The Crowd and the Cosmos: Adventures in the Zooniverse

by Lintott, Chris

  evolutionary one, rather like the familiar lineup that leads from

  crouching monkey to upright human. There was some support

  for this at the time Hubble was working, and it seems reasonable

  to believe that galaxies which started off as featureless balls of

  stars would slowly collapse under their own gravity to form

  discs. Within those discs would be spiral arms, the inevitable

  result of disrupting a rotating system, and these might well then

  unwind over time, completing the journey from one end of the

  tuning fork to the other. It’s an attractive picture, but one that

  turned out, sadly, to be complete nonsense. Galaxies don’t behave

  like that. Nonetheless, Hubble’s classification still tells us about

  the shape of galaxies, and it’s still used nearly a century later.

  The idea that we’re talking about classifying galaxies might

  seem archaic in and of itself. The discoverer of the atomic nucleus

  and pioneering popularizer of science, Ernest Rutherford, dis-

  missed such work as mere stamp collecting (physics alone, and

  his kind of physics at that, being exempt from being stamped

  with his philatelic scorn). But sorting things into categories often

  marks the first attempt to understand something scientifically,

  and it can be important even within the inner sanctum of phys-

  ics, or, in this case, astrophysics.

  Those taking a Rutherfordian view might expect classifica-

  tions based on mere observation to cease to be useful as we start

  52 The Crowd and The Cosmos

  to really understand the physics of galaxies. Simple labelling of

  what you see is fine to begin with, but Proper Science will pro-

  ceed differently, and we should expect applying labels as straight-

  forward as a shape to be left as a curiosity in scientific history.

  Just such an upheaval might be seen to be taking place right now

  in much of biology, for example, as genomic analysis rearranges

  our ideas about the relationships between species. A deeper and

  more useful classification of the tree of life can be found by look-

  ing at where the action really is—in the DNA—rather than by

  carefully observing anatomical features. But for a long time, all

  biologists had to work with was the power of observation (and

  plenty would still argue for traditional, rather than genetic work

  as a means of making progress).

  We might imagine that astronomers, too, will find some fun-

  damental measurements that record a galaxy’s history and

  explain its behaviour today. If such fundamental parameters do

  exist, though, we’ve yet to find them and thus, lacking any celes-

  tial equivalent to genetics, astronomers have little choice, more

  than eighty years after Hubble, than to carry on dutifully sorting

  galaxies into categories based on an antiquated tuning fork.

  The difference between elliptical and spiral galaxies isn’t a

  temporary one. Left in isolation, a typical spiral will have enough

  fuel, primarily hydrogen, to go on forming stars for billions of years.

  As mentioned earlier, recent star formation also means that most

  spiral galaxies will be blue and elliptical galaxies red; the most

  massive stars are blue but they are also short lived. This is some-

  what counter-intuitive, as you might expect the most massive

  stars to have more fuel for nuclear fusion on hand and thus to

  last longer. In fact, the rate at which nuclear fusion proceeds is

  exquisitely sensitive to temperature, and that in turn depends on

  the pressure exerted at the core by a star’s mass. More massive

  stars have much hotter cores, and so burn through their fuel

  The Crowd and The Cosmos 53

  much faster, and the brightest and bluest of them may live for

  only a few hundred million years. Spiral galaxies, and in particu-

  lar the spiral arms where most star formation takes place, are

  thus speckled with brilliant blue stars, while ellipticals are, for

  the most part, red.

  Distinguished by their properties, the two sets of galaxies are

  also separated by their environments. The two types of galaxy

  often live in separate places. Take the neighbourhood we happen

  to find ourselves in. The Milky Way and the Andromeda Galaxy

  are two of the three large galaxies in our Local Group. The Milky

  Way is a spiral, and Andromeda is also a disc rather than an ellip-

  tical. Its exact morphology is somewhat ambiguous, a situation

  not helped by its nearly edge-on presentation as seen from Earth,

  with the most prominent feature being a ring of recent star for-

  mation. But if we broaden our definitions to distinguish disc gal-

  axies, whether or not they have spiral arms, from ellipticals then

  the two comfortably sit within the same category.

