Image Processing Reference
In-Depth Information
other wavebands. Observing them at different wavelengths provides a much more
complete picture of their structure. Because of this, observational astronomers have
been a driving force in the development of multi-wavelength and invisible-light
imaging. In some cases, visible-light astronomy (known as optical astronomy)
is severely limited by the presence of light-obscuring matter between earth and
a celestial object of interest. In other cases, there is not very much visible light
emitted by the object of interest in the first place, yet the same object may be very
bright in other wavebands. These revelations about the sky were made only in the
latter part of the twentieth century, with the advent of invisible-light astronomical
instruments. High-resolution radio telescopes permitted precise mapping of the
radio sky, and orbiting telescopes made x-ray and gamma-ray astronomy possible,
since the atmosphere absorbs the shortest wavelengths of light. As mentioned
earlier, astronomers have now studied the heavens in twenty powers of ten in
wavelength of the electromagnetic spectrum!
The Milky Way is a particularly rich area of study for multi-wavelength
astronomy. It covers a full 360-degree field of view, and is quite wide in the narrow
dimension as well. The Milky Way is a cross-sectional view of our galaxy taken
from our vantage point about one third of the way from the center. Our galaxy is
shaped like a disc with a bulge in the center; the bulge is a region with a high
density of stars and other celestial objects. The comparative images shown in
Fig. 6.11 are all registered the same way, with the galactic center corresponding
to the center of the images. The radio images are all made with ground-based radio
telescopes. The infrared images are made by satellite-based instruments, as are the
x-ray and gamma-ray images. The optical image is a composite of long-exposure
photographs made from ground level. Note that these images have quite different
angular resolutions, owing to the vastly different imaging systems used to acquire
them. As a result, some of the images show much less fine structure than the optical
image, for example. Note the commonality of these images: the central area of the
Milky Way is bright at all wavelengths.
Scattering of electrons in interstellar plasma is the predominant mechanism that
causes the diffuse glow visible in the 73-cm-wavelength microwave image. This
waveband is called the radio continuum by astronomers. Plasma is gas that has
electrons stripped off of it either by intense heating or by absorption of ultraviolet
starlight. Strong radio emission by compact objects like Sagittarius A at the galactic
center is associated with the interaction of electrons with strong magnetic fields.
This image is courtesy of C.G. Haslam.
The second image is made with light with a 21-cm wavelength, which
corresponds to anemissionlinefound in atomic hydrogen. The presence of atomic
hydrogen traces the warm interstellar medium, which consists of large clouds of
gas and dust. This image is courtesy of W.B. Burton.
The third image at a 12-cm wavelength shows the radio continuum emission
from hot, ionized gas, but at a higher spatial resolution than the 73-cm wavelength
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