X-rays
As the wavelengths of
light decrease, they increase in energy. X-rays have smaller wavelengths and therefore
higher energy than ultraviolet waves. We usually talk about X-rays in terms of their
energy rather than wavelength. This is partially because X-rays have very small
wavelengths. It is also because X-ray light tends to act more like a particle than a
wave. X-ray detectors collect actual photons of X-ray light - which is very different
from the radio telescopes that have large dishes designed to focus radio waves!
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).
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When you get an X-ray taken at a hospital, X-ray sensitive
film is put on one side of your body, and
X-rays are shot through you. At a dentist, the film is put inside your
mouth, on one side of your teeth, and X-rays are shot through your jaw,
just like in this picture. It doesn't hurt at all - you can't feel X-rays.
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Because your bones and teeth are dense and absorb
more X-rays then your skin does, silhouettes of your bones or teeth
are left on the X-ray film while your skin appears transparent. Metal
absorbs even more X-rays - can you see the filling in the image of the
tooth?
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When the Sun shines on us at a certain angle, our shadow is projected onto
the ground. Similarly, when X-ray light shines on us, it goes through our
skin, but allows shadows of our bones to be projected onto and captured by
film.
This is an X-ray photo of a one year
old girl. Can you see the shadow of what she swallowed?
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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.
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!
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What does X-ray light show us?
Many things in space emit X-rays, among them are black holes, neutron
stars, binary star systems, supernova
remnants, stars, the Sun, and even some comets!
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.
Credit: Polar, PIXIE, NASA
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To the left is the first picture of the Earth in X-rays,
taken in March, 1996 with the orbiting Polar satellite. The area
of brightest X-ray emission is red.
The energetic charged particles from the Sun that cause aurora also
energize electrons in the Earth's magnetosphere. These electrons move
along the Earth's magnetic field and eventually strike the Earth's ionosphere,
causing the X-ray emission. These X-rays are not dangerous because they
are absorbed by lower parts of the Earth's atmosphere. (The above caption
and image are from the Astronomy Picture of the Day for December 30, 1996.)
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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.) |
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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.
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Many things in deep space give off X-rays. Many stars are in binary
star systems - which means that two stars orbit each other.
When one of these stars is a black hole or a neutron star, material is
pulled off the normal star. This materials spirals into the black hole or
neutron star and heats up to very high temperatures. When something is heated
to over a million degrees, it will give off X-rays!
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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.
Credit: X-ray (NASA/CXC/SAO); Optical (NASA/HST);
Radio: (CSIRO/ATNF/ATCA)
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This image is special - it shows a supernova remnant - the remnant of a star
that exploded in a nearby galaxy known as the Small Magellanic Cloud.
The false-colors show what this supernova remnant looks like in X-rays
(in blue), visible light (green) and radio (red).
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Credit: NASA/CXC/SAO
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This is the same supernova remnant but this image shows only X-ray
emission.
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