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).
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.
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?
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?
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!
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
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.)
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.
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!
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 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).
Credit: NASA/CXC/SAO
This is the same
supernova remnant but this image shows only
