Radio Astronomy FAQ
- What is radio astronomy?
- What is radio astronomy for?
- What have radio astronomers discovered?
- What do radio astronomers listen to?
- What are radio waves?
- What are radio telescopes like?
- Why was the Chajnantor site chosen to build ALMA?
What is radio astronomy?
Radio astronomy is the study of objects in space (such as stars and galaxies) that naturally emit radio waves.
Until the early 1930s, we did not know that many astronomical objects emit radio waves; astronomers observed the sky with the naked eye or with optical telescopes. Since then, astronomers have developed sophisticated systems to make pictures from the radio waves emitted by celestial objects.
Here are parallel pictures of the Horsehead Nebula in the optical / radio. The Horsehead Nebula at different wavelengths: â¨In the optical, dust obscures star-forming activity. â¨In the infrared, the hot, thin layer of dust around the cloud glows. â¨At radio wavelengths, both dust and molecules glow, providing a wealth of information on regions that are otherwise invisible in the optical range.
Many objects emit more strongly at radio wavelengths than they do at
visible wavelengths, so radio astronomy allows us to detect and picture
what had remained invisible for optical telescopes. Together, both
types of telescopes give us more complete a picture, and deeper an
understanding of the Universe.
What is radio astronomy for?
Radio observatories allow us to explore the fundamental nature and
history of our Universe. They study objects and phenomena that are
often invisible in other portions of the electromagnetic spectrum: the
earliest, coldest, most distant objects of the Universe appear dark in
visible light, but shine brightly in other parts of the spectrum – the
parts ALMA studies.
By studying the radio waves from these sources, astronomers learn about
their composition, structure and motion. This means we can study the
infant stars, early galaxies, the “Dark Ages” before the first stars
and galaxies, up to the edge of the observable Universe.
What have radio astronomers discovered?
Among many significant discoveries, radio telescopes allowed
astronomers to understand the phenomena occurring near the surface of
the sun. Solar flares are strong sources of radio emission; some are
potentially dangerous and can interrupt radio communications on Earth,
endanger sensitive equipment in satellites and even the health of
astronauts. Radio telescopes are used to measure the surface
temperatures of all the planets in our solar system, as well as some of
the moons of Jupiter and Saturn. Also, radio telescopes gave some of
the first hints for the presence of so called "dark matter".
These are only a few examples: many celestial objects emit more
strongly at radio wavelengths than at those of light, so radio
astronomy has produced many surprises in the last decades.
What do radio astronomers listen to?
Radio astronomers do not actually listen to noises!
Radio waves and sound are two different things. Sound waves are transmitted through matter, such as air or water. Radio waves are electromagnetic waves that can travel through a vacuum.
When you listen to the FM radio, your radio station transmitter turns sound waves (such as music or voice) into radio waves, so that they can travel long distances to your radio appliance. Your receiver then turns these radio waves back into sound waves for you to hear.
Radio telescopes do receive radio waves, but do not turn them into
sound waves—this wouldn’t be any use for scientific purposes. However,
they often produce images of objects that are otherwise not detectable
in the visible wavelength range.
What are radio waves?
Objects in space (planets, comets, stars, galaxies, or clouds of dust
for instance) emit electromagnetic waves. The full range of radiation
emitted by an object is called its electromagnetic spectrum.
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| Fig. The electromagnetic spectrum describes the range of electromagnetic waves according to their wavelength. Courtesy NRAO |
A small portion of the radiation is emitted in the visible range, i.e. we can see it with our eyes. But objects also emit signals in other portions of the spectrum: from very short wavelength and high energy frequencies (such as X-rays or gamma rays), to long wavelengths and lower energy frequencies, such as microwaves or radio waves.
For instance, the “coldest” places in the universe, such as the gas and dust clouds where the stars are formed, emit radio waves called “submillimetric” and “millimetric”. These radiations are not visible, but can be detected by adapted instruments. Thus, ALMA will study radio waves with a wavelength between 0.3 and 9.6 millimeters.
What are radio telescopes like?
In modern observatories, astronomers do not look directly through the
telescopes. The human eye has been replaced by electronic detectors and
powerful computers that analyze the signals emitted by distant objects.
They then turn them into images of the area of the sky that is being
observed, or else revealing the chemical analysis of the object
studied.
Radio telescopes and optical telescopes are very different. Radio telescopes are generally made of a large antenna and a sensitive radio receiver that amplifies the signal. The millimetric and submillimetric waves strike the main parabolic dish and from there travel to the tip of the telescope. Then, they rebound to the centre of the main dish to a receiver below the surface. The receiver converts the waves into signals that are stored in a computer.
Several antennas can be combined into an array, such as ALMA’s. In this
case, they act like one single telescope as big as the size of the
array; this is called interferometry. The signal is sent by fiber
optics to a computer in a large control building, located at a distance
from the array. The computer processes and combines the information
from all antennas to create an extremely clear image.
Why was the Chajnantor site chosen to build ALMA?
ALMA is being built in the Chilean Atacama desert - the driest
place on Earth, 5,000 meters (16,500 feet) above sea level. This is the
perfect place for a new telescope capable of detecting radio waves just
millimeters in wavelength, because the atmosphere is extremely dry.
Radio waves penetrate a lot of the gas and dust in space, and can pass through the Earth’s atmosphere with little distortion. However, if the atmosphere above ALMA contained water, the radio signals would be heavily absorbed – the tiny droplets of water scatter the radio waves in all directions before they reach the telescope, and would degrade the quality of the observations.
