What is Radio Astronomy?

A very brief introduction to Radio Astronomy

by Darrel Emerson (May 1997)


E-mail: [email protected]

[This short introduction is still being written, and will be expanded from time to time.

Any comments would be welcome.]

The first question is:

What is Astronomy?

Astronomy is a branch of fundamental physics. Astronomy is NOT to do with memorizing names of stars and of constellations. Astronomy gives physicists access to a laboratory where physical laws may be tested - and discovered - under conditions far more extreme than exist or can be created on Earth. There are higher temperatures, higher energies, higher vacuums, higher pressures, regions more dense, greater distances, and longer intervals of time available. By looking at very distant sources, the radiation may take so long (thousands of millions of years) to reach us that in effect we can look back in time.

The next question:

Why RADIO Astronomy?

Astronomers observe at all wavelengths where Nature and technology make observations possible. Because of the Earth's atmosphere and its ionosphere, radiation at certain wavelengths can not penetrate to ground-based observatories and so can only be observed from space. By convention, based on tecnology and not on Nature, the spectrum of radiation used by astronomers is divided roughly into the following categories. The numbers in the table are not intended to be precise definitions, but just to give a rough idea of the ranges of wavelengths involved.

 Radio:             100 meters to 0.1 mm.
 Infrared:           1 mm to 1 microns
 Optical:            2 microns to 0.4 micron
 Ultraviolet:      0.4 microns to 0.01 microns
 Gamma rays,   
    X-rays:           less than about 0.01 microns

   (NB a micron is a millionth of a meter, or a thousandth of a mm.
  For comparison, 25.4 microns equal one thousandth of one inch)

Note that the categories are rather vaguely defined, and in fact overlap.

The longest wavelengths which can reach the ground are about 100 meters - beyond that the ionosphere blocks the radiation. Even waves as long as 100 meters can only reach the ground in exceptional circumstances.

From the rough table above, it can be seen that "radio" observations range over a factor of about one million in the shortest to the longest wavelength. In contrast, "optical" observations only range over a factor of about 5 to 1 (our eyes are only sensitive to a range of wavelengths of about 2 to 1.) In some ways, this makes "radio" observations more interesting, since the wide range of wavelengths makes a wide range of physical phenomena accessible. However, observations in the different wavelength regions are really complementary: some phenomena are best studied in one particular wavelength range, while other phenomena are best studied by combining data from many different wavelength regions.

What signals do Radio Astronomers pick up?

There is measurable emission, at a variety of wavelengths, from every point in the sky. There are no silent spots. Radio Astronomy is particularly sensitive to emission from particles - atoms, molecules or electrons - in between the stars. High energy electrons spiralling in magnetic fields in space produce "synchrotron radiation" which can be measured over much of the radio portion of the spectrum. Planets (and the Earth's moon) are relatively strong thermal radiators at the shorter radio wavelengths, but are mostly undetectable at longer wavelengths.

The phenomena mentioned above (synchrotron, thermal) are very broadband. Subtle changes are detected if you, say, halve or double the wavelength, but mostly the picture of the sky (or planet) looks much the same. In contrast, atoms and molecules can be responsible for emission at very narrow, precisely determined wavelengths. By studying in detail how this emission varies with slight changes in wavelength, the temperatures, densities and motions (via Doppler shifts) of gas clouds in interstellar space, either in our own galaxy or in neighbouring or very distant galaxies, can be studied.

How do Radio Astronomers make pictures?

(Not yet written! Please be patient.)

Examples

Both of these examples show the entire sky (both hemispheres, in an RA-dec coordinate frame). The data were obtained from the SKYVIEW web site. The first is of the broadband ("continuum") emission at a wavelength of 74 cm (408 MHz) observed by Haslam et al. The Galactic Plane ("Milky Way") is obvious as the bright line of emission. Most of the radiation picked up is synchrotron emission, or high energy electrons spiralling in magnetic fields. Note the bright spots, corresponding to discrete radio sources. Cas A (a supernova remnant, the shell still expanding after a catastrophic stellar explosion, about 10,000 light-years distant) and Cygnus A ( identified with a remote galaxy at a distance of about 600 million light-years) are the two brightest, near the top of the field, to the right of center. The data are from Haslam et al.


The entire sky, imaged at a wavelength of 74 cm (408 MHz)

The entire sky, observed at a wavelength of 74 cm.


The image below is also of the entire sky, but with the receiver tuned to the wavelength of atomic hydrogen at 21 cm (1420.405... MHz). Note the absence of any discrete sources (the bright spots of the picture above) in this image - the continuum emission at this wavelength is much weaker than the emission from the hydrogen atoms, but what little broadband emission there is has been subtracted from the data, to give a measure just of the amount of cold, atomic hydrogen along each line of site. This image is from Dickey and Lockman.


The entire sky, tuned to the 21-cm hydrogen line wavelength

The entire sky, imaged at a wavelength of 21 cm, 
the characteristic wavelength of atomic  hydrogen.



The Radio Sky
SKYVIEW
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