If they study the same objects using different wavelengths, they will obtain images very different one from an other, allowing different analyses.
Click on image (© NASA) to enlarge image.

Jupiter

The planet Jupiter has a strong magnetic field which traps and holds very fast moving electrons (like the van Allen belts on the earth). As the electrons accelerate around the magnetic field lines, radio waves are emitted. This produces the extended features beyond the planet's disk that can be seen in the image on the right that was obtained at 21 cm with the Very Large Array.

Here is a view of the 3D imaging of Jupiter based on 21cm observations done with the Australia Telescope Compact Array. The "red ball" in the centre is the planet (emitting thermal emission), the green disk around it represents the radiation belts, synchrotron emission from high energy electrons moving in the magnetic field of Jupiter. See it rotating.

Stars

Stars are not strong radio emitters and our Sun is no exception. However, being so close we can detect much emission from it, in particular during periods of strong solar activity.

On the right, the active Sun is shown in an image obtained with the VLA at 20cm. The bright spots in the image correspond to the active regions, usually associated with sunspots.

Pulsars & Supernovae

Listen to three different pulsars:

• a very fast one which rotates 660 times per second
• a slower one which rotates 11 times per second
• a "very" slow one which rotates 1.4 times per second. The outer layers of gas blown off by the supernova explosion expand outwards, producing radio and other emission.

Radio images at very high resolution of the surpernova 1993J taken at different epochs after its explosion. The expansion of the shell of gas is clear from these images. The young supernova continues to expand for many years after the explosion.

Cassiopeia A, the remnant of a supernova event which may have been witnessed by Flamsteed in 1680 (image from VLA at 6 cm).

+ More about pulsars

The Milky Way and other galaxies

Thanks to the 21-cm line of neutral hydrogen, radio astronomers have a very powerful tool to study the motions of the gas in our Galaxy and in other galaxies. This is very important because this gives an independent way to measure the mass distribution. It turns out that the masses determined in this way are much larger than those based on the number of stars and the amount of gas visible in galaxies. In other words: there is much more matter in galaxies than we can see directly (the so-called dark matter).

What constitutes this dark matter is one of the main questions in astronomy. The neutral gas gives a completely different picture than the stars do of a galaxy, so the information about the gas adds a lot to the information obtained with optical telescopes. This is illustrated in the picture: it shows the stars (yellow) in the galaxy NGC 5055, while the blue shows the distribution of the neutral gas. Click on image to enlarge image.

Radio Galaxies

Some external galaxies produce an enourmous amount of energy at radio frequencies. This energy can be a thousand times more than that produced by all the stars in the galaxies taken together.

Astronomers believe that in these special galaxies, called "radio galaxies", this huge energy is produced in the "black hole" in the centre of the galaxy. Fast electrons and magnetic fields are produced in this region and then ejected. They then can travel to very large distances from the nucleus. The radio emission can therefore be very extended (as extended as the region where these fast electrons are present).

Here is a radio galaxy observed with the VLA.
( Radio Galaxy 3C296 ©NRAO/AUI 1999)
The region in red represents the radio emission while the region in light blue represents the optical emission (i.e. emission from the stars). It is clear that the radio emission is much more extended and has a completely different morphology than from optical emission.

One of the giant double-double radio galaxies. The left-most picture shows an area of the sky, about twice the size of the full moon, observed at radio wavelengths with the Westerbork telescope. The extended source in the middle, when looked at in more detail, consists of four bright components (middle picture, from an observation with the Very Large Array radio telescope in the US). The outermost components are a `normal' pair of radio lobes, which span a distance of roughly five million lightyears. The inner components, when viewed in even more detail (right-most picture, also from an observation with the Very Large Array) resemble the outer radio lobes remarkably well. They clearly form a separate pair of radio lobes, which is smaller than the outer pair of radio lobes, and which most likely result from a temporal halting of the two jets. The source in the middle, which is too small to show any details, is the so-called radio core. This is the place of origin of the two jets and thus the location of the central black hole. 

Satellites

Signals at radio wavelengths are, of course, also emitted by man-made instruments. Therefore radio telescope have been used to track satellites and receive information and pictures sent by them. The Parkes radio telscope, for example, received the TV images sent by Apollo 11 when Neil Armstrong first stepped on the moon.

Others

• The WSRT has been asked by NASA to participate in a search for the missing Mars Polar Lander. A very sensitive instrument like the WSRT was needed because only one of the transmitter on board of the Mars Polar Lander (the transmitter meant to comunicate wth the Mars Global Surveyor) was likely to be still working.

• The most extreme case of search for man-made signals with radio telescopes is the SETI project (Search for Extraterrestrial Intelligence). The characteristics of radio emission from man-made (or alien-made!) instrumentation are clearly different from those of astronomical objects and therefore radio waves are very suitable to distinguish between the two. The SETI project is now searching the regions around thousands of nearby Sun-like stars and has made use of many of the world's largest antennas.

So, you now know a lot more about radio astronomy. Want to know even more? Click on the link below:

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