Gamma-rays have the smallest wavelengths
and the most energy of any other wave in the
electromagnetic spectrum. These waves are
generated by radioactive atoms and in
nuclear explosions. Gamma-rays can kill
living cells, a fact which medicine uses to
its advantage, using gamma-rays to kill
Gamma-rays travel to us across vast
distances of the universe, only to be
absorbed by the Earth's atmosphere.
Different wavelengths of light penetrate the
Earth's atmosphere to different depths.
Instruments aboard high-altitude balloons
and satellites like the Compton Observatory
provide our only view of the gamma-ray sky.
Gamma-rays are the most energetic form of
light and are produced by the hottest
regions of the universe. They are also
produced by such violent events as supernova
explosions or the destruction of atoms, and
by less dramatic events, such as the decay
of radioactive material in space. Things
like supernova explosions (the way massive
stars die), neutron stars and pulsars, and
black holes are all sources of celestial
How do we "see" using gamma-ray light?
Gamma-ray astronomy did not develop until
it was possible to get our detectors above
all or most of the atmosphere, using
balloons or spacecraft. The first gamma-ray
telescope, carried into orbit on the
Explorer XI satellite in 1961, picked up
fewer than 100 cosmic gamma-ray photons!
Unlike optical light and X-rays, gamma
rays cannot be captured and reflected in
mirrors. The high-energy photons would pass
right through such a device. Gamma-ray
telescopes use a process called Compton
scattering, where a gamma-ray strikes an
electron and loses energy, similar to a cue
ball striking an eight ball.
||This image shows the
CGRO satellite being deployed from
the Space Shuttle orbiter. This
picture was taken from an orbiter
window. The two round protrusions
are one of CGRO's instruments,
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