  This reflects the fact that the shapes we see are essentially a

  result of the dynamics of the material within the galaxy. Disc gal-

  axies like the Milky Way are composed of material which orbits

  the centre in an organized fashion. The Sun, for example, travel-

  ling at a little over half a million miles per hour, completes one

  orbit around the centre of the Milky Way in just over 225 million

  years, a length of time that is sometimes referred to as a ‘galactic

  year’, and most of its neighbours will do the same. The disc of the

  galaxy is relatively thin. As Monty Python’s ‘Galaxy Song’ has it,

  the Milky Way measures 100,000 light years side to side, but is,

  out by us, only a few thousand light years wide. Think of the gal-

  axy as a large fried egg—ten centimetres across and a few milli-

  metres thick—and you won’t be far wrong. By contrast, individual

  stars in an elliptical galaxy are each travelling on their own orbit

  around the centre of the galaxy, but they are all inclined at

  54 The Crowd and The Cosmos

  different angles, producing the football or rugby ball shape we

  see. There is no organized set of orbits as there is in a disc, and

  this reflects the real physical difference between the two sets of

  galaxies.*

  M33, the third and final large member of our Local Group of

  galaxies, is also a spiral, and a beautiful one at that. Its multiple spiral arms are in danger of losing their unique identity, so dominated are they by the presence of young star clusters. This sort of

  arrangement is sometimes known as a ‘flocculent’ spiral, its

  appearance somewhat reminiscent of a scattering of tufts of

  wool. That detail aside, our local neighbourhood’s large galaxy

  population shows signs of segregation: three large galaxies, all of

  them spirals.

  Should we be surprised? The key is in understanding where we

  live. If you rank the environments in which galaxies are found,

  from the most densely populated regions to those which are

  emptiest, you find that we live in the cosmic suburbs. The Milky

  Way is not a hermit, in splendid isolation, but nor is our patch of

  space especially crowded. Looking around, this sort of environ-

  ment is typically dominated by spiral systems, which seem to

  thrive today in less dense environments. Ellipticals dominate the

  Universe
’s cities, mostly hanging out in vast clusters and super-

  clusters of galaxies.

  This talk of environments for galaxies would have surprised

  the astronomers of just a few decades ago. One of the most

  * I should probably point out that the use of these terms in the astronomical literature is often confusing, with things being made much worse by the presence of ‘S0’ galaxies that look like ellipticals but which have a disc hidden in them. People often use the terms ‘early-type’ for ellipticals and ‘late-type’ for spirals, the names deriving from the old mistaken idea that ellipticals eventually turn into discs. I’ll just use elliptical and spiral here, but really I’m trying to divide disordered systems from those galaxies, typically spiral, where a regular disc is the most prominent feature.

  The Crowd and The Cosmos 55

  profound and interesting discoveries of the last few years has

  been that there is structure in the Universe even on the largest

  scales that we can map. Take the million galaxies of the Sloan

  Digital Sky Survey, for example. The survey was critically able to

  make a three-dimensional map of the Universe, recording not

  only the position of galaxies in the sky but also their distance

  from Earth. Cepheids aren’t visible at these distances, but we can

  make use of another yardstick—the expansion of the Universe

  itself.

  I already mentioned that Hubble and others provided evidence

  that the Universe is expanding, and if you work backwards that

  leads you to the idea that everything began in a hot dense state

  just after something called the Big Bang which occurred 13.8 bil-

  lion years ago (give or take a hundred million years or so; the

  accuracy with which the age of the Universe can be determined

  still astounds me). By expansion we’re not talking about the

  movement of galaxies through space, but as the expansion of

  space itself; think not of actors rushing away from each other,

  but of a stage which itself expands, widening the space between

  everyone standing on it. That expansion of space, which pro-

  ceeds today at a rate such that every thirty billion billion kilome-

  tres’ worth of space grows by seventy kilometres a second, has

  an effect on the light travelling through it.*

  The expansion stretches the light to longer wavelengths,

  which correspond to redder colours. More distant galaxies cap-

  tured by Sloan, therefore, look red when compared to their

  local compatriots, purely because of this redshift. Their spectral

  * In more sensible if less comprehensible units, astronomers would write this as 70 kilometres per second per megaparsec. A megaparsec is 3.26 million light years, or just over thirty billion billion kilometres. In whatever units, this value is known as Hubble’s constant, though it isn’t constant, but rather something that will change through time as the contents of the Universe act on the expansion.

  56 The Crowd and The Cosmos

  pattern—what you see when you split the light up into its com-

  ponent wavelengths—shifts too. Elements emit or absorb light

  at particular wavelengths, and we can create a list of those wave-

  lengths by experiment in the laboratory. We still see the same

  pattern of lines corresponding to elements such as oxygen in the

  galaxy spectra, but shifted, and so we can compare the measured

  galaxy spectrum to the standard one and measure the redshift

  that way.

  This measurement contains information about how the

  Universe has expanded during the millions of years light from a

  galaxy has been travelling towards us. (When we want to talk

  about the intrinsic colour of a system, as when considering star

  formation, we usually have to adjust for this effect.)

