#2531Feature: Swift Mission
Like every other star in the universe, our sun is headed for an energy
crisis. In another six billion years or so, it will run out of gas. What
happens when the last of a star’s fuel is gone? Some stars die quietly,
as astronomers expect the Sun will do in the distant future. Others explode
with tremendous fury, briefly shining with the light and heat of a billion
or even a trillion stars. Orbiting telescopes like NASA’s Chandra
X-Ray Observatory and the soon-to-be-launched Swift Observatory are literally
shedding new light on how stars live and how they die.
On today’s program, we’ll explore the birth and death of stars.
We’ll also look at the powerful new tools that are opening our eyes to the amazing events in the life of a universe.
Without the heat and light of the sun, nothing could live on Earth. But other than supporting a planet crawling with life, our sun is a fairly ordinary star. Its core is made up almost entirely of two gases---hydrogen and helium. Like a giant controlled nuclear bomb, the sun fuses hydrogen into helium, turning matter into energy in the form of light and heat. Some of that energy travels to Earth as electromagnetic waves.
As you might guess from its name, an electromagnetic wave is made up of both an electric field and a magnetic field. Every type of electromagnetic wave can be described by at least three features---its amplitude, wavelength, and frequency. Think of a wave on a stormy sea. The height of the wave above the water’s surface is its amplitude. The distance from the top of one wave to the top of the next is the wavelength. Now, imagine yourself on a boat watching those waves go by. The number of waves that pass you each second is the frequency. If you’re in a sea of low frequency waves, you can probably expect a relatively calm ride. But as the wave frequency gets higher, watch out! Your boat will get pummeled with waves and the voyage could be violent.
A chart on which all electromagnetic waves are arranged according to their wavelengths and frequencies is called the electromagnetic spectrum. It ranges from very long, low frequency radio waves to microwaves, infrared waves, visible light, ultraviolet rays, X rays, and finally, to gamma rays which have incredibly high frequencies and short wavelengths. The human eye can detect just one tiny portion of the entire spectrum---visible light.
For most of human history, all we knew about our world is what we could see. Then, beginning about 500 years ago, our vision began to expand.
The invention of telescopes allowed astronomers to gaze upon far-off
planets and previously unknown moons in our own solar system. The next
leap in expanding our vision took place in the 20th century as scientists
developed tools to see in wavelengths other than visible light.
During the U.S. war with Iraq, television viewers in the United States saw scenes like this---it’s video footage taken with cameras equipped with night vision. The cameras detect what the human eye can’t---the infrared or heat energy emitted by people and objects.
Stars and other objects in space also emit infrared waves, not to mention x-rays, gamma rays and other wavelengths of energy. Engineers have developed telescopes and cameras that "see" the universe in these other wavelengths. That’s important because astronomers now believe that the universe we can see in visible light is just the tip of the iceberg.
Imagine that all you could see was the color red. Rainbows would have only one color and the sky would appear black, since it gives off very little red light.
The Sun, which emits just a fraction of its total light in the red wavelengths, would appear much dimmer and easier to look at even at noon. Now imagine you can see all colors again. Rainbows once again show their full spectral display. The sky is a glorious blue and the Sun becomes much brighter.
That’s precisely the view that astronomers are beginning to get of our universe. Some cosmic objects are fantastically bright in gamma rays, but are only meager sources of visible light. Since many objects emit most of their energy in wavelengths we can’t see, we need special telescopes that extend our vision, allowing us to perceive the invisible Universe. These tools help us to understand some of the common, yet unexplained events that take place every day in the universe.
One common, but spectacular event, is the death of a star. It happens
when a star uses up the hydrogen in its core. The star’s outer layers
expand and cool down. The cooler gases glow red, and the star is now known
as a red giant. Over thousands of years, the red giant blows off its outer
layers in a super-solar wind, until the naked core of the star is exposed.
The hot core illuminates the expanding gas, forming what is called a planetary
nebula. This is the fate of our very own star, the Sun.
But some stars do not go gently into that good night. Some stars rage against the dying of the light, exploding violently at the ends of their lives. A star like this is called a supernova.
NASA’s Chandra Observatory recently spotted such a star. Chandra detects X-rays, high-energy electromagnetic waves that can come only from very energetic objects. Exploding stars certainly fit that description! In March 2003, astronomers released this image of DEM L71, the remains of an exploded star called a supernova remnant. When the star exploded, it drove a high-speed wave of material into the gas and dust around it. This wave is visible in the visible light image on the right. What is invisible to our eyes, but obvious to Chandra’s X-ray eyes, however, can be seen on the left: the glow of gas heated to ten million degrees filling the supernova remnant.
