The Crowd and the Cosmos: Adventures in the Zooniverse Page 7
The Crowd and The Cosmos 41
Figure 7 The original Galaxy Zoo website, complete with Sloan Digital Sky Survey galaxy ready for classification.
At least on that particular day I had a good reason to be dis-
tracted. A few hours earlier we’d launched a website called Galaxy
Zoo (Figure 7), which asked anyone wandering past to help us
sort through the mountain of data that the Sloan Digital Sky
Survey had produced.* Having eventually found a connection, I
was slightly perturbed that the site wouldn’t load. Stranger still,
the email address we’d set up (asking people to get in touch if
they found anything particularly interesting) was acquiring mes-
sages faster than the computer could download them.
The culprit was the BBC News website, where the story
announcing our plea for help hovered among the top five most
shared articles. We were above a story with the headline ‘Garlic
“may cut cow flatulence” ’, which was gratifying for all sorts of
reasons, but behind ‘Man flies to wedding a year early’. Later in
* The original version of Galaxy Zoo is available at zoo1.galaxyzoo.org and the current version at www.galaxyzoo.org.
42 The Crowd and The Cosmos
the day, we slipped further, as ‘Huge dog is reluctant media star’
took over at the top, but despite this surge in interest in Samson
(at 6’5” Britain’s tallest dog but not one for the limelight) the fact that this many people were taking time to check out what was
supposed to be an interesting side project was clearly remarkable.
Galaxy Zoo asked people to sort galaxies by shape, a request
steeped in nearly a century of astronomical tradition. The galax-
ies that get all the press, the ones that show up at the start of any science fiction film with a decent budget for special effects, and
the ones whose images grace posters, are spiral systems, just like
our own Milky Way. We even call these systems ‘grand design’
spirals, a nod in nomenclature to their spectacular appearance.
These celestial Catherine wheels are the Universe’s dynamic
places, ever-changing discs lit up by the bright blue glow of
young massive stars. These stars, the most massive, most lumi-
nous, and hottest in existence, burn through their fuel at a much
faster rate than relatively puny objects like the Sun. Their youth-
ful presence therefore suggests a galaxy which is still capable of
star formation, and they are predominantly found in spiral sys-
tems (Figure 8).
A sprinkling of bright stars can mislead. All that’s important
does not glitter, and to concentrate only on spiral galaxies is to
miss the real action. The true heavyweights of the Universe are
giant balls of stars which often lurk in the centres of clusters of
galaxies. These are the ellipticals. Not much to look at, the repu-
tation of these systems is best summed up as ‘old, red, and dead’.
In other words, a typical elliptical galaxy is past its prime, devoid of the gas that is the fuel for star formation and missing as a result the young blue stars that give spirals their vim and vigour
(Figure 9). These differences show that a galaxy’s shape must
mean something. Pick at random two galaxies, an elliptical and a
spiral, and it’s a safe bet that the elliptical will be more massive,
The Crowd and The Cosmos 43
Figure 8 NGC3338 as seen by the Sloan Digital Sky Survey. This ‘grand design’ spiral has arms which are filled with clusters of young, blue stars.
have less gas and fewer young stars, and live in a more crowded
environment.
In fact, if you are only allowed to know one thing about a gal-
axy then go for its shape. Its shape—we’d normally say ‘morph-
ology’ to sound more scientific—contains a history of what’s
happened to the galaxy over its billions of years of existence, a
record of how it has interacted with its surroundings and how it
has grown over the years. The division between ellipticals and
spirals is really a split between galaxies with different stories to
tell, and is as fundamental to astronomers as the realization that
humans are broadly split into male and female would be to a
researcher studying the health of a human population.
44 The Crowd and The Cosmos
Figure 9 NGC1129 as seen by the Sloan Digital Sky Survey. This elliptical galaxy lives in a densely populated region, and lacks recent star formation.
This is hardly news. The systematic study of the shapes of
galaxies dates back to the most prominent and praised observer
of the telescopic age, Edwin Hubble. The man for whom the
space telescope is named was originally a scientific outsider, a
disappointment to parents who had expected him to go into the
family business of law. He got as far as a Rhodes Scholarship to
Oxford, but it didn’t take him long to realize that the life of a lawyer wasn’t for him. After a rather brief stint as a school teacher, he decided at the age of twenty-five that it was time to turn towards
his real interest—astronomy.
