| Observing at Other Wavelengths:
The Importance of the Atmosphere.
As we learned earlier, the Earth's atmosphere sets important limits on many avenues of astronomical exploration. Ultraviolet radiation cannot get through the Earth's ozone layer; X-rays and gamma rays collide with molecules high in the outer parts of the Earth's atmosphere and do not make it to the ground; much of the infrared radiation is absorbed by water vapour distributed in the air; and some of the radio radiation is reflected from the Earth's ionosphere. If we are going to study astronomical sources at these important and interesting wavelengths, then, we will need to overcome these limiting factors.
Putting an observatory on a mountaintop helps, but is not really enough. Space astronomy provides the answer in many cases. Let us look at some of the issues.
Satellite Astronomy: Looking Down.Astronomers use telescopes on small satellites to study remote stars and galaxies, but of course they can also be pointed downwards, towards the ground. Sometimes this is done for practical scientific purposes, as in the case of weather satellites and those used in the study of forested areas and the search for resources. At other times, there are military objectives in mind, as when `spy satellites' are used by the Americans and Russians in particular. In the lecture, I showed you a reproduction of an American 'spy satellite' photograph of a Russian shipyard, taken from about 100 km above the ground. Unfortunately, the figure, which was published in Scientific American magazine, did not reproduce very well, but you could see windows of buildings and so forth. The plain fact is that if you sat on a park bench reading a newspaper, a spy satellite hundreds of kilometres overhead could read the headlines at the moments of best atmospheric transparency! Of course these satellites take advantage of the fact that they are looking at sunlit ground, so there is abundant illumination and they can take lots and lots of very short-exposure photographs. Every so often there is a moment of superbly still atmospheric conditions and they can see resolve very fine detail with no smearing of the image by the turbulence of the air. Moreover, you will probably not be surprised to learn that the military have worked on and perfected techniques of adaptive optics which are beyond the powers (and budget!) of astronomers. Indeed, some of our recent advances in astronomical imaging came partly as a result of the declassification of once-secret military knowledge and instrumentation.
Infrared.Early attempts to study the sky at infrared wavelengths required getting to great altitude, to get above as much water vapour as possible. (Please remember that the infrared radiation is absorbed predominantly by the water molecules distributed uniformly in the air -- the humidity , if you like -- rather than just the water found in clouds in the form of actual droplets.) By the way, there is more to the problem than just the fact that the incoming infrared radiation is absorbed and unable to get through in much quantity. There is the extra consideration that the atmosphere is warm, and thus glows with infrared radiation itself. This provides a terribly bright background which makes it very hard for us to see the remote infrared sources in the universe -- stars, clouds of gas, and so forth. To appreciate this problem, imagine the difficulty you would have trying to see even the brightest stars if the entire sky were glowing as brightly with visible light as the face of the sun is! That is rather like the problem the infrared astronomers have; and indeed their problem is even worse, since even their telescopes and observatory domes are also warm and glowing in the infrared. We can counter this to an extent by using very lightweight telescopes, with skinny components which radiate minimal heat, and by actively cooling the telescope and its enclosure. To solve this problem completely, however, we really have to get above the bulk of the Earth's atmosphere. The first attempts were in balloons and then specialised aircraft, like the Kuiper Airborne Observatory (KAO), named after the great planetary astronomer Gerald Kuiper. (He was the first to recognize the great promise of Mauna Kea as a site for an observatory.) I showed you a picture in class of this flying observatory, one in which you could see a dark square just ahead of the wings of the aircraft: that was the opening out of which the telescope peers. More recently, new aircraft-borne infrared observatory has been developed. It is called SOFIA; a picture of it appears on page 185 of your text. Of course, this is not a perfect solution: there is still air up where these aircraft fly, and some water vapour. So, although such a plane can fly much higher than any mountaintop reaches, we still do better to get into space. There have been a series of infrared satellites, including the Infrared Astronomical Satellite (IRAS), ISO, and (most recently) SIRTF [the Space InfraRed Telescope Facility, subsequently renamed Spitzer, in honour of a great astrophysicist who championed the promise of space astronomy]. It is interesting to consider the design of such satellites. Because warm objects emit infrared radiation, any orbiting