{\rtf1\ansi\deff0{\fonttbl{\f0\fnil\fcharset0 Courier New;}} \viewkind4\uc1\pard\lang1033\f0\fs20\tab\tab\tab COSMIC RECYCLING CENTERS\par \par \par "So don't hang on--nothing lasts forever but the Earth and Sky...\par Dust in the wind--all we are is dust in the wind" --Kansas\par \par \tab The poets and songwriters have often characterized the heavens\par as unchanging. The stars are immortal, in contrast with the ordinary life\par processes on Earth. Day lilies and butterflies may be fleeting, but the \par Sun and stars will endure.\par \tab However, even the stars go through definite life cycles, and their\par story is a fascinating one. After all, every single atom of calcium in every \par bone in our bodies was produced inside an ancient star, which then exploded \par and added the processed material to the interstellar medium, which later \par went into forming our Sun, Earth and Solar System.\par \tab Astronomically, in human terms of discovery, the story begins a mere 75 years ago. \par Scientists, at that time, had no real understanding of the energy sources \par that allow the stars to shine.\par All known possibilities, from chemical reactions (like burning wood in a\par fire) to utilizing the gravitational potential energy stored in the star\par (i.e. having the star contract to smaller and smaller sizes as it radiated)\par fell woefully short of the required energy. All these energy sources could\par power the stars for a mere 1000 to 10,000,000 years. However, our knowledge\par of the age of the rocks and the Earth required several billion years for \par the Sun's existence.\par \tab Nuclear physics has provided the missing link in our chain of \par knowledge which determines the structure and evolution of the stars.\par So many observational puzzles have been explained once the hypothesis\par of nuclear burning was adopted, that there can scarcely be any doubt that\par these enormous balls of fiery gas are powered by insignificant, sub-atomic particles\par so small that it would take about 1 trillion of them lined up end to end to span\par the head of a pin! The universe is indeed a miraculous place....\par \tab The immense energies provided by the nuclear furnaces in the cores of\par stars are the result of one-way transmutations of elements, beginning with\par hydrogen to helium. It is these processes which cause the stars to evolve.\par As the star "cooks" the elements from hydrogen to helium to carbon and\par oxygen, there is progressively less and less energy available to extract.\par Once the core of the star reaches iron, the "jig is up". No longer can the\par star replenish its expenditure of radiation, and it must change its structure\par radically. Depending on its mass, it can either cool down gradually, dying\par like an ember in a fire, or go out in a blaze of glory with a catastrophic\par explosion, and become a supernova. During this explosion, which lasts only\par minutes, the released energy is so great that for a short while, the star\par outshines the entire galaxy of which it is part. Imagine, an object shining\par brighter than \par the Sun by a hundred billion times. Although the actual explosion lasts for\par less than a day, the effects linger for centuries. The gas from the explosion\par hurtles outward at speeds approaching that of light, and it begins to plow\par through the space between the stars. We can\par see the accumulation of material (called a supernova remnant)\par still expanding today, even when the original\par explosion occurred thousands of years ago. (Refer to Crab Nebula story here).\par \tab Often, the explosion leaves behind a strange object. The very center of the\par star does not disperse, but forms a new entity, a neutron star. This is an object that has\par more mass than the Sun, but occupies a volume no bigger than the city of Boston.\par Its density is truly astounding; one thimbleful of its material would weigh\par as much as 10 million full sized African elephants. This compact object usually\par spins on its axis ten to a hundred times per second, and is called a pulsar\par (even though it doesn't pulse at all, but rotates instead!). As the pulsar\par slows down over the centuries, it adds electron and other charged particles to\par the interstellar "soup" and provides the energy we see radiating towards us\par today from all parts of the remnant.\par \tab Since such high energies and temperatures are involved, it is not surprising that \par these objects radiate copious amounts of X-rays. The pictures we get from these\par objects tell us many things. Not only do we get an idea about the star\par that exploded, we also find out much about the interstellar medium itself as\par the star's energy sweeps up and accelerates the once calm environment\par surrounding the star. \par \tab The more detailed the picture we get from these objects, the better our\par understanding. So we try to get data from all parts of the electromagnetic\par spectrum, including x-rays. The problem is that x-rays are hard to focus.\par Instead of passing through lenses, or forming an ordinary image with mirrors, the\par x-rays get absorbed, and we see nothing. Indeed, our earliest x-ray\par "telescopes" did not focus or image x-ray light at all. They collected\par x-rays without making a picture at all. Imagine that you didn't have lenses\par in your eyes, but you still had a retina (your "detector") that was sensitive\par to light. How could you tell where objects were? What you could do is try\par to look for light through a long tube, such as a paper towel roll. While\par you couldn't see any details concerning the shape of the object you were\par looking at, you could at least determine the directions (roughly) where the\par light seemed to be coming from. Exactly the same problem happens with\par x-rays, and our earliest rockets and satellites could only tell where these\par x-ray emitting objects were in the crudest of ways.