The Sky at Night Phần 7 pptx

98 24 Scorpion in the Sky and contains several hundred thousand stars. The full diameter is 95 light-years, and near the centre the individual stars are so close together that collisions must sometimes occur. In May 1861, a nova flared up in the cluster and briefly outshone all the other cluster members combined; it was given a designation – T-Scorpii – but has never reappeared. It was probably an ordinary nova, a one-off, but M80 is worth monitoring. Adjoining Scorpius are the beautiful clouds of the star Rho Ophiuchi, and the whole region is exceptionally rich. So, when the sky is clear, go and seek out the Scorpion, with its red leader, its chain of bright stars, and its wonderful star fields. There is no other constellation quite like it. 99 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_25, © Springer Science+Business Media, LLC 2010 The Perseids provide the most reliable of all the annual meteor showers. Conditions in 2007 were expected to be good, so we planned a meteor watch at my observatory. It turned out to be a distinct success. Meteors can be seen on almost any clear night, and there can be few people who have not been impressed with shooting stars flashing across the sky and vanishing in a second or two. Yet not everybody knows what they are, and only during the past couple of centuries have we been able to learn much about them. They are phenom- ena of the upper atmosphere, and each streak indicates the last moments of a tiny object, generally no larger than a grain of sand, dashing into the upper air from outer Chapter 25 The August Perseids Perseids 2006 (Credit: Pete Lawrence) 100 25 The August Perseids space and burning away by friction against the atmospheric particles. It enters the air at a speed of anything up to 45 miles per second and is destroyed by the time it has penetrated to about 40 miles above sea level, ending its journey in the form of ultra- fine “dust.” Meteors are cometary debris. A comet has a nucleus made up of ice and solid particles; its mass is very slightly compared with that of a planet or even an asteroid, and I once described a comet as being “the nearest approach to nothing that can still be anything”. Some move round the Sun in elliptical orbits, in periods of a few years; these so-called short-period comets are old friends, and we know when and where to expect them. Others have much longer periods, so that we cannot predict them. I well remember the lovely green comet of 1996, discovered by the Japanese astronomer Yuji Hyakutake, and named after him. It will return to the inner Solar System in about 15,000 years time; remember to look out for it! As a comet draws in towards the Sun, its outer ices begin to evaporate, and the comet may develop a tail of immense length, so that it may become spectacular despite its negligible mass. However, a comet loses material at ever perihelion passage, and it wastes away; some periodical comets have been seen to disintegrate. The most famous case of this was that of Biela’s Comet, which is seen every 6 ¾ years – until 1846, when it broke in half. The pair came back on schedule in 1852, but this was their final appearance. The comet is dead, but when the Earth passes close to the place where it ought to be, we pick up particles which it has left behind – and the result is a shower of shooting stars. Actually, it is more accurate to say “when the Earth passes through the orbit of the defunct comet,” because dust particles have been left all along the orbit. Few periodical comets ever become bright enough to be seen with the naked eye even when they are relatively close to us, and only one – Halley’s Comet – can ever become brilliant. It returns to perihelion every 76 years, but at its last visit, in 1986, it was badly placed and some people were disappointed (it will be back again in 2063). But comets are clearly linked with meteor showers, and this brings me on to the August Perseids and our Sky at Night programme for that month in 2007. The particles left behind by a comet are moving along in parallel paths, and so the resulting meteors appear to emanate from one definite point in the sky, known as the radiant. The best demonstration I can give you is to picture the scene from the top of an arc overlooking a motorway. The lanes seem to diverge from a point near the horizon – the “radiant” of the lanes, and approaching cars will come from that point. The August meteor issue from a radiant in the constellation Perseus, and I always regard this as the richest of all annual showers – of which there are many. The Perseids were not identified until 1835, when attention was drawn to them by a Flemish scientist, Adolphe Quetelet, who was an interesting man; he was an enthu- siastic