Professional
astronomers usually have access to powerful telescopes, detectors, and
computers. Most work in astronomy includes three parts, or phases. Astronomers
first observe astronomical objects by guiding telescopes and instruments to
collect the appropriate information. Astronomers then analyze the images and
data. After the analysis, they compare their results with existing theories to
determine whether their observations match with what theories predict, or
whether the theories can be improved. Some astronomers work solely on
observation and analysis, and some work solely on developing new theories.
A. Observation
Observational
astronomers use telescopes or other instruments to observe the space. The
astronomers who do the most observing, however, probably spend more time using
computers than they do using telescopes. A few nights of observing with a
telescope often provide enough data to keep astronomers busy for months
analyzing the data.
A.1. Optical Astronomy
Until the 20th
century, all observational astronomers studied the visible light that
astronomical objects emit. Such astronomers are called optical astronomers,
because they observe the same part of the electromagnetic spectrum that the
human eye sees. Optical astronomers use telescopes and imaging equipment to
study light from objects. Professional astronomers today hardly ever actually
look through telescopes. Instead, a telescope sends an object’s light to a
photographic plate or to an electronic light-sensitive computer chip called a
charge-coupled device, or CCD. CCDs are about 50 times more sensitive than
film, so today's astronomers can record in a minute an image that would have
taken about an hour to record on film.
Telescopes may use
either lenses or mirrors to gather visible light, permitting direct observation
or photographic recording of distant objects. Those that use lenses are called
refracting telescopes, since they use the property of refraction, or bending,
of light (see Optics: Reflection and Refraction). The largest refracting
telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams
Bay, Wisconsin, founded in the late 19th century. Lenses bend different colors
of light by different amounts, so different colors focus slightly differently.
Images produced by large lenses can be tinged with color, often limiting the
observations to those made through filters. Filters limit the image to one
color of light, so the lens bends all of the light in the image the same amount
and makes the image more accurate than an image that includes all colors of
light. Also, because light must pass through lenses, lenses can only be
supported at the very edges. Large, heavy lenses are so thick that all the
large telescopes in current use are made with other techniques.
A.2.Gamma-Ray and X-Ray Astronomy
Gamma rays have
the shortest wavelengths. Special telescopes in orbit around Earth, such as the
National Aeronautics and Space Administration’s (NASA’s) Compton Gamma-Ray
Observatory and the Fermi Gamma-Ray Space Telescope, have been able to gather
gamma rays before Earth’s atmosphere absorbs them. X rays, the next shortest
wavelengths, also must be observed from space. NASA’s Chandra X-Ray Observatory
(CXO) is a school-bus-sized spacecraft that began studying X rays from orbit in
1999. See also Gamma-Ray Astronomy, X-Ray Astronomy.
A.3.Ultraviolet Astronomy
Ultraviolet light
has wavelengths longer than X rays, but shorter than visible light. Ultraviolet
telescopes are like visible-light telescopes in the way they gather light, but
the atmosphere blocks most ultraviolet radiation. Most ultraviolet
observations, therefore, must also take place in space. Most of the instruments
on the Hubble Space Telescope (HST) are sensitive to ultraviolet radiation (see
Ultraviolet Astronomy). Humans cannot see ultraviolet radiation, but
astronomers can create visual images from ultraviolet light by assigning colors
or shades to different intensities of radiation.
A.4.Infrared Astronomy
Infrared
astronomers study parts of the infrared spectrum, which consists of
electromagnetic waves with wavelengths ranging from just longer than visible
light to 1,000 times longer than visible light. Earth’s atmosphere absorbs
infrared radiation, so astronomers must collect infrared radiation from places
where the atmosphere is very thin, or from above the atmosphere. Observatories
for these wavelengths are located on certain high mountaintops or in space (see
Infrared Astronomy). Most infrared wavelengths can be observed only from space.
Every warm object emits some infrared radiation. Infrared astronomy is useful
because objects that are not hot enough to emit visible or ultraviolet
radiation may still emit infrared radiation. Infrared radiation also passes
through interstellar and intergalactic gas and dust more easily than radiation
with shorter wavelengths. Further, the brightest part of the spectrum from the
farthest galaxies in the universe is shifted into the infrared. The James Webb
Space Telescope is designed to observe over a wide spectrum of infrared
radiation.
A.5.Radio Astronomy
Radio waves have
the longest wavelengths. Radio astronomers use giant dish antennas to collect
and focus signals in the radio part of the spectrum (see Radio Astronomy).
These celestial radio signals, often from hot bodies in space or from objects
with strong magnetic fields, come through Earth's atmosphere to the ground.
Radio waves penetrate dust clouds, allowing astronomers to see into the center
of our galaxy and into the cocoons of dust that surround forming stars.
A.6. Study of Other Emissions
Sometimes
astronomers study emissions from space that are not electromagnetic radiation.
Some of the particles of interest to astronomers are neutrinos, cosmic rays,
and gravitational waves. Neutrinos are tiny particles with no electric charge
and very little or no mass. All stars emit neutrinos, but neutrino detectors on
Earth receive neutrinos only from the Sun and supernovas. Most neutrino
telescopes consist of huge underground tanks of liquid. These tanks capture a
few of the many neutrinos that strike them, while most neutrinos pass right
through the tanks.
Cosmic rays are
electrically charged particles that come to Earth from outer space at almost
the speed of light. They are made up of negatively charged particles called
electrons and positively charged nuclei of atoms. Astronomers do not know where
most cosmic rays come from, but they use cosmic-ray detectors to study the
particles. Cosmic-ray detectors are usually grids of wires that produce an
electrical signal when a cosmic ray passes close to them. Most often cosmic
rays are detected by showers of subatomic particles that result
when a high-energy cosmic ray strikes an atom high in Earth’s atmosphere.
B. Analysis and Theory
Whether astronomers
take data from a ground-based telescope or have data radioed to them from
space, they must then analyze the data. Usually, the data are handled with the
aid of a computer, which can carry out various manipulations the astronomer
requests. For example, some of the individual picture elements, or pixels, of a
CCD may be slightly more sensitive than others. Consequently, astronomers
sometimes take images of blank sky to measure which pixels appear brighter.
They can then take these variations into account when interpreting the actual
celestial images. Astronomers may write their own computer programs to analyze
data or, as is increasingly the case, use certain standard computer programs
developed at national observatories or elsewhere.
Often an astronomer
uses observations to test a specific theory. Sometimes, a new experimental
capability allows astronomers to study a new part of the electromagnetic
spectrum or to see objects in greater detail or through special filters. If the
observations do not verify the predictions of a theory, the theory must be
discarded or, if possible, modified.
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