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The progress of astronomy - or, more precisely, astrophysics -over the past century, and particularly the past generation, is not easily pigeon-holed.
On the one hand, profound truths have tumbled abundantly from the sky. Here are four diverse examples:
1. Our universe began some 14 billion years ago in a single cataclysmic event called the Big Bang.
2. Galaxies reside mainly in huge weblike ensembles.
3. Our neighbouring planets and their satellites come in a bewildering variety.
4. Earth itself is threatened (at least within politicians' horizons) by impacts from mean-spirited asteroids or comets.
On the other hand, ordinary citizens may well feel that astronomers are a confused lot and that they are farther away than ever from understanding how the universe is put together and how it works. For example, 'yesterday' we were told the universe is expanding as a consequence of the Big Bang; 'today' we are told it is accelerating due to some mysterious and possibly unrelated force. It doesn't help that the media dine exclusively on 'gee-whiz' results, many of them contradictory and too often reported without historical context. I can't help but savour the pre-1960s era, before quasars and pulsars were discovered, when we naively envisioned a simple, orderly universe understandable in everyday terms.
Of course, all the new revelations cry out for insightful interpretation. And that's why I'm delighted to introduce this brand-new edition. So much has been discovered since it first appeared in 1987 . . . so much more needs to be explained!
It's sobering to catalogue some of the objects and phenomena that were unknown, or at least weren't much on astronomers' minds, only a generation or two ago.
The one that strikes me most is that 90% (maybe 99%!) of all the matter in the universe is invisible and therefore unknown. We're sure it exists and is pervasive throughout intergalactic space (which was once thought to be a vacuum) because we can detect its gravitational influence on the stuff we can see, such as galaxies. But no one has a cogent clue as to what this so-called dark matter might be.
Masers, first created in laboratories in 1953, were found in space only 12 years later. These intense emitters of coherent microwave radiation have enabled astronomers to vastly improve distance determinations to giant molecular clouds and, especially, to the centre of our Galaxy.
A scientific 'war' was fought in the 1960s as to whether clusters of galaxies themselves clustered. Now even the biggest of these so-called superclusters are known to be but bricks in gigantic walls stretching across hundreds of millions of light-years. These walls contain most of the universe's visible matter and are separated from each other by empty regions called voids.
The discovery of quasars in 1963 moved highly condensed matter on to astronomy's centre stage. To explain their enormous and rapidly varying energy output, a tiny source was needed, and only a black hole having a feeding frenzy could fill the bill. Thus too was born the whole subdiscipline of relativistic astrophysics, which continues to thrive. Quasars are now regarded as having the highest energies in a diverse class called active galaxies.
Gamma-ray bursts, the most powerful outpourings of energy known in the universe, only came under intense scrutiny by astronomers in the 1990s (they had been detected by secret military satellites since the 1960s). The mechanism that leads to this prodigious output is still speculative, though a young, very massive star collapsing to form a black hole seems favoured.
A decades-long quest for extrasolar planets and closely related brown-dwarf (failed) stars came to an abrupt end in 1995 when the first secure examples of both entities were found. (By a somewhat arbitrary convention, planets are regarded as having masses up to several times that of Jupiter; brown dwarfs range from about 10 to 80 Jupiters.) Improved search strategies and techniques are now discovering so many of both objects that ordinary new ones hardly make news.
One of the greatest successes of astrophysics in the last century was the identification of how chemical elements are born. Hydrogen, helium, and traces of others originated in the Big Bang; heavier elements through iron derive from the cores of stars; and still heavier elements are blasted into space by the explosions of very massive stars.
The discovery of pulsars in 1967 confirmed that neutron stars exist. Born in supernova explosions, these bodies are only about 10 kilometres across and spin around as rapidly as 100 times a second. Whenever a pulsar's radiation beam, 'focused' by some of the strongest magnetic fields known, sweeps over the Earth, we see the pulse. In addition to being almost perfect clocks, pulsars have allowed studies as diverse as the interstellar medium and relativistic effects. Finally, unlike any other astronomical object, pulsars have yielded three Nobel Prizes!
Tantalizing though inconclusive evidence for extraterrestrial life accumulates impressively: possible fossil evidence in the famous Martian meteorite ALH 84001, the prospect of clement oceans under the icy crust of Jupiter's satellite Europa, and the organic-compound rich atmosphere of Saturn's moon Titan. And then there is the burgeoning catalogue of planets around other stars and the detection of terrestrial life forms in ever more hostile environments. All this suggests that we may not be alone. On a higher plane, despite many efforts to find extraterrestrial intelligence since Frank Drake's famous Ozma experiment in 1960, we haven't picked up E.T.'s phone call yet. But the search has barely begun.
The flowering of astrophysics stems from the development of ever larger, ever more capable telescopes on the ground and in space. All the electromagnetic spectrum - from the highest-energy gamma rays to the lowest-energy radio waves - is now available for robust scrutiny, not just visible light and long-wavelength radio emission as was the case in as recently as the 1950s.
Equally impressive has been the development of detectors to capture celestial radiation more efficiently. In the case of the CCD (charge-coupled device), trickle-down technology has allowed small amateur telescopes to act as though they were four or five times larger. Augmented by effective software, CCDs have caused a revolution among hobbyists, who, after nearly a century-long hiatus, can once again contribute to mainstream astrophysical research.
Increasingly, astronomers are no longer limited to gathering electromagnetic radiation. Beginning late in the last century, they started to routinely sample neutrinos, elementary particles that allow us to peek at such inaccessible things as the earliest times in the life of the universe and the innards of exploding stars. And the gravitational-wave detectors being commissioned at the time of this writing should allow glimpses of the fabric of spacetime itself.
Astronomy has involved extensive international collaborations for well over a century. The cross-disciplinary nature of contemporary research makes such collaborations even more compelling in the future. Furthermore, efforts to build the next generation of instruments on the ground and especially in space are so expensive that their funding will demand international participation.
Where do astronomers go from here? 'Towards the unknown' may seem like a cliche, but it isn't. With so much of the universe invisible or unsampled, there simply have to be many enormous surprises awaiting!
When it comes to the Big Questions, I don't know whether we are children unable to frame our thoughts, or teenagers at sea, or adults awash in obfuscating information. Researchers find the plethora of new discoveries - despite myriad loose ends and conundrums - to be very exciting, for it attests to the vibrancy and maturation of the science. Yet, as we enter the 21st century, astronomers are still a very long way from answering the two most common and profound questions people ask: what kind of universe do we live in, and is life pervasive?
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