Radio astronomy: a part of astronomy that, like optical astronomy, studies the celestial objects (planets, stars, galaxies etc.) by "capturing" the light that they emit, but that, unlike optical astronomy, cannot be seen with our eyes. With their instruments (radio telescopes) radio astronomers detect radio emission from these objects.
Light is a wave
Since the 19th century, thanks to Thomas Young, we know 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 colour 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 millimetres 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, Carl Jansky detected an unexpected signal: an emission from the centre 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 wavelengths?
There are several reasons to observe at radio wavelengths. Below we will show you the advantages and disadvantages:
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.
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.
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.
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.