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To the right is an image of a real X-ray detector. This instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. It looks very different from anything you might see at a dentist’s office!
X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident when experimenting with vacuum tubes.
|A week later, he took an X-ray photograph of his wife’s hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen called it “X” to indicate it was an unknown type of radiation. The name stuck, although (over Roentgen’s objections), many of his colleagues suggested calling them Roentgen rays. They are still occasionally referred to as Roentgen rays in German-speaking countries.|
The Earth’s atmosphere is thick enough that virtually no X-rays are able to penetrate from outer space all the way to the Earth’s surface. This is good for us but also bad for astronomy – we have to put X-ray telescopes and detectors on satellites! We cannot do X-ray astronomy from the ground.
How do we “see” using X-ray light?
What would it be like to see X-rays? Well, we wouldn’t be able to see through people’s clothes, no matter what the ads for X-ray glasses tell us! If we could see X-rays, we could see things that either emit X-rays or halt their transmission. Our eyes would be like the X-ray film used in hospitals or dentist’s offices. X-ray film “sees” X-rays, like the ones that travel through your skin. It also sees shadows left by things that the X-rays can’t travel through (like bones or metal).
We use satellites with X-ray detectors on them to do X-ray astronomy. In astronomy, things that emit X-rays (for example, black holes) are like the dentist’s X-ray machine, and the detector on the satellite is like the X-ray film. X-ray detectors collect individual X-rays (photons of X-ray light) and things like the number of photons collected, the energy of the photons collected, or how fast the photons are detected, can tell us things about the object that is emitting them.
What does X-ray light show us?
The Earth glows in many kinds of light, including the energetic X-ray band. Actually, the Earth itself does not glow – only aurora produced high in the Earth’s atmosphere. These aurora are caused by charged particles from the Sun.
|Recently, we learned that even comets emit X-rays! This image of Comet Hyakutake was taken by an X-ray satellite called ROSAT, short for the Roentgen Satellite. (It was named after the discoverer of X-rays.)|
|The Sun also emits X-rays – here is what the Sun looked like in X-rays on April 27th, 2000. This image was taken by the Yokoh satellite.|
The above image is an artist’s conception of a binary star system – it shows the material being pulled off the red star by its invisible black hole companion and into an orbiting disk.
|This is the same supernova remnant but this image shows only X-ray emission.|