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In
their quest to improve our ability to observe the universe,astronomers
have followed two lines of technological development. First,they
have designed and constructed increasingly powerful telescopes and
detectors,capable of observing progressively fainter sources with
greater clarity. During the next decade, astronomers will construct
telescopes with ever finer resolution, and will also link these
instruments to create systems whose observational precision equals
that of a single giant telescope as large as the maximum separation
of the individual instruments. Second,as a more subtle but equally
important way to widen our observational capabilities, astronomers
have attempted to study different regions of the spectrum of electromagnetic
radiation. This spectrum includes a variety of waves that differ
in their wavelengths of vibration.Only a tiny part of the spectrum
includes the waves that our eyes can detect.We call these waves
"light," and our eyes and brains recognize the different
wavelengths of light as different colors.
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Images
of the central plane of the Milky Way at five different wavelengths.
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Every
type of atom or molecule produces radiation, or blocks radiation
from other sources, only at particular wavelengths. Observations
over the total array of wavelengths therefore provide a "cosmic
fingerprint" that reveals the different varieties and numbers
of atoms and molecules in the objects whose radiation we can study
in detail.By measuring this fingerprint from any cosmic source of
radiation, astronomers can also determine the motions within the
source, as well as the speed with which the source is approaching
or receding from us. Visible light, however, can furnish only a
small portion of this fingerprint. On either side of the spectral
region that contains light waves, vast domains of the electromagnetic
spectrum extend to shorter and Radio Infrared Optical/Visible X
ray Gamma ray longer wavelengths. Though human eyes cannot perceive
this radiation, these domains are just as rich in information as
the visible light with which we view our world. During past decades,
astronomers have developed the tools to exploit most of these regions
of the spectrum. Each new spectral domain opened by a new set of
astronomical instruments not only has improved our understanding
of objects already known, but also has revealed entire new classes
of objects previously unsuspected. By studying cosmic sources of
radio waves, for example, astronomers found totally unexpected phenomena
such as pulsars, which produce rapid-fire bursts of radio emission;
radio galaxies, in which explosive events accelerate charged particles
to nearly the speed of light; and quasars, apparently the cores
of young galaxies, where matter continuously spirals around and
into a supermassive black hole.
Most
radio waves, the longest-wavelength and lowest-frequency domain
of the spectrum, can penetrate our atmosphere. But our atmospheric
veil blocks most radiation outside the radio and visible-light domains,
including gamma rays, x rays, ultraviolet light, and most infrared
radiation. To improve our understanding of the cosmos, we require
not only better ground-based observatories, which can detect visible
light, radio waves, and some infrared radiation, but also space-borne
observatories to study the cosmos in gamma rays, x rays, ultraviolet
light, and those portions of the infrared spectrum that cannot penetrate
Earth's atmosphere. By combining the results from these different
instruments, we can understand sources of radiation far better than
we could with observations made with any single telescope or within
any particular domain of the spectrum.
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