Radio Astronomy
Introduction
Within our galaxy, we cannot view more than 1,000 light years distant from us because of the obscuring effect of dust. Radio Astronomy can probe through this dust.
This form of astronomy traces its history back to Karl Jansky in 1931. Jansky's discovery appears to have been made in the traditional way for many scientific discoveries, i.e. by mistake when he was really investigating something else. As a radio engineer, he was studying 'static' (at the Bell Telephone Laboratories at Holmdel, New Jersey) and found he was picking up radio emissions from the Milky Way.
His discovery took a long time to get off the ground, it was only after the war that it really got going - it was probably wartime development in radar etc. that provided a great stimulus for the discipline, and indeed this discipline sems to have accounted for a large proportion of the ascent of astronomy after the Second World War, when the number of professional astronomers doubled in about 10 years. In Britain, two scientists who had worked on the radar program pioneered radio astronomy in the country - Martin Ryle at Cambridge University and Bernard Lovell at Jodrell Bank (Manchester University).
21 Centimeter Radiation
This radiation is produced by atoms of Hydrogen when the electron in its shell flips from spinning in one direction to the other. This is an extremely rare event, but the vastness of space contains enough hydrogen to produce detectable radiation of this type.
It was used to map the spiral arms and deduce that they are rotating.
Synchotron Radiation
This type of radiation is produced by relativistic electrons spiraling around, in a magnetic field, at close to the speed of light.
Seen as an excess at longer wavelengths whereas thermal radiation would tail off at this wavelength.
Continuous Radiation
Continuous radiation (analogous to the continuous radiation produced by the Sun) is emitted by large, hot gas clouds (hot HII regions).
Molecular Clouds
Molecules of Hydrogen do not emit strongly in radio waves. These clouds can be mapped by emission lines from Carbon Monoxide.
Observatories
Radio telescopes are bigger than optical telescopes
Jodrell Bank 76m dish. The telescope is owned by Manchester University and is situated on the South Cheshire Plain. It was constructed for about £ 500,000. It was originally to be built of wire mesh to detect in the meter range. During construction, 21 cm radiation was discovered, and in order to investigate this a solid surface was substituted. It was completed in 1957.
Mullard Radio Astronomy Observatory run by Cambridge University. Includes the Ryle Telescope which consists of eight connected telescopes - four fixed 13m dishes and four similar telescopes which are movable on rails for a distance of about 5km (actually along a disused railway line).
Araceibo, Puerto Rico. 305m dish, fixed in a natural bowl, so unable to move. Operated by Cornell University.
Green Bank West Virginia. The observatory first became operational in 1959. From 1962 with an aperture of 91 m, it unfortunately collapsed in 1988. Replaced by a 100m telescope, the largest steerable radio telescope in the world.
VLA (Very Large Array) in Socorro, New Mexico and operating since 1980.
It consists of 25 separate telescopes (each of 25m diameter) arranged in the shape of
the letter Y, with each arm of the Y being 21 km long. The array is equivalent to a
single radio telescope with a dish of 42 km diameter.
Discrete Sources
- Sagittarius A Strong radio source at the center of the Milky Way. This has been superseded as a candidate for the center of the Milky Way by the discovery of an even smaller intense source, Sagittarius A*.
- Cygnus A
- Taurus A the Crab Nebula
- Cassiopeia A remnants of Type 2 Supernova
- Virgo A M87
see also Radio Galaxies for Centaurus A
Interferometry
Individual radio telescopes have a very low resolution. By combining observations from two or more telescopes separated by a considerable distance, the resolution can be improved considerably. By using computers, these distances can be extremely large and the resolution impressive - equivalent to detecting a coin at a thousand kilometers, that type of thing.
Submillimiter Astronomy
These can be considerd as extremely short radio waves. Typically uses a telescope cooled by liquid helium - see James Clerk Maxwell telescope.