There’s a quotation often attributed to Hubble from this time
that has him grandly declaring that he’d rather be a third-rate
astronomer than a first-rate lawyer. As it turned out, he did
rather better than that. His first astronomical home was Yerkes
Observatory in Wisconsin, a strange place to build a facility
dependent on clear skies given its climate, but conveniently close
to the University of Chicago and at that time one of the world’s
The Crowd and The Cosmos 45
pre-eminent research facilities. Hubble’s PhD dissertation, based
on his work at Yerkes and published in 1917, laid the foundations
which would support his work for the next twenty years and
more. It was a detailed study of what were then called nebulae, a
word derived from the Greek for ‘cloud’ and normally used at the
time for a hodgepodge of objects. Star-forming regions like the
Orion Nebula I pointed my telescope at as a kid and distant gal-
axies were both ‘nebulae’, however different they now seem to us.
For a couple of centuries, observers had added to our cata-
logues of faint and fuzzy things, but Hubble’s contribution went
beyond merely finding more of such objects. A telescope isn’t a
complicated machine, not much more than a bucket for light
that obeys the basic rules of optics. One of these rules says that
how sharp the images an instrument produces will be (its ‘reso-
lution’) depends on the size of the mirror or lens being used (and
on the wavelength of the light, but that’s another story). A larger
telescope will, the blurriness and twinkling imposed by the
Earth’s atmosphere notwithstanding, always produce a sharper
image. Despite the Wisconsin weather, Yerkes gave Hubble
access to really big telescopes for the first time, and his newly
sharpened vision made it clearer than it had ever been that the
blanket category of ‘nebula’ concealed remarkable diversity.
It was a great time to be a young and enthusiastic observer.
New facilities were springing up, and after completing a brief
stint in the army Hubble found himself in California with acc
ess
to what was then the largest telescope in the world. This magnifi-
cent beast, now known as the 100-inch Hooker telescope, sits
atop Mount Wilson looking down on the sprawling city of Los
Angeles. The geography of the region conspires to create an
inversion layer, with cold air trapped underneath warmer air,
which is these days best known for trapping exhaust and producing
the smog that blankets that most car-worshipping of cities. If you
46 The Crowd and The Cosmos
Figure 10 Edwin Hubble—smartly dressed—observing at the Mount
Wilson 100-inch telescope.
can get above the inversion, though, you are rewarded with crys-
tal clear skies, and Mount Wilson is one of the sites with the best
seeing in the continental United States (Figure 10).*
It’s easy, I think, to imagine Hubble’s excitement on arriving in
this astronomers’ paradise, leaving frigid Wisconsin behind.
Whatever his state of mind, he quickly got to work, publishing a
* ‘Seeing’ is the term astronomers use to talk about the stillness of the air and hence the steadiness of the view provided. There are all sorts of technical ways to measure it, and a few non-technical ones too. For example, at Kitt Peak in Arizona, a count of circling vultures in the late afternoon is a reliable guide to how good a night it is going to be. As a rough guide, the deeper the blue colour of a daytime sky the better the seeing will be; think about the difference between the sky on a hazy summer’s day and the deep, crisp blue of a sunny day in winter.
The Crowd and The Cosmos 47
paper emphasizing the distinction between those nebulae which
merely reflect the light of stars embedded within them, like
Orion, and those which emit their own light. It seems obvious to
us now that these latter objects consist of stars, but even tele-
scopes as powerful as those at Mount Wilson refused to reveal
individual stars. The obvious explanation is that these systems
are far away, but then these nebulae appear bright enough that
they must incorporate an almost inconceivable number of stars.
This simple chain of logic results in the discovery (or, if you’re on the other side of the argument, the extravagant invention) of a
Universe of scattered galaxies, each faint smudge of light swim-
ming into view as important in its own right as what has until
this moment constituted the entire Universe.
Such grandeur requires equally extravagant standards of evi-
dence, and it’s not a bad rule of thumb that any theory which
requires a massive reimagining of our place in space is likely
wrong. The required clinching evidence that external galaxies
really existed soon arrived, thanks to a systematic study of galaxy
distances. Measuring the distance to something as far away as a
galaxy is not easy, but just as Hubble was beginning his study of
the nebulae astronomers had hit upon a useful method which
made use of a particular type of variable star—Cepheids.
Cepheid stars swell and then shrink in a regular pattern, and as
they pulse they also brighten and fade. That much has been
known since the late eighteenth century. They are also relatively
luminous, allowing them to be detected in distant galaxies, and
catalogues comprising hundreds of the things were quickly
assembled. One of the largest was put together by Harvard
astronomer Henrietta Swan Leavitt, hired at the college observa-
tory as a ‘computer’, back when that was a job title and not some-
thing that sits on your desk. Leavitt’s task was to measure the
brightness of stars that appeared on photographic plates obtained
48 The Crowd and The Cosmos
by Harvard’s telescopes, and she spent particular time on the
stellar population of the Magellanic Clouds. These two clouds are
satellite galaxies of the Milky Way, in orbit around (and probably
being consumed by) our own system, but for Leavitt’s purposes
they were useful because stars which belonged to the clouds were
far enough away that for all practical purposes they could be
assumed to all be at the same distance from Earth. So a Magellanic
star that appears brighter than another really is more lumi-
nous—we don’t need to measure its distance, something that
causes a lot of headaches when dealing with stars in our own
Galaxy. As a result, studying the Magellanic Clouds’ stars is key
to working out how the Universe is assembled.
