Ultraviolet
| Ultraviolet (UV) light has shorter
wavelengths than visible light. Though these waves are invisible to
the human eye, some insects, like bumblebees, can see them! (Image
of the bumblebee is courtesty of Mark Cassino.)
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Scientists have divided the ultraviolet part of the
spectrum into three regions: the near ultraviolet, the far ultraviolet,
and the extreme ultraviolet. The three regions are distinguished by how
energetic the ultraviolet radiation is, and by the "wavelength" of the
ultraviolet light, which is related to energy.
The near ultraviolet, abbreviated NUV, is the light closest to optical
or visible light. The extreme ultraviolet, abbreviated EUV, is the
ultraviolet light closest to X-rays, and is the most energetic of the
three types. The far ultraviolet, abbreviated FUV, lies between the near
and extreme ultraviolet regions. It is the least explored of the three
regions.
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Our Sun emits light at all the different
wavelengths in electromagnetic spectrum, but it is ultraviolet waves
that are responsible for causing our sunburns. To the left is an
image of the Sun taken at an Extreme Ultraviolet wavelength - 171
Angstroms to be exact. (An Angstrom is a unit length equal to
10-10 meters.) This image was taken by a satellite named
SOHO and it shows what the Sun looked like on April 24, 2000.
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Though some ultraviolet waves from the Sun penetrate Earth's
atmosphere, most of them are blocked from entering by various gases like
Ozone. Some days, more ultraviolet waves get through our atmosphere.
Scientists have developed a UV index to help people protect themselves
from these harmful ultraviolet waves.
How do we "see" using Ultraviolet light?
It is good for humans that we are protected from getting too much
ultraviolet radiation, but it is bad for scientists! Astronomers have to
put ultraviolet telescopes on satellites to measure the ultraviolet light
from stars and galaxies - and even closer things like the Sun!
There are many different satellites that
help us study ultraviolet astronomy. Many of them only detect a
small portion of UV light. For example, the Hubble Space Telescope
observes stars and galaxies mostly in near ultraviolet light. NASA's
Extreme Ultraviolet Explorer satellite is currently exploring the
extreme ultraviolet universe. The International Ultraviolet Explorer
(IUE) satellite has observed in the far and near ultraviolet regions
for over 17 years. |
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What does Ultraviolet light show us?
We can study stars and galaxies by studying the UV light they give off
- but did you know we can even study the Earth? Below is an unusual image
- it is a picture of Earth taken from a lunar observatory! This
false-color picture shows how the Earth glows in ultraviolet (UV) light.
| The Far UV Camera/Spectrograph deployed
and left on the Moon by the crew of Apollo 16 took this picture. The
part of the Earth facing the Sun reflects much UV light. Even more
interesting is the side facing away from the Sun. Here, bands of UV
emission are also apparent. These bands are the result of aurora
caused by charged particles given off by the Sun. They spiral
towards the Earth along Earth's magnetic field lines. |
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Many scientists are interested in studying the invisible
universe of ultraviolet light, since the hottest and the most active
objects in the cosmos give off large amounts of ultraviolet energy.
The image below shows three different galaxies taken in visible light
(bottom three images) and ultraviolet light (top row) taken by NASA's
Ultraviolet Imaging Telescope (UIT) on the Astro-2 mission.
The difference in how the galaxies appear is due to which type of stars
shine brightest in the optical and ultraviolet wavelengths. Pictures of
galaxies like the ones below show mainly clouds of gas containing newly
formed stars many times more massive than the sun, which glow strongly in
ultraviolet light. In contrast, visible light pictures of galaxies show
mostly the yellow and red light of older stars. By comparing these types
of data, astronomers can learn about the structure and evolution of
galaxies.
RETURN TO THE ELECTROMAGNETIC SPECTRUM
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