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).
|
|
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.
Credit: X-ray (NASA/CXC/SAO);
Optical (NASA/HST);
Radio: (CSIRO/ATNF/ATCA)
|
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 X-ray emission.
|
RETURN TO THE ELECTROMAGNETIC SPECTRUM
|
