Radio observations provide us with very different information from optical observations and use very different techniques. The long wavelength means that the wave nature of the radiation is very apparent in the observations. The long wavelength also corresponds to low energy photons. This means that radio regions can tell us about cool regions. For example, we will see how radio observations tell us about star formation in Part IV. We will also see that there are high energy sources that give off much of their energy at longer wavelengths. Thus, radio observations also give us a way of studying high energy phenomena.
Radio astronomy owes its origins to an accidental discovery by Karl Jansky, an engineer at the Bell Telephone Laboratories in New Jersey. In 1931, Jansky detected a mysterious source of radio interference. He noticed that this interference reached its peak four minutes earlier each day. This timing suggests an object that is fixed with respect to the stars. (This four minute per day shift is caused by the Earth's motion around the Sun. This and other aspects of astronomical timekeeping are discussed in Appendix G.) The time of maximum interference coincided with the galactic center crossing the local meridian. Jansky concluded that he was receiving radio waves from the galactic center. It was realized that astronomical objects can be strong radio sources.
The discovery was not followed up immediately. In fact, for a long time there was only one active radio astronomer. Grote Reber was an amateur radio astronomer in Illinois, who carried out observations on his back yard radio telescope in the 1930s and early 1940s. (When Reber submitted his first paper for publication in The Astrophysical Journal, it was sent to a referee, a normal procedure. To make sure that the data were to be believed, the referee, Bart Bok, a Dutch astronomer, then living in the US, took the abnormal step of visiting Reber and his telescope, and taking the editor along. Bok recommended publication of the paper, and was the first traditional optical astronomer to understand the importance of radio astronomy. Following WW II, radio astronomers benefitted from the development of radar equipment during the war. Radio observations were pursued by the British, Dutch, Australians, and a small group of Americans at Harvard. A major advancement was the ability to observe spectral lines in the radio part of the spectrum. We will discuss these lines in Chapter 14.
By the mid-1950s, it was clear that a major radio observatory had to be a cooperative effort, and the National Radio Astronomy Observatory (NRAO) was founded. (This was the first US national observatory, being formed a little before the optical observatory on Kitt Peak.) Bart Bok played a major role in the founding of the NRAO. The first telescopes of the NRAO were in Green Bank, West Virginia, far away from sources of man made interference (in the National Radio Quiet Zone). Since the Earth's atmosphere is virtually transparent through much of the radio part of the spectrum, it is not necessary to place radio observatories at high altitudes or clear sites. We can even observe through clouds. We can also observe day or night, since the sky does not scatter radio waves from the Sun the way it scatters light from the Sun, making the sky appear bright (blue).
We now take a look at how a radio telescope works. A radio telescope consists of some element that collects the radiation and a receiver to detect the radiation. Most modern radio telescopes have a large dish to collect the radiation and send it to
Far from Telescope
Resolution for a radio telescope. (a) The short dashed lines show what would happen if there were no diffraction. Only radiation traveling parallel to the telescope axis would reach the focus.The solid lines show the effects of diffraction. Radiation coming in at a slight angle with the telescope axis can still be reflected on to the focus.This means that when the telescope is pointed in one direction, it is sensitive to radiation from neighboring directions.This is shown in (b),as the telescope is sensitive to radiation coming from within a cone of angle approximately A/D (in radians).
a focal point. (They are like optical reflectors.) The long wavelength becomes important in this process. We have already seen that the resolution of a telescope depends on the size of the tele scope, relative to the wavelength (Fig. 4.26). (In the radio part of the spectrum, atmospheric seeing is not a problem.) Since the wavelengths are large, to achieve good resolution you need a large collector. However, that surface doesn't have to be perfect. It can have imperfections as long as they are smaller than approximately A/20. For example, at a wavelength of 20 cm, 1 cm diameter holes have no effect on the performance of the telescope. We are hindered by the fact that it is hard to make large telescopes with very accurate surfaces. Most large telescopes are made up of smaller panels that are easier to machine accurately. The panels are then aligned to produce the best surface. The alignment is at least adjusted for the effects of gravity as the telescope tilts at different angles, and techniques are being developed to control the surface actively by monitoring the panels at all times during observations. The best resolution for single radio telescopes is about 30 arc sec, slightly better than the naked eye for visible viewing.
Example 4.5 Strength of radio sources We measure the strength of radio sources in a unit called a Jansky (Jy). It is defined as 10~26 W/m2/Hz reaching our telescope. For a 1 Jy source, calculate the power received by a perfect antenna with an area of 102 m2, using a frequency range (bandwidth) of 106 Hz.
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