The death of the star that created DEM L71 was incredibly violent, but even that pales when compared to the explosions of some superstars. Less than two months ago, scientists witnessed what they believe was the death of a truly gigantic star, maybe twenty or thirty times more massive than our sun.
Gamma Ray Bursts
It’s been known since the 1970s that there are enormous explosions
of gamma rays called gamma ray bursts, occurring on a daily basis somewhere
in the Universe. These explosions are truly awesome: in a fraction of
a second, they release as much energy as the Sun will emit over its entire
Despite their violence, these bursts are among the most mysterious objects observed by astronomers. Not much is known about them, partly because the explosions can be over almost as soon as they begin; some last only a few milliseconds. Also, they usually don’t emit much visible light. Even though there are hundreds of bursts every year, they weren’t discovered until astronomers were able to detect gamma rays. These high-energy electromagnetic waves are absorbed by the Earth’s atmosphere, so it takes an orbiting satellite to see them.
In October of 2002, one such satellite managed to catch an elusive burst before it disappeared. Called HETE for High Energy Transient Explorer, the small satellite is designed to locate gamma ray bursts and to quickly alert robotic telescopes on Earth so they can make follow-up observations. NASA’s orbiting Chandra also made observations. Chandra saw tell-tale X-rays, supporting the popular theory that the gamma ray burst was in fact a type of super-supernova, called a hypernova. The evidence collected from around the electromagnetic spectrum has convinced many astronomers that the fleeting gamma ray bursts are mysterious messengers that announce the death of a star--- and perhaps the birth of a black hole.
A black hole is an object of unimaginable density. To even try to imagine
a black hole, you need to understand mass and gravity. Mass is a measure
of the quantity of material that makes up all things including you. Your
bones, skin, blood---every part of your body added together makes up your
mass. Everything with mass also has an attractive force called gravity.
Your weight is a measurement of the force of gravity between you and Earth. This force gets weaker with distance. If you traveled far enough away from Earth, the planet would have very little pull on you. But even though you’d weigh close to nothing, your mass would still be the same.
To escape the gravitational tug of something as big as Earth, an object needs to reach a very high speed. It’s known as the escape velocity. To escape Earth’s gravity, we need to travel at a speed of about 25 thousand miles or---40 thousand kilometers---per hour. To achieve that speed, humans traveling into space need a boost from powerful rockets.
Now consider the escape velocity needed to get away from a black hole. As far back as the 1600s the great scientist Isaac Newton predicted that if an object could be made very small without losing any mass, it would have an extremely strong force of gravity.
A black hole is an extreme form of such an object. A black hole forms when a star goes supernova. While the outer layers of the star explode outwards, the inner core of the star collapses into a stunningly dense object. What once was a giant star, far bigger than our own sun, may shrink to the size of your hometown. Yet, with so much mass packed into a small area, the gravitational force of the black hole is enormous.
To escape from a black hole, an object would have to reach a velocity greater than the speed of light! Consequently, not even light can escape from a black hole!
Astronomers look for clues to black holes by observing stars or gas clouds that appear to be affected by some unseen force. As matter falls toward the black hole, friction and magnetic forces heat the gas to millions of degrees. Such a hot gas emits X-rays. Chandra has observed otherwise normal stars that emit vast amounts of x-rays, indicating that the stars are actually pouring matter into nearby black holes.
Some astronomers believe that the recent findings of the HETE satellite may offer another signpost in the sky. For reasons scientists don’t yet understand, the presence of gamma ray bursts may signal the start of the process that leads to a black hole.
With much left to discover, scientists and engineers continue to design tools to extend our view of the universe. Next December, a new observatory will take its place in Earth’s orbit.
NASA’s Swift observatory is named after a kind of bird known for
its ability to perform amazing strategic maneuvers, turning very rapidly
to catch its prey. The robotic Swift observatory will likewise have some
Now in its construction and testing phase, the observatory features three telescopes. One is a Burst Alert Telescope that patiently waits for the flash of energy signaling the start of a gamma ray burst. When it detects a burst, the entire observatory literally swings into action, swiftly turning in under a minute to point its other telescopes at the burst while it is still in the process of erupting. These other telescopes on Swift collect visible light, ultraviolet light, and X-rays from the burst itself as well as from the afterglow of electromagnetic waves that sometimes linger for months following a burst.
Scientists working on the Swift mission expect it to provide the most detailed picture yet of a universe that lies mostly outside our field of vision.
For more on black holes and gamma ray bursts visit our web page. While you’re there, take our on-line poll. This week’s question: Should NASA reduce its human space flight program in favor of robotic missions?
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