Leavitt’s catalogues included more than 1,500 Magellanic
variable stars, twenty-five of which turned out to show the char-
acteristic Cepheid pattern. They revealed the Cepheids’ most
important property, an obvious relation between their bright-
ness and how fast they were pulsing. The brighter the star is, the
slower the pulse that powers its changes in brightness. This
makes some sort of sense, I suppose, as we know that the bright-
ness of a star is partly due to its mass, and it’s not hard to imagine ways in which the mass of a star could affect how it would pulse,
but it’s the use to which this new knowledge could be put that
makes it really important. Once the relationship between the
brightness and the period of Cepheids is understood, then all you
need to do to measure the distance to a galaxy is to find a Cepheid
within it. Record the period (the time for the star to complete
one cycle of brightening and fading) and you can deduce the
distance—a remarkably elegant technique for measuring dis-
tances which is as much a part of cosmology today as it was in
Hubble’s day.
Indeed, a large part of the reason that the Hubble Space Telescope is named after Edwin is that one of the high-priority tasks set for
The Crowd and The Cosmos 49
it was to find distant Cepheids, expanding the volume of space
throughout which we have solid, stellar distance measurements.
The experiment it carried out was precisely that upon which
Hubble’s contemporaries were embarked, and which provided
incontrovertible evidence that separate galaxies exist.
And that’s not all. Hubble used observations from the new
Californian telescopes to show that these galaxies appear to be,
almost without exception, moving away from us. The few excep-
tions that exist are all local. I’ve already mentioned our Milky
Way’s cannibalism of the Magellanic Clouds, and the nearest
large system, the Andromeda Galaxy, also seems likely to be on a
collision course with our own system. In our local neighbour-
hood, the gravitational pull between nearby systems such as the
Milky Way and Andromeda is more important than and can
overcome the expansion of the Universe, but on larger scales
nothing resists the Universal expansion. What’s more, thanks to
distances obtained from observations of Cepheids, Hubble
showed that the further away a galaxy is, the faster it is receding
from us. This observation, now often known as ‘Hubble’s law’,*
above all else provides solid evidence of what we would today
call the Big Bang. It is Hubble’s enduring legacy, although an
entertaining debate is underway to decide long after the fact
exactly how much of the
credit he deserves.
Others had published data sets of similar quality to Hubble, but
it does seem to have been his work that captured the imagination,
diverting the flow of the debate that was raging in the 1920s and
* As I was editing the book, the International Astronomical Union (IAU) formally recommended that it be known as the Hubble–Lemaître law, to recognize the contributions of Belgian astronomer George Lemaître, who predicted the effect before it was observed by Hubble and others. I am slightly mystified why the IAU decided to busy itself with such a matter, but there was a vote and everything, with 3,167 astronomers in favour and 893 against. You can call it what you like.
50 The Crowd and The Cosmos
1930s over the structure of the Universe. Yet Hubble himself
didn’t necessarily believe in anything like a modern Big Bang,
and, leaving the hard work of building the foundations of the
new cosmology to others, turned from using galaxies as particles
tracing the behaviour of the space in which they sit to consider-
ing them as objects of study in their own right. What he came up
with, which can still be found scattered through the pages of
today’s textbooks, was a tuning fork (Figure 11).
The tuning fork was a way to organize and think about the
diversity of galaxies that Hubble observed in the Universe. Along
the handle he placed the elliptical galaxies, arranging these
otherwise featureless galaxies by their shape. Starting with round
galaxies, he worked his way along to those which look like rugby
balls, and then to those almost cigar-like in structure. Along the
Figure 11 A modern version of Hubble’s tuning fork diagram, still used as the basis of galaxy classification today.
The Crowd and The Cosmos 51
tines of the fork come the spiral galaxies, arranged in order from
those with the most tightly wound arms to those where the arms
are much looser. One branch was for spirals with a distinct
straight ‘bar’ at their centre—so-called ‘barred spirals’—and the
other for those without. A few scrappy little irregulars like the
smaller of the two Magellanic Clouds apart, such a scheme could
account for the whole diversity of the galactic zoo.
What could account for the various shapes? Having sorted
them into a nice sequence, it’s tempting to see the diagram as an