Education Activities To Accompany Chandra Data Analysis Software Cas-A
Section I: Contextual Narrative
Cosmic Recycling Centers
"So don't hang on--nothing lasts forever but the Earth and Sky... Dust in the wind--all we are is dust in the wind" -Kansas
The poets and songwriters have always characterized the heavens as unchanging. The stars are immortal, in contrast with the ordinary life processes on Earth. Day lilies and butterflies may be fleeting, but the Sun and stars will endure.
However, even the stars go through definite life cycles, and their story is a fascinating one. After all, every single atom of calcium in every bone in our bodies was produced inside an ancient star, which then exploded and added the processed material to the interstellar medium, some of which later went into forming our Sun, Earth and Solar System, and finally us.
Astronomically, in human terms,the story begins a mere 75 years ago. Scientists, at that time, had no real understanding of the energy sources that allow the stars to shine. All known possibilities, from chemical reactions (like burning wood in a fire) to utilizing the gravitational potential energy stored in the star (i.e. having the star contract to smaller and smaller sizes as it radiated) fell woefully short of the required energy. All these energy sources could power the stars for a mere 1000 to 10,000,000 years. However, our knowledge of the age of the rocks and the Earth indicated several billion years of existence for the Sun.
Nuclear physics has provided the missing link in the chain of knowledge which describes the structure and evolution of the stars. So many observational puzzles have been explained once the hypothesis of nuclear burning was adopted, that there can scarcely be any doubt that these enormous balls of fiery gas are powered by insignificant, sub-atomic particles so small that it would take about 1 trillion of them lined up end to end to span the head of a pin! The universe is indeed a miraculous place....
The immense energies provided by the nuclear furnaces in the cores of stars are the result of one-way transmutations of elements, beginning with hydrogen to helium. It is these processes which cause the stars to evolve. As the star "cooks" the elements from hydrogen to helium to carbon and oxygen, there is progressively less and less energy available to extract. Once the core of the star reaches iron, the jig is up. No longer can the star replenish its expenditure of radiation, and it must change its structure radically. Depending on its mass, it can either cool down gradually, dying like an ember in a fire, or go out in a blaze of glory with a catastrophic explosion, becoming a supernova. During this explosion, which lasts only minutes, the released energy is so great that for a short while, the star outshines the entire galaxy of which it is part; imagine, an object shining brighter than the Sun by a hundred billion times. Although the actual explosion lasts for less than a day, the effects linger for centuries. The gas from the explosion hurtles outward at speeds approaching that of light, and it begins to plow through the space between the stars. We can see the accumulation of material (called a supernova remnant) still expanding today, even when the original explosion occurred thousands of years ago.
Often, the explosion leaves behind a strange object. The very center of the star does not disperse, but forms a neutron star. This is an object that has more mass than the Sun, but occupies a volume no bigger than the city of Boston. Its density is truly astounding; one thimbleful of its material would weigh as much as 10 million full sized African elephants. This compact object usually spins on its axis ten to a hundred times per second, and is called a pulsar (even though it doesn't pulse at all, but rotates instead!). As the pulsar slows down over the centuries, it adds electron and other charged particles to the interstellar "soup" and provides the energy we see radiating towards us today from all parts of the remnant.
Since such high energies and temperatures are involved, it is not surprising that these objects radiate copious amounts of X-rays. The pictures we get from these objects tell us many things. Not only do we get an idea about the star that exploded, we also find out much about the interstellar medium itself as the star's energy sweeps up and accelerates the once calm environment surrounding the star. The more detailed the picture we get from these objects, the better our understanding. So we try to get data from all parts of the electromagnetic spectrum, including x-rays. The problem is that x-rays are hard to focus. Instead of passing through lenses, or forming an ordinary image with mirrors, the x-rays get absorbed, and we see nothing. But about 20 years ago, we learned how to focus x-rays using grazing incidence mirrors. The results were astonishing. And the improvements kept coming until now we have the superb optics of the CHANDRA satellite.
Section II: Loading Cas-A
Start ds9, connect to the Virtual Observatory through the Analysis menu.
Select "he" for the color display, and "linear" for the Scale. (This is just to provide a convenient way of looking at the object.) You can also play around with the different display parameters.
Center click on the bright dot near the upper left part of the display. This will center the object. You have just clicked on the "pulsar", seen for the first time with the Chandra satellite!
