Page 288 - NS-2 Textbook

ASTRONOMY 283

bodies also emit radiation in the radio-frequency portion Radiotelescope technology has also showed that not
of the electromagnetic spectrum. This discovery gave rise all radio \-\Taves from space come fronl s'\virling, excited
to the development of radiatelescapes. In a radio telescope, a gases and celestial bodies. A particular kind of wave was
dishlike antenna is used to gather the radio waves from fotmd to be emitted by the cool, quiet hydrogen clouds
space-much like an objective nillror gathers light waves strewn throughout space. Mapping these hydrogen radio
in a reflecting telescope. AshullOlners can thus measure signals has enabled astronomers to pierce the dusty re-
and record the characteristics of incoming radio ,,,,aves. gions of space through which light rarely passes. Such
Radiotelescopes must be very sensitive in order to de- maps told them that our Milky Way galaxy is shaped like
tect faint radio 'waves from space. Also, because the ·wave- a pinwheel, and that it rotates, carrying the Stm and the
length of radio waves can be thousands of times longer planets with it. Until the radiotelescope, scientists could
than those of light waves, radiotelescopes must be very only speculate that this was the case, as it is with many
large. Since radio waves are so long, ho\vever j radio re- other galaxies.
flectors do not require the precision of optical telescopes. The same radio telescope teclmiques have also fotmd
The mirrors on large reflecting telescopes are polished to molecules in space. Amino acids, the basis of all living
one-millionth of an inch, the tolerance for light waves. things, were found all through space by means of the ra-
But a radio reflector for long radio waves can be made of diotelescope. No optical device could have accomplished
iron mesh, with a tolerance of half an inch. this feat.
The world's largest steerable radiotelescope is the The spectroscopes and radiotelescopes have fotmd
Robert Byrd radio telescope at Green Bank, West Virginia. that hydrogen and helium account for more than 99 per-
Its dish is oval-shaped, 328 by 361 feet. Other very large cent of all matter in the universe. It is believed that 93
ones are at Eifel Mountain near Emu1, Germany, 328 feet; percent of all the atoms in the universe are hydrogen, ac-
and Jodrell Bank in Cheshire, England, 250 feet in diam- counting for 76 percent of its mass. Almost 7 percent are
eter. The world's largest stationary radiotelescope, with a helium atoms, totaling 23 percent of the mass. All the rest
diameter of 1,000 feet, is near Arecibo, Puerto Rico. of the elements add up to only a fraction of 1 percent of
In recent years, it has been fOlmd that an array of the total.
several radiotelescopes can work together to form a giant No matter where ash·onomers have searched in
radio telescope. Such an array of about thirty receivers at space, the universe appears to be made up of the same el-
the National Radio Astronomy Observatory at Socorro, ements. This is a very important fact. It tells us that if life
New Mexico, can produce images of the radio sky that exists elsewhere in the universe, such life almost certainly
rival optical telescopes in precision. would consist of the same elements we have on Earth.
Because some celestial bodies are too far away or too Recently, radiotelescopes have also been used to con-
cold to radiate visible energy, the radio "star map" or trol and receive data from spacecraft exploring our solar
radio source map of the sky does not correspond with an system. In the controlling mode, they send very strong
actual map of the stars. Radio astronomers have found signals radiated at high power levels (300,000 to 400,000
many huge regions of high-speed gases and the remnants watts) to reach the distant spacecraft, then listen for the
of celestial supernovae explosions. Sunspots also give off faint replies and data transmissions, which are often only
radio waves, as does the corona of the sun. Radiotele- a few fractions of a millionth of a watt strong.
scopes have discovered quasars. These objects look no In October 1992 the National Aeronautics and Space
larger than a single star, but they emit hundreds of times Adrmnistration (NASA) began using radiotelescopes in a
more energy than most galaxies. They have also discov- systematic search for any possible radio signals being
ered pulsars, which are bodies that radiate energy at reg- broadcast in the microwave band by extraterrestrial life in
ular intervals. Once thought to be artificial beacons, they our galaxy. Federal funding for the program was termi-
are now considered to be rapidly rotating compressed nated by Congress in 1993 due to budget pressures, but a
stars in the last stages of stellar life. private group called the SETI (an acronym for Search for
Special Uses of tlle Radiatelescape. While radio telescopes Extraterrestrial Intelligence) Institute was able to obtain
normally are used only to receive radio waves, it is possi- enough ftmding to keep the project going. Called Project
ble to modify these devices for other purposes. The ra- Phoenix, its goal was to scan about a thousand of the clos-
diotelescope can direct powerful radio beams at a celestial est stars for radio transmissions that might indicate the
object and then receive them when they rebound toward presence of intelligent life. The project was concluded in
Earth. Radiotelescopes equipped with such transmitters 2004 after having observed more than 800 stars out to a
are often cal1ed radar telescopes, Because radio ·waves havel distance of 240 light years. No intelligent signals were
at the speed of light, radar telescopes can furnish accurate ever found. As a follow-on project, SETI scientists have
data about the distance of celestial bodies near Earth. By begun a yearlong search for laser light pulses beamed to-
using various wavelengths, infonnation about the compo- ward Earth from nearby stars, using the University of
sition of these bodies can be obtained. California's Lick Observatory near San Jose, California.