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HOW
ASTRONOMERS USE THE ELECTROMAGNETIC SPECTRUM
The development of instruments to measure wavelengths
of light in all parts of the Electromagnetic
Spectrum
has contributed immensely to science.
We now know that certain animals
snakes) can "see" infrared light. This allows them to
find prey in the dark because thermal energy is emitted in the infrared.
Scientists have developed cameras that allow us to "see" infrared
light. Below is an infrared image of an engineer holding a burning match.
Photo to right: Courtesy of Infrared
Information
Processing Center, California Institute of Technology, Jet
Propulsion Laboratory
"False colors" have been
used to indicate temperature. Note the white and deep red in the
flame and the engineer's palm (where his warm blood vessels are
close to the surface of the skin) and the blue of his cool glasses.
This kind of imaging has been very
important to scientists. These images of the Earth
showing ocean temperatures were taken
from a satellite.
Infrared imaging has aided astronomers,
too. These images show the constellation Orion as it is seen with
visible light (left) and with infrared light (right).
(Photo above: Courtesy of Infrared Information
Processing Center, California Institute of Technology, Jet
Propulsion Laboratory)
The yellow regions in the right
image are hottest and show scientists the hot gasses around stellar
nurseries. The clouds of red are relatively cooler interstellar
gas and dust. Little of this information is available from visible
light.
Other animals (like bees) can "see" ultraviolet. A bee's view
of flowers is remarkably different than ours. The following image
shows a flower of a species of plant pollinated by bees.
In many ways this flower is unremarkable. However, a picture of the
same flower taken with film sensitive to ultraviolet radiation looks
very different. The pattern in ultraviolet guides a
bee to pollen and nectar. (Photo's: Courtesy of Nectar
Guides)
Detectors sensitive to ultraviolet light guide astronomers to new information.
Peter Bunclark took this image of Galaxy M101 in visible light.
The image below is another view of Galaxy M101 taken by the Ultraviolet Imaging
Telescope (UIT) onboard the Space Shuttle Endeavour. UIT is an instrument
sensitive to ultraviolet light. The image has been computer processed
so that the colors represent the intensity of ultraviolet light.
This shows mainly clouds of gas containing newly formed stars many times
more massive than the sun, which glow strongly in ultraviolet light.
The ultraviolet image shows astronomers much more about new star
formation than the photograph taken in visible light. (Photo: courtesy
of Ultraviolet Imaging Telescope)
The Solar
and Heliospheric Observatory (SOHO) is a project of international
cooperation between the European Space Agency and NASA. Among
its many scientific contributions, it enhances our knowledge by
photographing the Sun in several wavelengths nearly at the same
time. The following images were taken within 2 hours of each other.
All colors are "false colors" chosen to help scientists
know what wavelength is being viewed. The brightness corresponds
to intensity of radiation.
The Michelson Doppler Imager (MDI) aboard SOHO took the image below.
This image shows the Sun as it looks in the visible wavelengths.
The scale at the bottom relates color to temperature.
The dark areas are sunspots and the bright regions are faculae. These are
regions of complex
solar activity.
The Extreme Ultraviolet Imaging
Telescope (EIT) took this image at an ultraviolet
wavelength of 304Å.
Scientists
know that light emitted at this wavelength is predominantly emitted
by helium that has lost one electron (ionized) at 60,000 to 80,000°C.
Knowing the wavelength of light emitted tells the scientists something
about the temperature, the chemistry and the layer of the Sun being
viewed because helium can only exist in a particular state and temperature
in specific locations. Notice that the regions of more intense radiation
at 304Å correspond to regions where sunspots and faculae occur.
The following image was also taken by EIT at 171Å. Light
emitted at this wavelength is predominantly emitted by iron that
has lost 8 or 9 electrons that happens at 1,000,000°C. The features
seen here are in the corona.
Compare the areas of most intense radiation at 171Å with those
at 304Å and the pictures taken by MDI.
The following image was taken by EIT at 195Å. Light
emitted at this wavelength is predominantly emitted by iron that
has lost 11 electrons at 1,500,000°C. The features seen here
are in the corona. Compare these areas of most intense radiation
with those in the other images.
The following image was taken by EIT at 284Å. Light
emitted at this wavelength is predominantly emitted by iron that
has lost 14 electrons at 2,000,000 to 2,500,000°C. The features
seen here are in the corona. Compare these areas of most intense
radiation with those in the other images.
