Date: February 21, 2009
Title: An Introduction to Chandra and the X-ray Universe
Podcaster: April Hobart
Organization: Chandra X-Ray Center http://chandra.harvard.edu
Description: NASA’s Chandra X-ray Observatory, in orbit since 1999, studies the high-energy Universe, where black holes, exploding stars, and mysterious matter hold sway. X-ray telescopes like Chandra are not like the telescopes you find in backyards or at the local observatory. In addition to being above the Earth’s atmosphere, they need to have special mirrors to detect the X-rays that pass through most objects.
Bio: Chandra X-ray Center is operated for NASA by the Smithsonian Astrophysical Observatory.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Bobby Lee Litchfield, and dedicated to the Chandra X-ray Observatory and all X-ray images, medical and astronomical. Bobby Lee works at Masterplan as a Senior Radiology Specialist, and reminds us that in X-ray, it’s all about the image.
Transcript:
Narrator: When the Chandra X-ray Observatory was launched aboard the Space Shuttle Columbia on July 23rd, 1999, it began a new era of X-ray Astronomy.
NASA: 5, 4, 3, we have a go for engine start, zero, we have booster ignition and liftoff of Columbia, reaching new heights for women and X-ray Astronomy.
Narrator: But what do X-rays from space tell us, and why do we build telescopes to study them? Most people think of light as what we can see with our human eyes. But in truth our eyes can detect just a small fraction of the radiation emitted by objects in space. Martin Elvis, a senior scientist at the Chandra X-ray Center in Cambridge, Massachusetts, discusses why astronomers want to study the sky in X-ray light.
Martin Elvis: So what’s X-ray astronomy? You look up at the night sky and you see stars all over the place, stars like our own sun, big ones, small ones, old ones, young ones, but basically all stars shining with fusion power, nuclear fusion. We only see stars as dominating the night sky because we look in this really narrow band of the whole electromagnetic spectrum called the optical range. If you look well outside this range in either long wavelengths or short wavelengths, you don’t see the same stars at all, so we tend to talk about X-ray sources. And they’re really not stars. The sky in X-rays is not dominated by stars. Instead, it’s dominated by something entirely different, that’s powered not by nuclear fusion but by another process which is many times more efficient at converting mass into radiation than nuclear fusion is. Since nuclear fusion is the most efficient process we have around us on earth, that’s pretty impressive. So X-ray astronomy in one sense is just trying to answer the question, “What is this stuff? When we look at the sky in X-rays, what do we see?” And it’s something entirely new, something you would never have guessed just by looking up at the night sky.
Narrator: One of the most exciting things about X-ray astronomy is that it’s really in its infancy. The first X-ray satellites weren’t launched until the 1960’s. Why the late start? The answer is because X-rays from space are absorbed by the Earth’s atmosphere. Therefore, X-ray astronomy had to wait for the Space Age and the development of rockets to get its detectors into orbit. And what does Chandra see in the X-ray universe? When matter is heated to really high temperatures, as in millions of degrees, X-rays are produced. Here’s more from Martin.
Martin Elvis: The energy of each photon of light in X-rays is about a thousand times that in optical light, visible light. So it also follows that the temperatures that we deal with in X-rays are about a thousand times hotter, so you’re looking for processes that are a thousand times more energetic.
Narrator: So in order to understand what is happening in the hot, turbulent regions of space, we need to have X-ray telescopes. Chandra is using its powerful telescope to study such things as material around black holes, the debris from exploded stars, and hot gas that pervades the space between the galaxies. These are phenomena and objects that would simply be invisible in other wavelengths.
Narrator: NASA’s Chandra X-ray Observatory, in orbit since 1999, studies the high-energy Universe, where black holes, exploded stars, and mysterious matter hold sway. X-ray telescopes like Chandra are not like telescopes you find in backyards or at the local observatory. In addition to being above the Earth’s atmosphere, they need to have special mirrors to detect the X-rays that pass through most objects. Let’s listen to scientist Martin Elvis explain more about Chandra’s technology.
