Skip to content
Skip to navigation menu

Background to Radio Astronomy

by Luke Woolfenden

Radio telescope on roof of Physics building

Stars, the sun, planets, hydrogen clouds, pulsars and many other sources radiate not only in the visible waveband of the electromagnetic spectrum but also in the radio region. Some objects emit only dimly in the visible wavelengths and are undetectable using even the most sophisticated of optical telescopes. However these objects can be strong radio sources and so easily detected in these longer wavelengths. Radio photons also have the advantage of being relatively low in energy, being emitted in larger numbers than shorter wavelength photons for the same energy output.

In 1931 a radio engineer, Karl G Jansky, was given the task of identifying the arrival direction of thunderstorm static. Working at 14.6 m he identified two sources caused by thunderstorms but was still left with a continuous "hiss" type static. The position of the source appeared to change in a uniform fashion throughout the day. Jansky hypothesised the source to be the sun, but after noting its coordinates, reviewed this stating that it was from a source outside of the solar system. These coordinates were later discovered to coincide with the position of the centre of the galaxy.

In 1937 another radio engineer, Grote Reber, began work with a parabolic reflector antenna 9.7 m in diameter. It relied on the earth's rotation to alter Right Ascension but was steerable in declination. He initially observed at 9.1 m and 33 cm but detected no celestial emissions. His first detections were made at 1.87 m, the wavelength he later mapped the sky in, finding a dominance of emissions in the plane of the galaxy.

Radio telescope

An astronomer, Professor Jan Oort of Leiden Observatory, became interested in Reber's work and after hypothesised that the detected radiation was part of a continuum. He predicted that if a monochromatic (single wavelength) line radiation could be found within the radio window, the emission could be strong enough to greatly further ground based observations. A young colleague of Oort, professor Dr Hendric Van De Huls, took on the challenge of finding a mechanism that might produce such radiation. He reported that neutral Hydrogen, an atom that might be found in abundance in interstellar space, had a hyper fine energy transition associated with electron spin/magnetic dipole orientation. This small transition would correspond to a wavelength of 21.2 cm (1420MHz). Within weeks the first detections of 21 cm radiation had been made (Kraus, Radio Astronomy, 2nd edition, Cygnus-Quasar Books).