The last three images show loop
structures in the corona. The presence, structure and dynamics of
these loops give scientists important insights into solar
processes.
People are also familiar with the
use of x-rays to allow us to "see" things that are not
visible. But astronomers use x-rays far differently than doctors
do. An x-ray of a broken bone does not show us the source of the
x-ray. What we see in an x-ray picture shows different intensities
of the x-rays coming through the object between the emitter and
the film. William Conrad Roentgen discovered x-rays in 1895 and
took this 'picture' of the hand of his wife, Bertha.
The bone of her hand and the metal ring blocked x-rays from the film.
You can see a faint outline around the bone that is the skin and
muscle. However, astronomers look directly at the source of x-rays
and create images of the object emitting the x-ray. The Chandra X-ray
image of Supernova 1987A shows an expanding shell of hot gas produced
by the supernova explosion.
Photo: The colors represent different intensities of X-ray emission,
with white being the brightest.
Recent optical observations of SN
1987A with the Hubble
Space Telescope have revealed gradually brightening hot spots
from a ring of matter that was ejected by the star thousands of
years before it exploded. Chandra's X-ray image shows the cause
for this brightening ring. A shock wave, traveling at a speed of
4,500 kilometers per second (10 million miles per hour), is smashing
into portions of the optical ring. The gas in the expanding shell
has a temperature of about 10 million degrees Celsius, and is
visible only with an X-ray telescope.
Closer to home Yohkoh is
looking at the Sun in X-ray. The 
image below is in a lower energy - or soft - x-ray.
Photo: False color image courtesy of Yohkoh. Brightness
indicates high intensity of x-ray. 2001/11719:52
The Soft X-ray Telescope on board
the Yohkoh satellite was designed to study the hottest part of the
Sun's atmosphere (the corona). Because the corona is so hot (2 million
degrees Celsius) it is best studied by detecting the X-rays it emits.
Scientists are able to compare x-ray data from Yohkoh to images
from SOHO in ultraviolet and visible light. The image above was
taken within a comparable time frame to the SOHO EIT images shown
in the UV section of this page. Images taken at the same time in
different wavelengths allow scientists to look at different parts
of the Sun. They can then make connections between events in different
parts of the Sun.
The Solar-B satellite,
soon to be launched, will carry a coordinated set of instruments
that will measure the Sun's magnetic field and simultaneously its
ultraviolet and X-ray emissions. This data will improve our understanding
of the Sun's variability and will contribute to a deeper understanding
of the Sun's influence on life on Earth.
Electromagnetic radiation with wavelengths
longer than infrared include radio waves and microwaves. We are
most familiar with these types of radiation as carriers of radio
or television signals or to heat our food. However, in 1931 scientists
discovered that radio waves were coming from space and these radio
waves were not carrying an extraterrestrial version of "I Love
Lucy". Instead they were caused by electrons being accelerated
by collisions or interactions with magnetic fields. Other radio
wavelengths come from the behavior of molecules such as rotations
or vibrations. Scientists are able to use radio emissions from space
to determine the molecular structure and the surrounding environment
of objects and clouds of dust and gas in space. One of the least
understood of all radio sources are quasars (quasi-stellar
radio source).
The following image shows Galaxy
M81 
taken at a wavelength that shows radio emissions from neutral hydrogen.
The most intense radiation is red
and the least intense is blue. Scientists learn very important information
about the structure and dynamics of clouds of dust and hydrogen
around stars.
Scientists use an extremely powerful
technique to study objects in space. They combine
information from several widely different wavelengths. Click here for
images of the Sun in all the different wavelengths.
Astronomers are also studying our
own galaxy, the Milky Way. Our solar system lies about two-thirds
of the way out away from the center of our galaxy. 
The Multiwavelength
Milky Way shows our galaxy in 10 different wavelengths representing
each major portion of the electromagnetic spectrum. Since the images
show 360º we can see the entire Milky Way.
Photo Left: Courtesy of Multiwavelength Milky Way
It is well worth visiting The Multiwavelength
Milky Way web site and seeing each part of the above poster in detail
- or even ordering your own poster!
The development of instruments capable
of extending our 'sight' into all wavelengths of the electromagnetic
spectrum enables scientists to know so much more about the structure
and dynamics of the universe. Multiwavelength
Astronomy reveals the universe in all of its light.
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