Authors: Kitty Ferguson
In the existing three-dimensional maps, such as Geller and Huchra’s, the largest structures visible are about the size of the volume surveyed. To get a good statistical sample of these enormous structures and to find out if there are even larger structures, it is necessary to map larger volumes.
A British and Australian project called the 2DF Galaxy Red Shift Survey used the 3.9-metre telescope at the Anglo Australian Observatory on Siding Spring Mountain in Australia. By 2000 they hoped to have measured a quarter of a million galaxy red shifts. Already they were seeing indication of the walls and voids. James Gunn at Princeton was heading another new project, the Sloan Digital Sky Survey. Even in today’s science world, where teams are the norm, SDSS is a remarkable conglomerate. It includes astronomers at the University of Chicago, Princeton, Fermilab near Chicago, Johns Hopkins, the Institute for Advanced Study, the US Naval Observatory,
the
University of Washington and the Japan Participation Group.
The Sloan Telescope itself, at the Apache Point Observatory in the foothills of the Sacramento Mountains north of El Paso, Texas, is modest in size but, linked to an impressive array of technology, it is powerful enough to undertake the largest and most comprehensive census of the visible universe that has ever been attempted, a survey of a quarter of the northern sky. Part of the Sloan apparatus consists of CCDs or ‘charge-coupled devices’, silicon chips that convert light from the night sky into digitized images that can be poured on to magnetic tapes and into computers. The project began surveying the sky in the summer of 1998, after nine years of organization, design and construction, and over the next six years produced images in five colours of 50 million galaxies, 100,000 quasars, millions of individual stars in the Milky Way, and other assorted items that might turn up. ‘A field guide to the heavens’, Mike Turner (one of the Chicago contingent) calls it. More than a field guide. A three-dimensional scale model of a large part of the universe.
Even when these projects have succeeded in mapping the universe in three dimensions, the situation will still be not unlike that in the 17th century when astronomers had catalogued and charted what was ‘out there’ with greater and greater precision but were waiting for a Newton to come along and discover the dynamics that would reveal
why
things should have turned out as they have.
Not that the ingredients that will go into that understanding are entirely unfamiliar. Gravity and relativity are part of the explanation. Quantum theory – the theory of the very small (atoms, molecules and elementary particles) – is also certain to be an essential ingredient, because as large as the clusters and superclusters are today, in the early universe the material of which they are made was compressed in an area as small as anything in the quantum world. Seeds of the most colossal structures were probably sown by quantum fluctuations at a
time
less than a tiny fraction of a second after the Big Bang. The patterns of the ripples in the early universe, found in the cosmic microwave background radiation, would have depended on the way interactions occurred inside the fireball before it even reached the size of an apple.
Chaos and complexity theorists – who study randomness, the borders between what is random and what is predictable, and the patterns that emerge out of what seems to be chaos – also believe their theories have something to say about the dynamics that formed the large-scale structure of the universe. It would be difficult not to notice the resemblance between the universe on the largest known scales, the ‘sponge’ level, and the graphs and fractals produced in chaos theory.
Perhaps the best way to put together an overall mental picture of the structure of the Galaxy and the large-scale structure of the universe, as state-of-the-art astronomy describes them, is to take an imaginary tour. At the start, travellers should remind themselves that light travels at a rate of approximately 186,000 miles or 300,000 kilometres per second. It takes light 1.3 seconds to travel from the Moon to the Earth, 8.3 minutes to travel from the Sun to the Earth, four hours to travel from the Sun to Pluto, and about 4.3
years
to travel from the Sun to the nearest star. Four hours to 4.3 years is an enormous leap, but within the range that light from the Sun can reach within 17 years, there are only about 50 stars.
Whether the Galaxy is large or small is, nevertheless, a relative matter. Taking into account the entire universe would mean not only considering planets and solar systems, but things far smaller yet – microbes, molecules, atoms, quarks, neutrinos. Next to those,
you and I
are by no means small, and compared to us the Galaxy is huge beyond comprehension. But among galaxies, it is mid-sized. Compared with the largest scale of structure known in the universe, it is vanishingly tiny.
