Radio Astronomy

Mankind has always been fascinated by what one can see in the heavens for thousands of years. However, the human eye is sensitive to a narrow band of the electromagnetic spectrum from about 4000 to 6600 Å wavelength. Astronomical bodies, however, emit electromagnetic radiation across most of the spectrum from meter wavelengths in the radio band to sub-angstrom wavelengths in the x-ray/gamma ray bands. The radio band, which traditionally covered meter and centimeter wavelengths, but more recently has expanded to shorter wavelengths as radio instrumentation has been developed for millimeter and sub-millimeter wavelengths.

If one were to have eyes sensitive only to radio waves, the Sun, the planets, and the Milky Way would be recognizable, yet different. The Sun would still be extraordinarily bright. However, the tenuous and extremely hot plasma comprising the solar corona would cast a diffuse, luminous haze around the Sun. The planets would shine on their own, instead of in reflected light from the Sun. The Milky Way, the spiral galaxy in which we live, would still glow. However, the radio light would not come from stars too numerous to discern, but instead it would come from highly energized electrons.

Karl Jansky made the first radio astronomy observations in the early 1930s.  Jansky was working for the Bell Telephone Laboratories, investigating the sources of interference which might affect transatlantic phone calls. He built a rotatable antenna sensitive to emissions at a wavelength of 15 m and found sources of ‘static’ coming from local thunderstorms, distant thunderstorms in the tropics and an unknown steady source. Jansky had discovered synchrotron emission associated with energetic electrons accelerated in the magnetic field of the Milky Way. The astronomical community did not fully appreciate the significance of Jansky’s discovery for decades. One of the few who followed up on Jansky’s results was Grote Reber. Reber built a 9m diameter parabolic reflector to investigate the strength of the radio emission from the Milky Way at meter and centimeter wavelengths. His antenna–receiver system was not sensitive enough to detect the Milky Way at centimeter wavelengths, but he did detect and map the Milky Way at 1.9 m wavelength.

Radio astronomy grew rapidly following the Second World War, aided by the significant wartime investment in radio equipment for use in radar systems. Some very large telescopes were built. The Sun was shown to be a strong radio source, along with some other isolated sources.  The discovery of radio galaxies is one of the many achievements of radio astronomy. Other significant achievements of Radio Astronomy include discoveries of Quasars and superluminal motion, The cosmic background, Pulsars and supernova remnants, The galactic center, Gravitational lensing, Molecular clouds and Masers.

Questions to Ponder

• What are the largest radio wavelengths observed from radio sources in the sky?
• How can radio waves travel astronomical distances without any attenuation i.e. energy loss?