For the astronomy buff, recent decades have been a time of unprecedented and, at the same time, baffling discovery. Advances in the physical sciences have meant that we now recognise a whole host of celestial phenomena which, even 40 years ago, when Fred Hoyle's popular Frontiers of Astronomy was published, were unknown or poorly understood. Some are old friends reclassified: types of variable star, for example, which seem to have bred like rabbits. Some are objects, such as black holes, previously merely suspected to exist. Many are new discoveries: pulsars and quasars, for instance, which required a great deal of research before we began to understand them.
Such has been the torrent of new information, concepts and theories that even the well-informed have had trouble piecing it all together. Moreover, while our understanding of many elements of the solar system has improved—details of the solar wind, the interplanetary medium and the arrangement of galaxies—our knowledge of the geography of our own galaxy has remained fragmentary. Although we have known directions and distances of various objects, and the "poached egg" picture of the galaxy has been developed to show both the spiral structure of the disc and the arrangement of the objects within, it has been difficult tc relate all this information to the night sky as we view it.
Seeing depth in the night sky is optically impossible, for whether by eye or telescope the objects beyond the solar system simply appear to be at the same very large distance from us, as if they were pasted on the surface of the celestial sphere. Indeed, the Moon, planets and satellites appear nearer only because they move across the background. For the stars, clusters and bright and dark nebulae there has been no easily comprehensible account of galactic geography (glactography?) which would enable us to "see" them in three dimensions.
Only in the last few years have the various strands of evidence been multiplied and then reconciled one with the other to give us a reasonable model of the Milky Way galaxy. There is still much to discover, much to debate, but at last we have a description in plain English. The generously illustrated The Guide to the Galaxy by Nigel Henbest and Heather Couper (Cambridge University Press, 1994) is an anatomy of our galaxy, showing the spatial relationships and significance of its parts. We are shown for the first time the night sky as a three-dimensional structure which is but the present state of an evolving system. The static celestial sphere has been banished forever.
The first of the book's eight chapters traces the discovery of the fact that we inhabit a galaxy, one huge star system amongst very many spread out through the entire volume of the observable Universe. The story runs from myths such as that of Juno spilling her milk as she fed the infant Hercules (and thereby spawning the Milky Way) to the work of Edwin Hubble, who laid to rest the last shreds of belief in the primacy of "our" galaxy. This great tale is clearly told and well illustrated. Here is Galileo, whose telescope first revealed the Milky Way to be a multitude of stars rather than a smear of luminosity, and Thomas Digges, who boldly conjectured that the celestial sphere might be no sphere at all but a slab or stratum of stars with the Sun in the centre. This picture accounted for the appearance of the Milky Way and the relative paucity of stars as we look out at right angles to its plane.
Then there is the work of the father figure of amateur astronomers, Sir William Herschel, one-time professional musician, who discovered Uranus and became one of the greatest telescope makers and observers ever. Most of his career was devoted to an attempt to plumb the depths of space—to estimate the distance of the furthest stars visible in his great 1.2-metre-diameter telescope, and by so doing to arrive at the shape and size of the Universe. In this he was defeated when he realised that since the stars are not all of the same inherent brilliance, a simple measurement of their apparent brightness from Earth was no indication of their distance from it. Yet for all the failure of his great project he left us a catalogue of discoveries unequalled by any other observer.
The story was advanced by the American giants of astronomy at the turn of the century. There was Harlow Shapley, the thwarted student journalist who chose astronomy as his subject because it came second in the university calendar after archaeology, a word he could not pronounce. Using the then finest telescope in the world, the 60-inch at Mount Wilson, he examined those dramatic balls of stars, the globular clusters, and deduced that the Sun was not at the centre of things, but rather well out towards the edge of a system which he estimated to be 300,000 light years across. Such an enormous system led him to believe that this was the Universe itself, and that Herschel's nebulae, which we now know to have been, in the main, separate galaxies, lay within the Milky Way.
Two men were to correct Shapley's mistake. First was the patrician Rhodes scholar Edwin Hubble, who dropped a brilliant career at the bar to become an astronomer, declaring that he would rather be a second class astronomer than a first class lawyer. In the early 1920s he was able to observe Cepheid variables—luminous stars 10,000 times brighter than the Sun whose true brightness is a function of the frequency with which it changes—in the galaxies closest to us, and show that they were at distances which placed them far beyond the Milky Way. As Shapley remarked on reading a letter from Hubble announcing these results, "He has destroyed my universe," and indeed he had, for it was now far bigger and more complex than any astronomer had suspected.
The other correction was made by the immigrant Walter Baade, who took advantage of World War II blackouts to make deep surveys that can only be made in a really dark sky. He established that there are two separate populations of stars in the Andromeda galaxy, one making up the nucleus and the other, younger and bluer, in the disc. Later, he refined this work to show that there are also two types of Cepheids corresponding to these populations. Those of the disc are inherently brighter than those of the nucleus, which are also found in the globular clusters that Hubble had assumed that all Cepheids were of this last type, and when this mistake was corrected Baade's measurements doubled the size of the Universe at a stroke.
The stage now set, Chapter 2 deals with the Local Group, that collection of some 30 galaxies clustered around the two great spirals, the Milky Way and Andromeda galaxies, which dominate this corner of space.
The remaining six chapters concentrate on the anatomy of the Milky Way. This is no ramble through the parts of a gigantic celestial carousel spinning slowly and majestically in the depths of space. Rather, it is a guided tour down the Shotover in spate. This is an unfamiliar, even disturbing, exercise, but then the galaxy is an evolving dynamic system, a boiling and roiling of combinations, coalescences, associations, dissociations, disruptions and disasters. Even that guzzling monster, the putative black hole at the very centre, slowly but surely capturing the lesser players and consuming them, is ephemeral in the face of eternity.
First there is a discussion of the enormous difficulties inherent in observing our galaxy from within, for it is both tumultuous and turbid. The foreground blocks the distance, structures overlap and intermingle, and traces of old events are distorted by the effects of more recent cataclysms. The information gleaned from visual wavelengths, which within the disc are blocked by clouds of gas and dust, had to be supplemented by infra-red and radio evidence before astronomers could construct a general picture of the spiral form of the galaxy. And only when the technology evolved to place instruments in space were we able to see at X-ray, ultra-violet and many of the infra-red wavelengths which are blocked by the Earth's atmosphere.
The next four chapters deal with our sector of the galactic disc: the Perseus arm which lies rimward of us, the Orion arm in which our solar system is located, the Local Neighbourhood—everything within about 800 light years of the Sun, and hence most of what is visible to us, and the Sagittarius arm, which lies immediately hubward of us. These are the chapters which give us the third dimension for our view of the night sky, a grasp of the reality so long hidden behind the two-dimensional screen of the constellations. I am tempted to say that if you have not seen the maps and associated photographs of our Local Neighbourhood in chapter six, then you ain't seen nothing yet, for here is the real picture of the sky visible to the naked eye or with binoculars. Here the apparent, almost random, scatter of stars and wisps of nebulosity are shown to have structure and meaning, to tell of events that were old when life first stirred on Earth.
The final chapter deals with the centre of the galaxy, and here the picture is much more conjectural, in spite of recent data. Why this should be so is made clear, with discussions of such phenomena as "The Mouse," a comet-shaped radio source, and the "Great Annihilator," probably a black hole tearing matter from a companion star. Shldovskii's "outstanding peculiarity" remains a splinter in the astronomical mind.
The Guide to the Galaxy an excellent and exciting introduction to astronomy which approaches the subject in an entirely novel and rewarding way. If you are contemplating buying your first, or another, book on astronomy, this should be it.