Ignoring the fact that no human has ever viewed the Milky
Way
Galaxy from a distance, imagine that one portion of the tour does start outside the Galaxy, facing the great spiral, then approaches it like a moth attempting to fly through the blades of a fan. Our itinerary takes us through the disc, not the central bulge. First we pass through a ‘layer’ several thousand light years thick, probably consisting of extremely hot gas and faint elderly stars. This region has been difficult to study and, so far, has not been well explored. There is some uncertainty about what is there.
Next we come to a region called the main disc, about 2,000 light years in thickness. Here are many of the Galaxy’s stars, and if we choose the right trajectory through the main disc, we’ll pass through our own solar system. If our route doesn’t take us through one of the ‘arms’ of the spiral but instead goes through what look like much more sparsely populated areas in between, we’ll be surprised to find not as much empty space as we might have expected. It won’t be like flying through a fan and missing the blades, because the ‘arms’ are nothing so solid and distinct as blades of a fan. The number of stars per cubic light year is not much greater in a spiral arm than in the areas between the arms. However, passing through an arm we would be more likely to encounter extremely brilliant massive, short-lived stars and glowing nebulae lit by the light of the young stars.
Continue travelling and we’ll come to the thinnest layer of the Galaxy. If the Galaxy were a chocolate-mint wafer, the main disc would be the chocolate and the inner disc we’re now entering would be the mint cream layer. This is a disc of gas and dust, only 500 light years thick, and it is the Galaxy’s nursery. It houses the youngest stars and is the birthplace of new stars. The resemblance to the chocolate-mint wafer is not exact. This thin disc of gas doesn’t end out where the main disc layers end (as the filling of a chocolate-mint wafer does) but instead stretches out beyond that to a distance more than a third again as far from the centre of the Galaxy. At its extremes the gas disc bends like the rim of a hat. At one side of the Galaxy the edge of the hat
brim
curves upwards. On the other side of the Galaxy it curves downwards and then, further out, back upwards.
Continuing our journey out the other side of the Galaxy disc we find the same layers in reverse. Compared to its diameter along its plane (looking at it edge-on), the Galaxy is very thin indeed. A chocolate-mint wafer is too thick to be an accurate comparison, not large enough in diameter in relation to its thickness. The Galaxy has more the proportions of a gramophone record.
One thing astronomers now know – to follow up on an earlier attempt to map the Galaxy – is that Harlow Shapley was right to conclude that globular clusters outline the extent of the Galactic halo, though there are also some of them well beyond it. The Omega Centauri cluster, a globular cluster that Ptolemy catalogued (though not as a cluster), that Halley recognized as a cluster, and about which Herschel spoke in superlatives – ‘richest . . . largest . . . truly astonishing . . . whose stars are literally innumerable’ – does indeed turn out to be extraordinary. It is the brightest, largest and most massive cluster in the Galaxy, with tens of millions of stars. There are some 100 billion stars in the whole Milky Way Galaxy, many of them congregated in the central bulge, the bright yolk of the fried egg. If our journey had taken us directly through the bulge, we might have seen, or even ended up in, a massive black hole that is suspected to lurk at the very heart of all that brightness.
The Milky Way, which is now behind us as we move further afield, is only one among many billions of galaxies, not all of which are the same size and shape. They range from about 10 million to 10 trillion times the mass of the Sun, but that includes the extremes. A typical galaxy (the Milky Way is one of them) is midsize. We first pass among the other members of the ‘Local Group’ of galaxies. This group measures about five million light years across and is rather flattened in shape.
The designation, ‘group’, is part of the terminology astronomers use to describe the structure hierarchy of the universe,
though
this ‘hierarchy’ is not rigid, and thinking of the universe this way ignores the rich diversity of its structure. There are, moving to larger and larger scales, first galaxies, then groups, clusters, clouds, superclusters and supercluster complexes or ‘walls’.
