For more on galaxy rotation, I would recommend all ZookeeperKate's posts on the Galaxy Zoo Blog.) But I don't think we need worry that we've already found out everything. There is a lot more left to come!

Object of the Day: Inspecting Spectra

I thought you might enjoy this nice new angle to the blue ellipticals/red spirals discovery made by Galaxy Zoo: "Tuesday 9th December 2008", today's Object of the Day, written by me and Fluffyporcupine. I'm no expert on spectra - I know more about why we shouldn't use them as classification tools than about spectra themselves. I love the theory behind them, but as soon as I'm shown one and asked to apply it to a real-life situation I make a lot of mistakes.

I didn't think I'd be any good at collaborative writing, because I'm so bossy and inflexible - I can't resist wanting everything to be done my way. That didn't matter this time, because Fluffyporcupine is saintly as well as clever. She's the one who knows how to find great galaxies with telling spectra, and I'm the one who has had the practice of putting it into a story. If you write a lot, I'd actually recommend collaborating with someone to see what new things you learn. I certainly learnt a lot from Chris after he sent me his comments on my CERN article. And I learnt a lot this morning too - and was completely absorbed. In fact, I was more entranced writing something with someone, than I generally am when writing on my own.

I've been bowled over by the zooite contributions to Object of the Day - it's wonderful to give our hardworking, knowledgeable and very witty and imaginative zooites the stage for a day; and it's fascinating to see what they come up with. That's one of my favourite things about Galaxy Zoo - we're all students and teachers together, all at different levels. I think some Objects of the Day have been written jointly, and it's a great exercise. You can play to your strengths - and then perhaps later could try swapping over!

One advice I'd give anyone writing Object of the Day is not to try and tell an entire story. In astronomy, one story leads to another, which leads to another again. If you start talking about spectra, you could start talking about chemical elements. That might then go on to sizes of stars and nuclear reactions. That might go on to supernovae or quasars. Or maths. Or telescopes. Or . . . In any case, every year we'll have about 365 posts, so there is plenty of room for stories. That's one way you can have the stage, but you don't have to write a novel. I hope more zooites will volunteer to write an Object of the Day once the busy Christmas period is over. It's great fun, and hopefully useful and enthralling too.

The Story of the Red Spirals (part 1)

I'll be writing a lot about the colours of galaxies in several different posts; here's Chapter 1, as it were. Kind of hoping the scientists will pop in occasionally and point out any glaring errors, and anyone who'd like to will give me feedback on the clarity and style . . .

A few years ago, Kevin was a PhD student at Oxford examining a new breed of galaxy. It's well known that there are two basic galaxy types: spiral and elliptical. It has also long been well known that spirals are usually blue, and ellipticals are usually red. The beautiful arms in a spiral galaxy are really density waves, like a traffic jam on a motorway. And like the areas of congestion, they don't necessarily move along with the stars - a car might cruise along at 70 m.p.h. for a few hours, reach the traffic jam, get very cross, but eventually get to the other side of it and continue on its way. (The spiral arms do indicate a lot about the galaxy - which way it is spinning round [except in the exceptions], how big the black hole in the centre is, etc.) This means that the environment the stars and gas live in is continually changing and disrupted - which triggers off star formation.

When stars are young, they are hot - and blue. Blue light has the shortest wavelength, and the highest energy. The equivalent, in sound, is a high-pitched note. (Sound and light both have wavelengths way higher and lower than what we can hear and see, but that's another story.) In any case, a blue colour in galaxies is a good indicator of a lot of heat and star formation.

Elliptical galaxies, on the other hand, don't have star formation. There seem to be two reasons for this. One is that the orbits of the stars are "radial" - they all go their own way - so there are no large-scale disruptions to trigger clouds of gas to condense. Another is that they seem to have run out of gas anyway. Come to think of it, one thing I don't know for a fact is whether these two points are related.

Ellipticals appear dull at first, but they're incredibly peaceful, beautiful sights. SDSS pictures usually show them as a clear gold or vanilla colour. They sort of "fade out" towards the edge, and have a bright core. Of course, this bright core often contains a supermassive black hole, such as in the case of the giant elliptical galaxy M87, which won't be the pinnacle of serenity. Elliptical galaxies are thought to be remnants of past mergers. They're generally found in giant clusters of galaxies, where mergers would be more likely to take place, and they are often larger than spirals. Mergers create massive disruption and trigger off spectacular star formation, which might use up all the gas free to form stars.

So "blue galaxy" came to mean "spiral", and "red galaxy" came to mean "elliptical". It worked fine before SDSS. Then we found it wasn't that way at all.

(Is it just me or is most science based on "People have always said that things are this way. But we have found it might actually be this way!"? After you've heard that in every lecture at university for a year or so, it gets highly tedious and you wonder if all the academics want to do is prove each other wrong, rather than make new directions for themselves. On the other hand, "Eureka!" moments are . . . somewhat immature. Curiosity and research and painstaking surveys have gone on for centuries. [There are always exceptions - apparently there are no budding fungi taxonomists left in the UK, but not all British fungi have been classified yet! Who's up for filling this niche?] Most of science now seems to be refining, redefining, extending and building on what we already know. Perhaps it's irksome because you wonder what there is left to do, other than prove someone wrong and then get proved wrong by the next upstart next year. Or perhaps I'm the only one irk-able enough to be irked. Let me know.)

Kevin was studying a strange new breed of galaxy: the blue elliptical. In other words, an elliptical galaxy which was still undergoing plenty of star formation. Because these are rare, and studying a few probably wouldn't tell us the whole story of what was going on, Kevin was aiming to find as many as possible. He spent a week sorting through 50,000 galaxies, categorising them into spiral or elliptical. His office-mate, Tom Zlosnik, wrote in the Galaxy Zoo Forum that he was getting through about one per second! It must have been a trial, though, and he doesn't recommend it. "Your PhD student," Chris said in a lecture, "will classify 50,000 before telling you exactly what you can do with the other 850,000."

And so Galaxy Zoo was born. But that's definitely another story.

Many of our classifiers wrote to us to ask us, "I've been noticing that spirals are usually blue and ellipticals are usually red. Have you noticed that? Is it just me? Why is that?" I wonder if they knew what an exciting question they were asking? I didn't receive any of these myself, so I can't tell you what the team answered. The most important thing was to encourage people to classify by shape, not colour.

The blue ellipticals featured heavily on the forum, particularly in the autumn. "Here's a blue elliptical for Kevin!" we kept grinning, often posting dwarfs, irregulars and fuzzy spirals. In fact, the way they were really extracting the data wasn't using that thread but using the computer to check the colours, and extracting the ones that came out as "blue" from the computer and "elliptical" from us. (And that is why you shouldn't classify by colour, folks! If you'd followed convention you'd have clicked "spiral" and all these would have been missed!) Kevin nevertheless kept us up to date, and made us a special thread about his paper.

And that turned out to be only some of the story. Because then people started spotting the opposite: red spirals. Which I'll write about next time. One thing was getting clear - studying a few galaxies in depth won't tell you what's going on at all! Only by examining galaxies in vast quantities, on the scale of SDSS and Galaxy Zoo, can you get a whole picture. It's like studying a few animals for years until you think you know all about them - and then going and looking at the whole ecosystem. Suddenly everything we saw seemed to be introducing new exceptions.