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 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 wich 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, Carl 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.

Go further: Why observe at radio wavelengths?