‘Groups’ typically include three to six conspicuous galaxies and a number of smaller, dimmer ones. The Local Group is no exception. The Andromeda galaxy is its dominant spiral, with an estimated 400 billion stars. The Milky Way ranks second, and there is a smaller but still impressive spiral called M33. These three giants are all that an observer in, for instance, the Virgo Cluster would be able to see of our Local Group, but lesser galaxies in the group outnumber them ten to one, and there are probably others not yet observed because they’re hidden from Earth by dust clouds within the Milky Way. Andromeda has two small companions, M32 and NGC205, both elliptical galaxies. The Milky Way holds court with a retinue made up of its two best-known companions, the Magellanic Clouds, which are irregular galaxies, and three other satellite galaxies. Two of these, the Carina galaxy and the Sextans galaxy, are dwarf galaxies that are spherical in shape. The Sextans dwarf, discovered in 1990, is a little further away than the Magellanic Clouds and the total luminosity of its stars combined is only about 100,000 times the luminosity of the Sun – less than some single stars in the Milky Way. A more recently discovered satellite galaxy, the Sagittarius galaxy, is the closest neighbouring galaxy found so far. First recognized in 1993, it’s a dwarf that looks likely to be cannibalized by the Milky Way. It has already lost some of its outer stars to the Galaxy’s gravitational pull.
The distance to the Andromeda galaxy is fairly well established at some two and a quarter million light years, but there is still dispute about the distance to M33, the third largest Local Group spiral. When Edwin Hubble first measured it in the 1920s, he estimated that M33 was about as far away as the Andromeda galaxy. Allan Sandage, however, has reinterpreted
Hubble’s
data on the Cepheids used for that measurement, employing more modern techniques, and has concluded that M33 is more like three million light years from us, well beyond the Andromeda galaxy. Astronomers who have studied the same Cepheids at infrared wavelengths disagree with Sandage. They estimate that the two galaxies, though further away than Hubble estimated, are, as he thought, both about the same distance from us.
Groups like our Local Group have no particular structure or shape; in fact, they are also known as ‘irregular clusters’ and their galaxies are a hodgepodge of all types. However, that doesn’t mean their existence is a random occurrence, with all these galaxies just happening to be passing close to one another on their way to somewhere else. All the galaxies in the Local Group are bound together by mutual gravitational attraction, and all are orbiting a common centre of gravity. Andromeda and the Milky Way are, at the moment, approaching each other at a speed of 300 kilometres per second. Slipher’s discovery of the blue shift in Andromeda’s light was not an error. There could eventually be a head-on collision, but that is not something to write about on the walls of the underground yet. The event is still a few billion years in the future, and the merger of the two galaxies will take another several billion years after that to complete. In the end there will probably be, instead of two spiral galaxies, one huge elliptical galaxy. Then again, Andromeda and the Milky Way may only circle one another in a polite do-si-do and then move apart again. Which will it be? In 2005, NASA plans to launch the Space Interferometry Mission, a spacecraft carrying an array of telescopes capable of determining, among other things, the exact angle of Andromeda’s approach.
Obviously, relationships among galaxies are not always placid. Andromeda seems to have stripped M32 of a good number of its stars, while M32 has in turn caused distortion in the spiral structure of Andromeda. NGC205 is twisted by the
pull
of Andromeda. Most astronomers also believe that the Small Magellanic Cloud is being torn apart, probably by the gravity of the Milky Way Galaxy. There is a long, narrow ribbon of hydrogen gas, streaming half of the way around the sky and seeming to begin in the large pool of hydrogen that surrounds the two Magellanic Clouds. This may be gas from the ripped Small Cloud, left behind along its orbit around the Milky Way.
We would have to travel several million light years outside the Local Group to get to the nearest galaxies beyond it. But at this point we can learn more about the large-scale structure of the universe not by travelling further away from Earth, but by moving to larger and larger scales. Bear in mind the ‘hierarchy’ of the large-scale structure, but also be aware that the universe is not nested as neatly as Russian dolls, with galaxies within groups within clouds, within clusters, within superclusters, within supercluster complexes. There is far too much complication out there for such a simplistic picture to be anything but a distortion.
The Local Group is not far outside the border of the Coma-Sculptor cloud, a large cloud which in turn lies near the outer limits of the Virgo supercluster. The Virgo cluster, which is huge compared with the Local Group, is the giant heart of the Virgo supercluster and about 60 million light years from Earth. It is an enormous swarm of thousands of galaxies and a lot of hot gas – a ‘regular cluster’ because it doesn’t seem to have such a mix of galaxy types as are to be found in motley assortments like our Local Group. Instead, it contains more than 1,000 prominent galaxies centred on a pair of giant elliptical galaxies, with probably many more less prominent galaxies that astronomers haven’t yet detected, but very few spirals.