astronomer, but was also a leading criminologist and statistician. After he drew attention to the August shower, others took notice of it, and in 1864 the Italian astrono- mer Giovanni Schiaparelli (best remembered, perhaps for his observations of the “canals” of Mars) found that the meteors moved in the same orbit as a comet, Swift- Tuttle, which had been discovered in 1862, and was thought to have a period of well 10125 The August Perseids over 100 years (Lewis Swift and Horace Tuttle were both renowned comet hunters). The inference was obvious; Swift-Tuttle was the parent comet of the Perseids. Every year, the shower meteors begin to appear around July 23 and become more and more plentiful until maximum activity is reached on August 12–13. After that subsides, though a few Perseids may still be seen as late as August 20. The ZHR, or Zenithal Hourly Rate, may be as high as 80 (the ZHR is defined as the number of naked-eye shower meteors which would be seen by an observer under ideal conditions, with the radiant at the zenith or overhead point. In practice, these conditions are never attained, so that the observed rate is always less than the theoretical ZHR). There are three more points to be noted here. First, although the meteors radiate from Perseus – but are not confined to Perseus – they flash along to any point in the sky. Second, not all the meteors that you see will be Perseids; some belong to other, less prolific showers, while there are also sporadic meteors, which may appear from any direction at any moment, and are not linked with known comets. You can identify Perseids by plotting their paths against the stars and tracing them “backwards” to their starting point. Finally, what we see in the sky is not the particle itself, which is too small to be visible, but the luminous effects which it produces during its headlong dash through the atmosphere. For 2007, the prospects were as good as they could possibly be. The weather forecast was favourable, and – most important of all – there was no interference from moonlight; the Moon was new. Maximum was due at 2 a.m. on the morning of August 13. So for a Sky at Night “special,” we assembled a group of experienced meteor observers, headed by John Mason, plus others (such as me) who were less dedicated, but were determined to enjoy the display of cosmic fireworks. The two groups were doing different programmes. I was merely carrying out a television commentary plus counting the meteors, noting their magnitudes, their colours (if any) and other exceptional features; some meteors leave trains which persist for some seconds, or even less frequently, a minute or two. The more serious members of the party were carrying out photography. The method here is to point the camera in a suitable direction and take time-exposure. You will record star trails, and perhaps an artificial satellite or two. We were lucky; we had barely settled down when the ISS – the International Space Station – passed overhead. You could not possibly miss it; it shone more brightly than Venus. What did we hope to see? Well, a decent shower; the Perseids are usually co-operative. There was always the chance of a “fireball” which would light up the landscape; I have seen a few of these, produced by particles the size of grapes or even golf-balls, but they are rare. Otherwise, we had to wait and see. The sky was cloudless and transparent, and the lawn surrounding my observatory is shielded from any obtrusive artificial lights, so that it was pleasingly dark. Meteors began to appear; apart from one brief period, the clouds stayed away. Gradually activity increased. There were some bright meteors, though no fireballs. None of us felt inclined to give up; as usual on these occasions, there was rather a party atmosphere. Coffee was most welcome, and we 102 25 The August Perseids remained until the approach of dawn, when we adjourned indoors to refresh ourselves with drinks which were rather more potent than coffee. The end of an enjoyable evening. What had we achieved? Scientifically not a great deal, though routine observations are always useful; our main point was that we had spoken to a large audience of around a million people, some of whom went outside and watched a spectacle which they would otherwise have missed. That in itself, I feel, justified our special programme. Of course, the Perseids will be with us again next year, but in 2008 the conditions will not be nearly so favourable, as meteor watchers will have to contend with strong moonlight. Finally, spare a thought for Comet Swift-Tuttle, responsible for it all. It was not seen for many years after 1862, and calculations indicated that although it had been missed at several intervening returns, it should be back in 1981. Careful searches gave negative results, and I thought that it had simply been overlooked. John Mason disagreed; he believed that the orbit had been wrongly worked out, so that the real date of the next perihelion passage would be early in the 1990s. We had a modest bet about this (a bottle of Irish Whisky, I recall) and I was confident – until 1992, when the comet turned up. The period is now known to be 135 years, Swift-Tuttle has not been conspicuous lately, but a good deal will be heard about it in the twenty-second century, when it will pass alarmingly close to us. Wait for an end-of-the world scare then – but don’t blame the Perseids! 