\par \tab But about 20 years ago, we learned how to focus x-rays using\par grazing incidence mirrors (see RU applet on x-ray telescopes here).\par The results were astonishing. And the improvements kept coming until\par now we have the superb optics of the CHANDRA satellite. First, look at the\par x-ray light from Cas-A, a bright supernova remnant, using the Roentgen Satellite (ROSAT, figure 1) launched\par 10 years ago. You can see some features where the x-rays seem brighter in\par some places.\par \pard\li1440\par \par Now, we will analyze the recent CHANDRA observations of this\par object, and compare the two results.\par \par File: acisf01512N001_evt2.fits\par \par 1) Open this file in DS9\par \par You will see in the lower left hand corner a piece of the SNR.\par The object is so big that the regular display is not suitable for \par looking at the whole remnant. So we will change the display...\par \par 2) Select the "BIN" button, and change (at the bottom) the display from 1024x1024 pixels to 2048x2048. Now you can see the entire Cas-A x-ray region.\par \par 3) Center the SNR by left-clicking and dragging the blue box in the "snapshot" region in the upper right hand part of the DS9 window display.\par \par \par \pard Compare this picture with the ROSAT result. The differences are\par remarkable. The clarity and resolution of the CHANDRA images will allow\par us to map in detail these fascinating objects. And look at the very\par center of the CHANDRA image. The pointlike object is the\par "pulsar", seen for the first time ever with this satellite! But, we haven't been able to find the\par "clock" period yet. Why? We don't know. But stay tuned, the answer will\par undoubtedly be exciting.\par \tab This remnant has been expanding for over 300 years at incredibly high\par speed, so by now\par it is quite large. One would expect that such a large region\par (extending over a distance equal to that of the Sun to the nearest\par stars) could not change its brightness very rapidly. To see this more clearly,\par imagine you are in a football stadium, and a team scores a touchdown. A roar\par goes up from the crowd, and even though they might stop yelling at about the\par same time, it will take some rather long interval for the noise to subside,\par since some parts of the stadium are farther away from you, and hence will "send"\par you that information later than the nearer parts. So even if our supernova \par remnant was dying out (and it is, over many, many centuries), we would expect\par that the light curve (the plot of brightness vs. time) that we see from the \par whole expanding ball would show little if any variation. And in fact, that\par is the case, as we see in Figure 2. So unlike our "clock in the sky"\par x-ray source, the emission from Cas-A, our cosmic recycling center, seems\par very dull and uninteresting.\par \tab But that is not all the information we collect about these objects.\par We can also tell the "color" of the x-ray light. Just as a blue flame is\par hotter than a red flame, x-rays can tell us the state of the emitting\par region too. And when we look at the energy of all the x-ray photons that Chandra can collect, we get a remarkable result. \par \par \pard\li1440\par \par 4) Left click and hold on the pulsar; drag the mouse outward so the circle that is displayed encompasses all of the remnant. You have now created a "region" that will allow you to look at only those x-ray photons that come from within it, and hence from within the remnant itself.\par \par 6) Go to "ANALYSIS" and select "Load Analysis commands"\par Select "funtools.ds9.orig" as the command file\par \par 7) go back to "analysis" and select "Do energy spectrum (.1-10 kev)\par \par \pard\par \par \par What we see here is that superimposed \par on a continuous background of x-ray light, there are fingerprints of the\par elements in the remnant. Like a prism that takes sunlight and makes a\par "rainbow" out of what we think is only yellow light, so the detectors on CHANDRA examine the rainbow of x-rays. And just as that visible rainbow contains information about the\par chemical composition of the Sun (refer to spectral line formation here?), so \par the CHANDRA energy spectrum tells us in x-rays about the recycled material from\par our supernova. It's all there, the building blocks of life: Calcium, oxygen,\par iron.... To see where all this material is in the remnant, we can look at regions in different colors, each representing a different part of the \par spectrum. In figure 3, we can actually see the regions where the different elements predominate. This type of analysis is tremendously useful in our quest to understand the processes taking place in the enrichment of the interstellar medium. \par \par \par \tab So, we have come full circle; the ancient star that once shone in the\par night sky billions of years ago, has exploded and sent out the material\par for future stars into space, where someday, possibly billions of years\par from now, the calcium we see in the spectrum may ultimately form into\par alien bones, and the oxygen may form part of a planetary atmospere where life\par may flourish. So even though our Sun may\par die, the seeds for its rebirth are contained within itself, to be recycled,\par possibly endlessly, through the vastness of space and time.\par \par "To everything, turn, turn,turn\par There is a season, turn, turn, turn\par And a time to every purpose under heaven...\par A time to be born, a time to die...\par A time to build up, a time to break down...\par A time to cast away stones,\par A time to gather stones together..." ---The Byrds\par \par \par \par }