infrared telescope would give off so much infrared light itself that it would swamp the detectors if it were not cooled. (Remember that the satellites are bathed in sunlight and thus reach an ambient temperature which can be considerable.) Thus they are launched complete with a supply of a refrigerant -- liquid helium. The IRAS satellite surveyed the whole sky, and also did "pointed" observations, where objects of special interest were studied especially carefully. But eventually it ran out of coolant, since it slowly leaks out, and is now defunct (as is the much more recent ISO satellite, for the same reason). By the way, the loss of cryogenics (the coolant) may not be the only thing that besets satellites. They need to be pointed at the astronomical targets of interest, which requires a mechanism for moving the satellite around. Often this is provided by little `jets' which squirt out small amounts of gas to push the satellite the other way. (Remember Newton's third law! This is just like the `jetpack' an astronaut might use.) But of course when that fuel runs out, the telescope can no longer control its orientation, and often begins to drift aimlessly in its direction of pointing. We can no longer control what we are observing. Even worse, we may not be able to position the telescope in such a way that data can even be sent back down to Earth, since its antenna may now point aimlessly into space. The mission has reached the end of its useful life. Infrared radiation tells us a lot about not-so-hot regions, like clouds of cool gas and dust which might be in the process of forming stars and planets. Indeed, such images reveal that at least some nearby stars are surrounded by disks of cool material -- perhaps analogs of the disk which is believed to have surrounded the sun in the formative stages of our own solar system.
Ultraviolet.The best-known ultraviolet satellite is the IUE - the International Ultraviolet Explorer - which lasted for about twenty years but which was later `decommissioned' (shut off) to save money. This kind of satellite can often last much longer than an infrared satellite since it does not require the cryogenic coolant which gradually evaporates. Ultraviolet radiation tells us about quite hot objects, such as the most massive and hottest stars, or stars which have blown off their outer materials and left the inner hot core exposed. We will learn more about those later in Phys 016, when we discuss stellar evolution.
X-Rays.X-rays do not make it through the Earth's atmosphere. This may surprise you, if you think of the way X-rays pass through your body. But remember that not all the X-rays go right through you. After all, you see bones and so in "in silhouette" because some of the X-rays are in fact absorbed by the bones! The atmosphere is transparent to visible light, but X-rays interact with individual particles high up and do not pass through unscathed. Thus we need to launch X-ray-sensitive satellites if we want to study any X-rays which may be emitted by celestial sources. You may wonder how it is that X-rays can be brought to a focus to form any kind of an image. If we build a conventional telescope with a curved mirror, surely the X-rays will pass right through the material of which the mirror is made (or penetrate part way and be absorbed deep inside, as with your bones). How can the X-rays ever be directed to a focus? This is in fact a sensible worry, but the solution is to use what is called "grazing incidence optics," as is shown in the figure on page 188 of your text. The X-rays glance off the mirrors rather as a flat stone skips off water, and by careful design of the mirrors you can bring them to a focus. The first example of this was the so-called Einstein Observatory, and there is now an even more technologically advanced X-ray telescope in orbit. It is called Chandra, and it is named after the Nobel-prize winning astrophysicist Subrhamanyan Chnadrasekhar. We will encounter him again in Phys 016 when we discuss the white dwarf stars that represent the end-points of the lives of stars like the sun. X-rays tell us about very hot regions (clouds of gas with temperatures of a million degrees or more) and about exotic objects like neutron stars and black holes. The way they produce X-rays will be part of our discussion of star deaths in Physics 016, but let me anticipate that by reminding you how a dentist provides X-rays. In the dentist's office, an `electron gun' emits electrons which are attracted to a positively-charged piece of metal (typically tungsten). The electrons build up enormous speed thanks to being electrically attracted to the tungsten target, and arrive with a real bang. The energy released in the impact shows up in a variety of ways -- the target gets quite hot, for instance -- one of which is the release of large numbers of energetic photons: X-rays! Neutron stars can sometimes be found near other stars (as in a binary pair), and if gas from the other star falls onto the neutron star, attracted by gravity rather than electrical charge, it can, on impact, lead to the release of many X-ray photons in a way exactly analogous to what happens in the dentist's office. So X-ray satellites allow us to study these exotic high-energy objects.