103 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_26, © Springer Science+Business Media, LLC 2010 Everybody is fascinated by black holes. Who better to give us the latest news than professor John Brown, the Astronomer Royal for Scotland, who as well as being a world-renowned solar physicist is also an expert amateur magician. Joined by Drs Fiona Spititz and Chris Lintott, we attired ourselves suitably and did our best to forge a link between black holes and Dark Forces. Chapter 26 Black Holes: And Black Magic Einstein’s cross (Credit: ESA) 104 26 Black Holes: And Black Magic Black Holes and Black Magic? No true connection, needless to say, but black holes are so bizarre that they really do seem to be magical. Even now we cannot pretend that we have anything like a full understanding of them. Then first concept of them seems to have been due to an English amateur scientist, the Rev. John Michell, who lived from 1724 to 1793 (of course, most people mis- spell his name as “Mitchell”). His activities were very varied – someone ought to write a really good biography of him – and one of his suggestions was that a body of sufficient mass would pull hard enough to prevent even light escaping from it. A similar comment was made later by the great French mathematician Laplace, but the term “black hole” dates from only 1968, when it was introduced by the American scientist John Wheeler. It caught on and is now part of our language, but in a way it is misleading because a black hole is not black at all. It emits no light and so cannot really be said to have any colour. If we cannot see a black hole, we have to locate it by means of its gravitational pull upon objects that we can see. Black holes are incredibly massive – thousands of millions of times more massive than the Sun – so that they can certainly make their presence felt. We have every reason to believe that there is a black hole in the centre of our Galaxy, because we can measure the speeds at which objects fairly close to it whirl around it, and this allows us to calculate its mass. It seems that a black hole is the end product of a very massive star. Normal stars create their energy by nuclear reactions taking place inside them, as our Sun is doing now. Eventually the supply of available nuclear “Fuel” will run out, and the whole situation must change. A modest star such as the Sun will lose its outer layers and subside into a white dwarf, where the atoms are crushed and packed together so tightly that the star is extremely dense; the final state will be as cold, dead black dwarf. We know plenty of white dwarfs, one of which, the faint companion of Sirius, is no larger than the Earth but is as massive as the Sun, but the whole course of stellar evolution is so slow that the universe may not yet be old enough for any dwarfs to have formed. After all, the Big Bang happened a mere 13.7 thousand million years ago! A star much more massive than the Sun will die in a much more spectacular fashion; it will explode as a supernova, blowing much of its material away into space while the remnant, now composed of neutrons, will spin round and send out beams of radio emission. If these beams sweep over the Earth, just as the beams of a rotating lighthouse will sweep across the watcher on the beach, we pick up pulsed radio waves – hence the term “pulsar” (there is a pulsar in the famous Crab Nebula, 6,000 light-years away; the supernova responsible was seen to blaze out in the year 1054). Pulsars, too, must end up as black dwarfs. But if the mass of the dying star is greater still, it cannot even produce a pulsar. Once the final collapse starts, nothing can stop it. The star becomes smaller and smaller, denser and denser – and the escape velocity goes on increasingly until it reaches 186,000 miles per second, the speed of light. Light is the fastest thing in the universe, at least so far as we know, and so nothing can break free from the doomed star. It has become a black hole. The underlying principle is not the same as was believed by Michell, because the modern idea of gravity is different. To