  Conversely, if you understand the expansion then the redshift

  becomes a measure of distance to a particular galaxy. The Sloan

  survey’s efforts thus included taking a spectrum for each of

  nearly a million galaxies, a task made more complicated by the

  fact that only the best nights, when the air is stillest, could be

  used for this delicate work. Sloan did, however, have an advan-

  tage over previous surveys, in being designed to take hundreds of

  spectra at once. For each patch of sky it might observe, a metal

  plate was prepared, drilled with holes corresponding to the posi-

  tion of each galaxy. Into each of these holes, a fibre optic cable

  was plugged, carrying the light from a distant galaxy on the last

  few metres of its journey to the instrument that would analyse it.

  Compared to the old method of pointing the telescope at each

  galaxy in turn, this provided for great efficiency, but at the cost of having to complete the laborious task of plugging fibres into the

  holes in the plate. A rival survey, called 2dF, spent great time and

  effort producing a fibre-handling robot to do the job. It’s a better

  long-term solution, but Sloan just relied on the efforts of junior

  The Crowd and The Cosmos 57

  astronomers, whose labour proved an easy if unsatisfying solu-

  tion to the problem night after night and plate after plate.

  The results of all of that effort were spectacular. Sloan and

  similar surveys revealed with clarity what previous data sets had

  only led astronomers to suspect. The Universe around us is a

  honeycomb, a cosmic web of clusters and of filaments which

  wind their way around enormous empty voids. Each of these

  superstructures is made up of thousands or hundreds of thou-

  sands of galaxies; the largest of them, the Sloan Great Wall, is

  more than a billion light years across.* The fact that there are dif-

  ferences even on these enormous scales has consequences.

  Jumping millions of light years in any direction from the Milky

  Way could place you in a very different place, surrounded by dif-

  ferent types of galaxies or even, if you end up in a void, with very

  little company at all.

  In the densest parts of these superstructures, a spiral galaxy

  like our own would look rather out of place. Clusters and super-

  clusters of galaxies are the realm of enormous galaxies which are

  almost uniformly elliptical. Even smaller examples like the Virgo

  Cluster, at 54 million light years away our nearest example, have

  plenty of ellipticals among their population. The densest part of

  that cluster, centred on a galaxy called M87 which itself weighs in

  at 200 times the mass of the Milky Way, is almost entirely popu-

  lated only by ellipticals, with spirals mostly relegated to a few

  surrounding groups of galaxies which might still be in the pro-

  cess of being absorbed by the main cluster.

  * Studying such a vast structure is rather difficult, and arguments about the Sloan Great Wall still rage. There’s some evidence that it isn’t one, but rather three different structures superimposed onto each other. The problem is not merely of cartographic interest; understanding the odds of such a large structure forming provides neat constraints on cosmological theories.

  58 The Crowd and The Cosmos

  Given these facts, galaxy classification may seem to be a simple

  matter. Ellipticals are red, and live in great galactic cities. Spirals are blue, and live in the sticks. Anyone wanting to select one at

  the expense of the other could just sort by colour. Pick a handfulr />
  of red galaxies, and they’re likely to be ellipticals. Stick to blue

  ones, and you’ll probably come up with a fistful of spirals.

  Alternatively, we could take a bunch of elliptical galaxies and

  expect them to be red, or collect spirals and then look and find

  they were almost all blue.

  This close connection between colour and shape makes it

  sound like visitors to the Galaxy Zoo website were wasting their

  time, but things were much more interesting than that. I’d first

  had an inkling that things might not be simple when, nearly a

  year to the day before the launch of the site, I’d gone up to Oxford

  for a job interview. I was coming to the end of my PhD (and more

  practically, to the end of the grant I’d been awarded to complete

  it which was paying the bills). I was nervous, and intimidated by

  the place and the department, and the most critical part of the

  day consisted of giving a seminar on my work to a busy crowd.

  I’d rather more of them had been distracted by whatever was on

  their laptops than listening to me blather on, trying to demon-

  strate my deep and abiding desire to work on extragalactic astro-

  physics by reviewing work I’d just completed.

  I’d been working on updating a classic, simple model of how

  the details of the expansion of the Universe affects the galaxies

  that form within it. Less than a decade earlier, careful measure-

  ments had revealed to astronomers that the expansion of the

  Universe is not slowing down under the influence of gravity, as

  might have been expected, but rather that it was speeding up. We

  The Crowd and The Cosmos 59

  still don’t know why this is happening,* but it has profound con-

  sequences for every aspect of our understanding of how the

  Universe evolves.

  The point of my work was to reconsider the formation of ellip-

  ticals. This sort of work, more connected to theory than my nor-

  mal stuff, is fun, but not my natural territory—hence being

  worried about the talk. As it turned out, I’d barely started talking, and was still feeling exposed, nervous, and uncomfortable, when

  I was interrupted, loudly and insistently, from the other end of

  the room.

  I’d just been going through the motions of explaining where

  my data came from and how I’d selected a sample of reliably