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 often 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, which later
went into forming our Sun, Earth and Solar System.
Astronomically, in human terms of discovery, 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 required several billion years for
the Sun’s existence.
Nuclear physics has provided the missing link in our chain of
knowledge which determines 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, and become 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. (Refer to Crab Nebula story here).
Often, the explosion leaves behind a strange object. The very center of the
star does not disperse, but forms a new entity, 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. Indeed, our earliest x-ray
“telescopes” did not focus or image x-ray light at all. They collected
x-rays without making a picture at all. Imagine that you didn’t have lenses
in your eyes, but you still had a retina (your “detector”) that was sensitive
to light. How could you tell where objects were? What you could do is try
to look for light through a long tube, such as a paper towel roll. While
you couldn’t see any details concerning the shape of the object you were
looking at, you could at least determine the directions (roughly) where the
light seemed to be coming from. Exactly the same problem happens with
x-rays, and our earliest rockets and satellites could only tell where these
x-ray emitting objects were in the crudest of ways.
But about 20 years ago, we learned how to focus x-rays using
grazing incidence mirrors (see RU applet on x-ray telescopes here).
The results were astonishing. And the improvements kept coming until
now we have the superb optics of the CHANDRA satellite. First, look at the
x-ray light from Cas-A, a bright supernova remnant, using the Roentgen Satellite (ROSAT, figure 1) launched
10 years ago. You can see some features where the x-rays seem brighter in
some places.
Now, we will analyze the recent CHANDRA observations of this
object, and compare the two results.
1) Open this file in DS9
You will see in the lower left hand corner a piece of the SNR.
The object is so big that the regular display is not suitable for
looking at the whole remnant. So we will change the display...
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.
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.
Compare this picture with the ROSAT result. The differences are
remarkable. The clarity and resolution of the CHANDRA images will allow
us to map in detail these fascinating objects. And look at the very
center of the CHANDRA image. The pointlike object is the
“pulsar”, seen for the first time ever with this satellite! But, we haven’t been able to find the
“clock” period yet. Why? We don’t know. But stay tuned, the answer will
undoubtedly be exciting.
This remnant has been expanding for over 300 years at incredibly high
speed, so by now
it is quite large. One would expect that such a large region
(extending over a distance equal to that of the Sun to the nearest
stars) could not change its brightness very rapidly. To see this more clearly,
imagine you are in a football stadium, and a team scores a touchdown. A roar
goes up from the crowd, and even though they might stop yelling at about the
same time, it will take some rather long interval for the noise to subside,
since some parts of the stadium are farther away from you, and hence will “send”
you that information later than the nearer parts. So even if our supernova
remnant was dying out (and it is, over many, many centuries), we would expect
that the light curve (the plot of brightness vs. time) that we see from the
whole expanding ball would show little if any variation. And in fact, that
is the case, as we see in Figure 2. So unlike our “clock in the sky”
x-ray source, the emission from Cas-A, our cosmic recycling center, seems
very dull and uninteresting.
But that is not all the information we collect about these objects.
We can also tell the “color” of the x-ray light. Just as a blue flame is
hotter than a red flame, x-rays can tell us the state of the emitting
region too. And when we look at the energy of all the x-ray photons that Chandra can collect, we get a remarkable result.
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.
6) Go to “ANALYSIS” and select “Load Analysis commands” Select “funtools.ds9.orig” as the command file
7) go back to “analysis” and select “Do energy spectrum (.1-10 kev)
What we see here is that superimposed
on a continuous background of x-ray light, there are fingerprints of the
elements in the remnant. Like a prism that takes sunlight and makes a
“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
chemical composition of the Sun (refer to spectral line formation here?), so
the CHANDRA energy spectrum tells us in x-rays about the recycled material from
our supernova. It’s all there, the building blocks of life: Calcium, oxygen,
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
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.
So, we have come full circle; the ancient star that once shone in the
night sky billions of years ago, has exploded and sent out the material
for future stars into space, where someday, possibly billions of years
from now, the calcium we see in the spectrum may ultimately form into
alien bones, and the oxygen may form part of a planetary atmospere where life
may flourish. So even though our Sun may
die, the seeds for its rebirth are contained within itself, to be recycled,
possibly endlessly, through the vastness of space and time.
“To everything, turn, turn,turn
There is a season, turn, turn, turn
And a time to every purpose under heaven...
A time to be born, a time to die...
A time to build up, a time to break down...
A time to cast away stones,
A time to gather stones together...” ---The Byrds