Serendipity in Astronomy: The Discovery of X-Rays.The earliest astronomical X-ray observations were made long before artificial satellites were reliable technology, and in fact were carried out using small rockets which could be fired briefly high into the atmosphere before they fell back to Earth. One early flight demonstrated that the Sun was at best a very weak source of X-rays. (They come from the tenuous but extremely hot corona of the sun, and are also produced in solar flares from active regions on the sun.) Given this, astronomers were able to determine that if all stars were like the sun, then their enormously larger distances would mean that we would never detect X-rays from them at all. Consequently, it was confidently predicted by some that there would be no interesting sources of X-rays in the universe. This turns out to be wrong -- no one then knew about neutron stars and black holes! -- but the error was only realised by chance. A rocket flight was sent up to study the moon (to see, in fact, if energetic electrons from the solar wind might cause the lunar surface to emit X-rays, just like a gigantic version of the dentist's X-ray machine). On that flight, a strong source of X-rays was noted in the constellation Scorpius, from a source now called Sco X-1. This was the rather flukey beginning of modern X-ray astronomy. Many thousands of sources, of a great variety of kinds, are now known.
Gamma Rays: More Serendipity.Gamma rays are photons which are even more energetic than X-rays, and they are produced only rarely by astronomical sources. They tell us about the most energetic events going on, but are not yet well understood. Speculation has arisen, for instance, that gamma rays may be produced when comets and asteroids run into neutron stars, or when neutron stars run into each other. Whatever is happening, it releases vast amounts of energy! Of more interest for our purposes is the fact that astronomical gamma ray sources were once again found in a completely serendipitous way. A couple of decades ago, the American military launched a satellite which was carrying gamma-ray detectors. The purpose was to monitor Earth-based events, and in particular to look for bursts of gamma rays which might indicate that the Russians were carrying out atmospheric tests of hydrogen bombs. (Gamma rays are emitted in thermonuclear explosions.) No such emissions were seen, but the first astronomical gamma-ray sources were detected out in space. Part of being a good scientist is to have an open enough mind that serendipitous events of this sort are recognized and capitalised upon, rather than being dismissed as irrelevant or an error "...because we know better." Many scientists have lived to be scooped in various great discoveries because they ignored some clear evidence which did not fit into their preconceptions.
Skipping Stones.In describing the way in which we can bring X-rays to a common focus, I discussed skipping stones off water -- a simple analogy -- and showed you a snippet from the 1950's movie `The Dam Busters.' This movie recreated a real-life WWII air raid of May 1943, one in which special bombs were used to destroy German hydroelectric dams. The bombs skipped repeatedly over the bodies of water above the dams, as the actual film footage showed, and had the desired effect of destroying a couple of major dams and disrupting German industry and armaments. That discussion had some fascinating physics in it. Let us take a moment to consider the way in which the bombs work. Here are the interesting aspects: The bombs were cylindical in shape, rather like big oil drums, and had a total mass of about five tons. The bombers flew in low (sixty feet above the water) and level, moving very steadily in their paths at a speed of about 240 miles per hour. The low altitude and high speed meant that the bombs met the water at a very shallow angle and were sure to `skip' along the surface. Just before the bombs were released, they were set spinning backwards (imagine a rather short log spinning rapidly about its long axis) at a speed of about 500 revolutions per minute. The point of this is to help to maintain the orientation of the bomb as it falls and skips - remember how the conservation of angular momentum helps a spinning body maintain a stable orientation in space. By careful design and timing, each bomb reached the face of the dam just about as its skipping action had finished, and you might expect that it would now merely sink into the lake. But the rapid backwards spin of the bomb meant that it actually `crawled' down the face of the dam. When it reached a sufficient depth, the growing water pressure triggered a fuse which exploded the bomb. The bomb's proximity to the concrete dam, and the cushioning effect of the surrounding water, meant that an enormous shock wave propagated into and through the dam itself - and destroyed it.