Michell, and also to Newton, gravity was a 10526 Black Holes: And Black Magic force which enabled one body to affect another even when the two were widely separated; this “action at a distance” was often regarded as a form of scientific black magic. Albert Einstein changed all this and interpreted gravity as a distortion of “space-time.” Consider a bowl with paper stretched across its top; roll a marble across the paper, and it will follow a straight line. Now imagine an object inside the basin which could in some way pull the sheet of paper downwards (I admit that I do not see quite how this could be managed, but never mind). The marble will no longer roll straight; its path will be distorted. I know this is a poor analogy, but it is better than nothing. In space, the object at the bottom of the bowl represents our black hole. Black holes can therefore affect the paths of light-beams, and this is shown by the phenomenon of gravitational lensing (though other massive bodies can act simi- larly; large galaxies and clusters of galaxies, in particular). A good example of this is what is termed the Einstein Cross (because it was Einstein who realised that this sort of thing could occur). At a distance of 8,000 million light-years, we find a quasar, catalogued as Q2237 = 030; quasars, as we know, are the immensely luminous cores of active galaxies. En route to Earth the quasar’s light passes by the very massive galaxy called Huchra’s Lens. The light from the quasar is “bent”; we see four images of the background quasar, and you will agree that the effect is remarkably striking. Many other cases of gravitational lensing are known, though not many are as symmetrical as the Einstein Cross. I must not forget to say something about Hawking radiation, first proposed by the famous Cambridge cosmologist Stephen Hawking. What we call a vacuum is not actually empty; it is seething with “virtual particles” which appear in pairs, but vanish again so quickly that they are truly ghostlike. A particle and its antiparticle will anni- hilate each other – but if a pair appears at the extreme edge of a black hole, the so-called event horizon, one of the pair may enter the black hole, leaving its partner marooned outside. With no partner, the stranded member of the pair becomes a “real” particle, and the black hole is forced to emit a certain amount of radiation, which results in loss of mass. It has been suggested that the emission of enough Hawking radiation might finally make the black hole explode and destroy itself, but whether a major black hole has yet perished in this way seems somewhat uncertain…. Exotic theories about black holes are plentiful. They have been regarded as passages between our universe and a completely different universe, in a different dimension, with which we can normally have no contact whatsoever; on the “multiverse” picture, there may be many of these – perhaps an infinite number. Approaching a black hole in the hope of a free ticket to the universe next door would be rather hazardous, and there would be all manner of curious effects involving both space and time. Incidentally, what is the fate of a star which collapses to produce a black hole? Does it crush itself out of existence altogether, or does it turn up elsewhere, either in our universe or in another? Will we Earthmen ever be threatened by a maverick hole able to creep up and take us by surprise? At present these are the problems which we cannot solve. Perhaps we will find the answers eventually, but meantime it is not hard to see why some people still feel that there must be at least a tenuous link between black holes and black magic! 107 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_27, © Springer Science+Business Media, LLC 2010 In October 2007, the great radio telescope at Jodrell Bank had been in action for half a century. Together with Chris Lintott, I went there and talked not only to Bernard Lovell but also to Bernard Baruch, Ian Morrison and Phil Diamond of the Jodrell Bank team. Bernard and I are very old friends. I remember thinking – would this be the last time we would meet face to face? Bernard, now 94, is totally fit in every way; thanks to the activities of our gallant German allies, long ago, I am not really mobile. We must see. Incidentally, both Bernard and I are cricket fanatics. He played to a really high standard; I did my best to spin my unorthodox leg-breaks. Sadly, we never actually took the field together, and now fear it is rather too late! Go to Jodrell Bank, near Macclesfield in Cheshire, and you will see the 250-ft Lovell radio telescope. In fact you cannot possibly miss it, because it dominates the entire landscape. It is a miracle of engineering even today and was even more so in Chapter 27 Jodrell Bank: Fiftieth Anniversary The Lovell Telescope (photo by Patrick Moore) [...]