- 1931 - Karl Jansky at Bell Labs discovers signals due to
"extraterrestrial origin." Today the unit of radio flux
density - is
called a Jansky and is given by 1 Jy = 10-26 W m-2Hz-1. Many stellar sources are the order of a few 10s of Jys.
- The second strongest source in the sky, after Sol, is Cygnus A. At 440 MHz (75m) its spectral flux density is 4500 Jy.
The power collected by a 5 MHz bandwidth on a 25 m diameter radio telescope is
P= S(f) Pi (d/2)2 df = 4500 Jy*3.14*(25 m/2)25x106 = 11.0 x 10 -14 Watts.
- Transparency of the Earth's Atmosphere. The allows investigation at the surface of the planet of a limited region of the electromagnetic spectrum.
Courtesy NASA/JPL-Caltech.
- What kind of processes delimit and exist( are sources) in the Radio
Astronomy region of the E&M spectra?
a)
Molecular Rotation - jumps between rotational levels in many molecules are in
the microwave region of the spectrum. They mostly lie in regions short of of a wavelength of 1 mm.
Remember, E = hf where h = Planck's constant and f is the frequency in Hz and E is the energy. The relationship between frequency f and
wavelength l is given by
fl=c = speed of light. In microwaves one usually speaks of frequency MHz or GHz or
wavelengths like 21cm.
b) On the other end of the radio spectrum the free electrons in the path of the beam scatter and absorb the radiation - that is for frequencies below
the plasma frequency, the transmission is cut off. This cut-off plasma frequency is given by:
f cut-off = sqrt ( Ne*qe2/(e0 mass electron) MKS Units
- Sources of Signals in The Microwave
In addition to a) and b) above, we have also the Cosmic Blackbody radiation T=3 Kelvin, spin flip transition in atomic hydrogen at 21 cm, general
back body radiation of cool objects,
stimulated radiation from astronomical masers - mostly dust clouds, and other processes.
Microwave Astronomy can be passive or active depending on the application. The active applications
generally fall under the name Radar Astronomy - see below.
Some Passive Applications:
- Use the line emissions for MASERS located in dusty regions in spiral arms to map out the arms of galaxies and using the Doppler Effect
determine the rotational curve of a galaxy.
See this reference for a review article.
This line of evidence yields an important idea about the distribution of dark matter around galaxies.
- Monitoring and discovery of pulsars and enhancing our knowledge of their evolution.
Follow this link to Pulsars 101.
- Doing experimental general relativity using radio astronomy - another example is a double pulsar system.
- Discovery of planets in orbit about a pulsar.
Some active Applications - radar astronomy.