Martin Elvis: The main thing Chandra does is take these superb, sharp images. How does it do that? Well, X-ray telescopes are different from optical telescopes. They have a very different shape, although, in fact, they’re really reflecting light in just the same way as optical light, its just that with X-rays, you have to coax them into being reflected. If you have a normal mirror, you look at yourself in the mirror, and the light’s going in and straight back, so it’s being bounced through 180 degrees. If you try that with X-rays, they just get absorbed, but you can get specular reflection if you come in at a grazing angle of a degree or less. Once you get that specular reflection, X-rays act just like optical light. You can concentrate them and focus them, no problem. Trouble is you’re only bending the light through one degree on each reflection and we end up having two reflections in our mirrors. That means that the light’s only coming together very, very slowly, so we tend to have very long telescopes, most of it being just empty space. We’re just waiting for the light to converge down to its focus. The bad thing about these mirrors is you’re looking almost edge or end-on at a cylinder, so the area of glass that the light’s reflecting off is only a thin annulus. So what we do is pile a whole bunch of telescopes nested one inside the other to build up the area, but basically you still have to polish a hundred times as much glass as you would for a normal optical telescope. So this 1.2 meter diameter Chandra mirror is focusing light down onto an exquisite point just a thousandth of an inch across. That’s why Chandra’s powerful.
Narrator: In addition to its special mirrors, Chandra also travels in an unusual orbit around the Earth. Unlike its partner mission, the Hubble Space Telescope, Chandra cannot be serviced by astronauts. That’s because it does not circle relatively closely to Earth as Hubble does. Martin Elvis explains more about why Chandra travels in unusual circles, or, more accurately, ellipses.
Martin Elvis: Chandra doesn’t just go above the atmosphere. It goes a third of the way to the moon, getting well away from the Earth. But it can’t get above the atmosphere twice, so why do we bother? The answer is to be much more efficient at observing. It’s only a small telescope and we tend to have to observe a long time. But if we’re down where the space station is or the shuttle can get to, then wherever you want to look, half the time the Earth is in the way, not what you want if you’re looking with an X-ray telescope. So instead, if you can afford the energy to push you way out there, the Earth looks very small, and you can point almost anywhere without it getting in the way. That’s very useful for many observations, but mainly it doubles the efficiency of Chandra.
Narrator: Now that we’ve heard a little about how Chandra works, let’s listen to Martin give us an introduction to how X-rays are produced in the Universe.
Martin Elvis: There are three different ways you can get matter to be that hot. One is simply an explosion, like a supernova, such as the one in 1987 in the large Magellanic Cloud. What we see there is very fast-moving gas that has hit material outside and is now glowing with a shock at a few million degrees. The next way you can make X-rays is a more complicated process, and that’s by having very fast-moving charged particles in a magnetic field. A basic law of physics is that any charged particle moving in a magnetic field gets swirled around and in doing so it’s accelerating around a bend, and accelerating charge radiates. It turns out there are lots of places in the universe where we get magnetic fields and very fast-moving, (we call them, relativistic) particles. They’re moving very close to the speed of light. The Crab Nebula, for instance, is powered by a pulsar at the center which has so much energy in it that the little wisps and things you see in the image which look like they’re sort of swirling around the nebula, they aren’t swirling at all. They’re moving outwards at extraordinary velocities. This type of X-ray-making mechanism we find very commonly also in quasars and blazars, which are things with these huge jets that come out, maybe many times the size of a whole galaxy, and these are powered in X-rays by the same mechanism. The third way of making X-rays is perhaps the least likely. It’s just dropping something down a hole. If you have a lot of mass somewhere, like a planet or, better still, a neutron star or a black hole, and you drop something in, then it speeds up, and when it hits the surface or some other gas coming from a different direction, it heats up. A spaceship reentering the Earth’s atmosphere does the same thing. It starts glowing very hot. The spacecraft is generating heat through friction with the air and slowing down. And thats just transferring the energy of its motion into heat. So it’s a very simple process really. It just turns out that most of the X-ray sources in the sky are powered this way.
Written by: Smithsonian Astrophysical Observatory/Center for Astrophysics/Megan Watzke & Martin ElvisProduced by: SAO/CFA/April Hobart
End of podcast:
365 Days of Astronomy
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