... hit it, in the manner of two cars meeting head-on They will first orbit each other and take part in what we may call a cosmic waltz; only then will they merge, and individual stars will seldom collide – after all, a star is a comparatively small target in the vastness of space The situation may be likened to that of two orderly crowds passing through each other But the material between the stars will... was violent, with material sent out from the tiny, icy, 2.2-mile nucleus so rapidly that by the 1st week of November the coma had expanded to become larger than the Sun It lay in the constellation of Perseus, and completely altered the look of the whole of that part of the sky; with the naked eye it looked like a slightly fuzzy star, and on 25 October I estimated the magnitude to be 2.4, much brighter... looked rather like a globular cluster, but telescopically it was clear that the brightest part of the huge coma was displaced from the centre It was strange to realise that this flimsy object, of negligible mass, had become the largest member of the Solar System! What makes the event even more remarkable is that perihelion had been passed as long ago as 4 May, that the comet was receding from both the. .. and the nearest star, Proxima Centauri, will be another (even smaller) pinhead over four miles away But is we represent the Milky Way Galaxy by a dinner-plate, Andromeda will be another plate on the far side of the dining-room table We can see other encounters, too Look at the Antennae Galaxies in Corvus (NGC4038-9) which are well within range of amateur owned telescopes; the images shown here tell the. .. what is happening, even though the Mice are 3,000 million light-years away The long streamers are due to the relative difference between the gravitational pulls on the near and far parts of each galaxy 112 28  The Grand Collision Making forecasts is always dangerous, but so far as the future career of our Galaxy is concerned, we are confident that there is no major mistake Go outdoors tonight, if the. .. Slipher, at the Lowell Observatory in Arizona, had examined the spectra of galaxies and had found that almost all of them showed red shifts; the dark absorption lines were shifted over to the red or long-wave end of the background rainbow This is the familiar Doppler effect; a red shift indicates a velocity of recession, and Hubble concluded, correctly, that the entire universe is expanding Yet there... the USSR only the Jodrell Bank telescope could track the rocket, and overnight Lovell was transformed from a reckless spendthrift into a national hero Lord Nuffield paid the outstanding debts; the crisis was over and Jodrell Bank was safe Since then research has been constant; the 250-ft dish is still the third largest in the world and arguably the best Quasars, pulsars, gravitational lenses and other... and the Earth; at the time of the outburst it was more or less at opposition, so that when a definite tail developed it pointed away from us, and was so foreshortened that it was difficult to make out at all Visually it was never conspicuous, but images from the Hubble Space Telescope in late November showed a bluish tail; at one stage the tail became disconnected, and there was also a section of material,... schedule Comets are the most erratic members of the Solar System We never quite know how they are going to behave, and one of them, known officially as Comet 17P/ Holmes, has given us a real surprise Last October, it suddenly flared up from P Moore, The Sky at Night, DOI 10.10 07/ 978 -1-4419-6409-0_29, © Springer Science+Business Media, LLC 2010 113 114 29  Holmes’ Comet magnitude 17 to 2.5, and in less... line of stars extending from Alpheratz in the direction of Capella Not far from the second brightish star in the line, the orange Mirach, you will find the dim blur that marks the position of the Andromeda Galaxy, M31 It is just visible to the naked eye when the sky is really dark, and binoculars show it clearly, but photographic or electronic images obtained with adequate telescopes are needed to bring . and black magic! 1 07 P. Moore, The Sky at Night, DOI 10.10 07/ 978 -1-4419-6409-0_ 27, © Springer Science+Business Media, LLC 2010 In October 20 07, the great radio telescope at Jodrell Bank had. conditions, with the radiant at the zenith or overhead point. In practice, these conditions are never attained, so that the observed rate is always less than the theoretical ZHR). There are three. fields. There is no other constellation quite like it. 99 P. Moore, The Sky at Night, DOI 10.10 07/ 978 -1-4419-6409-0_25, © Springer Science+Business Media, LLC 2010 The Perseids provide the most