I seem to have spent my life mapping the sky. Jemima Public might find this strange. As a scientist paid by her taxes, surely I should be doing something useful, like curing cancer or teaching her son how to integrate ? As a human being, surely there are more enjoyable or profound things – reading King Lear, or throwing darts at a picture of Donald Rumsfeld ? Even some of my professional colleagues are a bit snooty about it – sounds a bit of a dull slog, Lawrence … shouldn’t you get back to quasar variability studies ? Well, my colleagues, my fellow humans, I find that I must disagree.
Maps of the sky are of considerable cultural significance; they have been hugely productive scientifically; and they have been the engine of discovery, revealing an unknown universe step by step. Lets take each of these in turn, starting with the human impact of exploring the cosmos.
So.. the Cosmic Explorer boards his ship and sails forth into the dark unknown, observing, sketching, and writing as he goes, a modest supply of the King’s gold jingling in his pockets. He does this for the Advancement of Learning, but when he returns from the tropical glare to the grey skies of home, weary and bearded, the people want to hear his story. Draw us a map, they say, with the coastlines, cities and mountains you have found, that we may know the true extent and form of our world. Tell us of the other nations and peoples, how rich, how powerful they are, that we may know our place. Tell us of the wonders that you found – what strange beasts, what customs – that we may appreciate the true variety and beauty of the world.
Where do I live ?
Schoolboy answer : 33 Ironside Villas, Edinburgh, Midlothian, Scotland, UK, Europe, Earth, Solar System, Western Arm, Milky Way, Local Group, Virgo Supercluster, Universe..
Turn left just after Barnard’s star, you can’t miss it. Yes its silly and its whimsical; but it springs from a true need to know.
There is a long tradition of trying to produce a map of everything. This is now very hard in astronomy as the universe is soooo big. One needs to show a series of expanding views : the solar system, the solar neighbourhood of stars, the Milky Way, the Local Group, the Virgo Supercluster… There are some interesting web sites trying to do this. Probably the best is the Atlas of the Universe. This has lots of other good links. Another nice one is Tour the Universe, part of the online presence of the PBS Nova TV programme. There is a web version of the famous Powers of Ten There are also several downloadable programmes for roaming round the Universe on your PC – one from the Hayden Planetarium, a big open source project called Celestia, and a more home grown one just called Universe. One of the most interesting recent professional attempts to get the whole Universe across has been led by Gott and Juric who try to put everything on a single logarithmic strip.
One of my favourite pictures, shown here, is a whole sky map made in the infra-red by the 2MASS project. . It is cleverly done to show both the Milky Way and the sea of other galaxies that it is lost within, colour coded by distance. Regular visible-wavelength light is extinguished by the obscuring muck in the interstellar medium, so that we can’t even see the centre of our own Galaxy. (Note : if another astronomer uses the word “extincted” I shall scream). In the infrared, you can see clean through, and we can finally see that yes, the Milky Way really is like two fried eggs back to back.
My own project, UKIDSS, is the successor to 2MASS – an infrared survey that doesn’t cover the whole sky but is much deeper. In fact one piece of it, the Ultra Deep Survey (UDS), led by my old chum Omar Almaini, is intended to build up over several years to map out a volume at redshift three that is big as the map of the local Universe that 2MASS has made. The UDS isn’t finished yet, but here is a picture showing the map so far – the faint red blob by the arrow is an object pinned down by Ross McLure, that he believes is a luminous “Lyman break” galaxy at redshift 6.
What is our place in things ?
We all know that Copernicus knocked us off our self-important pedestal. But the obsessive star mappers of the 18th and 19th centuries made an arguably bigger change : we live in a vast sea of stars at huge distances – we are insignificant. Temporarily we reverted to believing we were at the centre of a structured Universe, as star maps seemed to show that we live at the centre of a flattened swirling disc. But this was an illusion caused by the obscuring muck; now we know we live in an unfashionable suburb of the Milky Way. Next came two great twentieth century revolutions. The Milky Way is only one of many island universes; the Universe was again unimaginably vast and formless. For decades now we have been mapping the realm of the galaxies, but we have not uncovered another layer of structure – no Metagalaxy – the galaxies just go on and on and on and on. They are clumped and clustered, and measuring this clumpiness has filled many careers (including mine) as it is a diagnostic of rival cosmological theories … but no new landmass is emerging through the mist. We are just a dot within a dot within a dot.
However .. Slipher and Hubble and friends found all those galaxies rushing away from us. The Universe is expanding; running the movie backwards, the Universe had a beginning, in a violent explosion : what Hoyle scornfully termed the Big Bang. (Here is a free plug for Simon Singh’s book about the Big Bang.) This is philosophically unsettling, and Hoyle wriggled bravely, but every year the facts pile up. The Universe was different in the past and began 14 billion years ago. Much of our mapmaking now, like the UKIDSS-UDS, is aimed at mapping very very faint galaxies, to directly study the past, and we are struggling towards an understanding of the era of galaxy formation. But the philosophical discomfort remains. The Big Bang universe is one of the central features of our time.
What’s out there?
Astronomical surveys are not just about structure. They are about content. It is surveys that have gradually revealed the true variety of the cosmic bestiary. With the naked eye we see only stars and planets. Telescopic surveys revealed fuzzy patches that are either clouds of gas, or separate island universes. The patchiness in star counts revealed that the space between stars is not empty, but filled with smoke. But the real action started from the middle of the twentieth century, as technology allowed us to map the sky at different wavelengths. Radio surveys revealed the existence of pulsars, and radio galaxies squirting out jets millions of light years across. X-ray astronomy revealed collapsed objects – neutron stars and black holes – swallowing material from companion stars. Infra-red astronomy revealed new stars in the process of forming, and far-infrared astronomy showed us ultraluminous starburst galaxies. Not only did we find new objects that we didn’t know existed, but even new states of matter – relativistic plasma, degenerate matter, gas at ten million degrees between the galaxies.
Because of multi-wavelength surveys, twentieth century astronomy was a story of never ending surprises. But could it be that no windows remain to open .. Are there any surprises left ? Back soon ..