Radio astronomy is a part of astronomy that, like optical astronomy, studies the celestial objects (planets, stars, galaxies). This is done by "capturing" the light that these objects emit, but unlike optical astronomy, this 'light' cannot be seen with our eyes. Radio astronomers use our instruments to detect radio emission from these objects.
Astronomy is a science that is concerned with observing and studying all kinds of objects in the sky. You can think of stars but also planets, galaxies, etc. Such objects are also referred to as celestial bodies. Although people cannot see the other wavelengths (radio, X-rays, ultraviolet and infrared), they are very important for astronomy. There are celestial bodies that emit more powerful and simply spectacular signals at these wavelengths; in the last 40 to 50 years, astronomers have been faced with numerous surprises in their sky observations at these wavelengths. Radio astronomy is part of astronomy that studies celestial bodies by catching radio waves through a radio telescope. A radio telescope detects radio waves from different celestial objects. By catching radio waves with a radio telescope, a celestial body is looked at in a different way.
What are radio waves?
Since the 19th century, thanks to Thomas Young, we know that light behaves as a wave. As in the ocean, "light waves" can have different sizes depending on the distance between its beginning and its end. That is why we define light according to its "wavelengths".
Visible light / Invisible light
The human eye can only detect wavelengths in a range from 400 to 700 nanometres (that is to say any color between purple and red which all comprise the visible light spectrum; Nanometre - nm: used for very small distances, 1nm = 0.000 000 001 m). But if you look at the so called "electromagnetic spectrum", you will notice visible light is only a small part of it.
Before purple and after red are ultraviolet and infrared, and then a lot of other "invisible light" having shorter and longer wavelengths (from 0.0001 nm to 0.000 000 000 001 nm). Among longer wavelengths are Radio waves which have wavelengths in the range of a few milimetres to several metres. Therefore radio wavelengths are up to a million times longer than visible light!
Receiving celestial signals
In 1932, while investigating radio disturbances which might interfere with transoceanic telephone signals, Karl Jansky detected an unexpected signal: an emission from the center of our Milky Way. This was the beginning of radio astronomy.
To receive celestial emission, radio astronomers need equipment similar to that used to receive other radio waves. That is why the development of radio astronomy was closely linked to the technical development of radio communication and radar.
We can compare a celestial signal to TV signals. As a spectator, the first thing astronomers need is an antenna to receive the wave (that is to say the light) they want to "see". Then, they need something to analyse the signal and make it visible : some have a TV set, astronomers have computers. Finally, the image can be seen on a screen.
Why observe at radio waves?
There are several reasons to observe at radio waves, below you find a short list with advantages of observing at radio waves.
Radio astronomy can be done from the earth without being too much affected by the weather (although the quality of the observations is better with good weather)! However, there is now also a radio antenna in space, to further improve the resolution of the observations.
- Day and night
Radio telescopes observe day and night (although for some observations the influence of the sun is negative!)
About 90 % of the visible matter in the Universe is Hydrogen (wavelength : 21.106114 cm). With a radio telescope we can study the most abundant element in the Universe.
- No absorption
Radio waves are not affected by absorption. Optical waves are absorbed by e.g. dust clouds that are floating between the stars (like a sort of interstellar fog ). Radio telescopes see straight through these dust clouds.
Besides the advantages, there are some disadvantages too.
On the negative side, to get good quality images that show all the details of the celestial objects it is more complicated than, e.g. at optical wavelengths. This has to do with long wavelengths of radio waves. To get good angular resolution requires large telescopes.
- Complicated procedure
A complicated procedure is required to produce the images of the observed objects (in other words, the observer does not see the images straight away). This procedure uses very powerful computers and it is necessary because of the way the observations are done.