The Interesting Physics of Water.Water has some very interesting and important properties that I wanted you to be aware of. In its liquid form, water is almost completely incompressible. If you take a lot of everyday materials, like wood or snow, you can compress them under pressure and collapse them to a smaller volume. This is not the case for water. This explains something which used to perplex me: how can people survive a dive to hundreds of feet below the surface of the ocean? Why are they not completely crushed by the extreme external pressure of the surrounding water? The short answer is that most of the cells in their bodies are water-filled and resist the compressional effects. What does get compressed, of course, is any air-filled parts of the body: the lungs, the sinuses, and so on. (That is why we have to breathe air under very high compression using a regulator, so that the lungs can be filled with high-pressure gases.) When cooled, most materials decrease slightly in volume (this is true for water), and they decrease still more in volume when they freeze. Ice is remarkable in that it actually expands upon freezing - the water molecules take up a rigid crystalline orientation in which there is a lot of empty space, as if the molecules are all holding each other `at arms' length.' One consequence is that ice is less dense than liquid water, which is why icebergs float in the sea. This is of considerable importance for life on Earth, since it means that lakes freeze from the top down - and once an ice layer forms, any heat in the water trapped beneath cannot readily be radiated away to the air. If water were like other fluids, and lakes froze from the bottom up, no fish would survive Ontario winters! The increase in volume of water as it freezes explains any number of everyday things, such as why water pipes will burst if they freeze. It also explains why frozen fingers and toes can be so badly damaged: the frozen liquid in your cells expands and actually ruptures the cells, causing irreparable damage. Finally, these properties of water allow us to understand skating. When we stand on metal blades, our body weight is focussed onto a tiny area of ice and subjects it to enormous pressures. For many liquids, this compression would correspond to what happens in the freezing process itself, but for water the compression collapses the crystalline rigidity of the ice and restores the water to a liquid state. In short, you skate on tiny amounts of liquid water which act to lubricate your progress - you are on a film of water (which freezes again as you skate away).
The Trouble With Hubble.Some of you may know the story of the Hubble Space Telescope, an orbiting telescope which is named after a famous astronomer who we will encounter in Physics 016. (It is Hubble who is credited with discovering the expansion of the universe.) Plans for the HST began many years ago, but it took a long time to bring it together. Its original launch date was postponed following the Challenger disaster, the explosion of a Shuttle in which seven astronauts died. But eventually it was launched, with great excitement and fanfare. Tragically, it then became apparent that there was something wrong. The first images taken with the telescope were simply not very good. As it turned out, the problem was that the mirror had been exquisitely crafted - but was the wrong shape, just as if your optometrist had carefully designed and polished your glasses to the wrong prescription. The mirror in the HST was made by the American engineering firm of Perkin-Elmer, and was the most perfectly formed mirror in astronomical history. Indeed, if you were to expand the mirror to the size of the United States (five thousand kilometers across), you would see only a beautiful smooth curved surface, with no little dips or bumps any bigger than a few centimeters in size! But the overall shape was not the correct one to bring all the rays of light to a perfect focus. Needless to say, this caused a lot of grief and finger-pointing. Who was to blame? Subsequent reconstructions of the procedures followed showed that Perkin-Elmer carried out various optical tests, at least one of which showed that there were problems. But these warning signs were ignored in the face of other apparently more reassuring test results. For several years, HST worked with its optics in an ``out-of-focus" way. There were still some astronomical programs which could be carried out. (In fact, I and some colleagues had some HST time awarded to us, but we had to tell the telescope schedulers to postpone our observing until after the repair mission since our scientific objectives really required the best images. Eventually a solution was found -- but not, as you might think, the extreme one of replacing the primary mirror, which would be too big and expensive a job to carry out in space. Instead, a telephone-booth sized piece of complicated optics, with various mirrors and lenses, was designed to be inserted into the HST. (This module was named COSTAR, an acronym that stands for C orrective O ptics...etc.) Then, when various observations are to be made using the different instruments on board, the appropriate piece of `correcting optics' is automatically inserted into the path of the light to put the photons back into the right places. The repair mission, now quite a few years ago, was a great success, and the HST has continued to carry out lots of exciting and important science. (We will encounter a lot of that astronomy later in Phys 016 when we discuss galaxies and cosmology in particular.) Let me remind you of its advantages: it is unaffected by atmospheric turbulence, so can give images which are as good as the wave nature of light permits. As noted, however, we can now do this from the ground, thanks to adaptive optics. Moreover, from the ground we can use even bigger telescopes (the Shuttle could not lift into orbit anything much bigger than the present HST) so that we can collect more photons than the HST, which is an advantage in some applications. it can work at wavelengths which we cannot study from the ground - the ultraviolet part of the spectrum is particularly important in this respect it is above all of the `airglow', the faint light emitted by the atmosphere of the Earth. This reduces the background and helps the HST see even deeper into space. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Friday, 27 November, 2020.)