Radio Astronomy Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 18, 2011

The Radio Sky

When a radiotelescope with sufficient resolving power is used to map the sky, certain regions of greater and lesser radio emission are found. The center of our galaxy, located in the direction of the constellation Sagittarius, is a powerful radio source. The Sun is a fairly strong emitter of radio waves, as is the planet Jupiter.

A strong source of radio waves is found in the constellation Cygnus, and it has been named Cygnus A . The Australian radio astronomers J. Bolton and G. Stanley determined that Cygnus A has a very tiny angular diameter, and they also found many other localized radio sources. This led to the development of a system for naming radio sources. A significant celestial source of radio waves is designated according to the constellation in which it is found, followed by a letter that indicates its relative radio intensity within that constellation. The letter A is given to the strongest source in a given constellation, the letter B to the second strongest, and so on. Cygnus A is the strongest source of radio emissions in Cygnus and also, it so happens, in the entire sky. It is so small in diameter that its output fluctuates because of effects of Earth’s ionosphere as the signals pass through on their way to the surface. Cygnus A is a radio galaxy .

Using radio telescopes, maps of the sky have been made, in the same way that optical astronomers make star and galactic maps. Radio maps do not look like optical maps. Instead, they appear like topographic maps used in geologic surveys or like computerized abstract art. Regions of constant radio emission are plotted along lines, which tend to be curved. Or they can be rendered as pixelated images in color or grayscale, as shown in Fig. 18-4, an image of a hypothetical radio galaxy viewed edgewise. (The smaller objects are hypothetical foreground stars within our own galaxy.) The better the directivity of the radio telescope, the greater is the number of discrete radio objects that can be defined on such a map.

Observing the Invisible Radio Astronomy
The Radio Sky

Figure 18-4. Radio map of a celestial object, in this case a hypothetical galaxy viewed edgewise. Shades of gray indicate relative radio brightness.

In radio maps of the entire sky, the Milky Way shows up as a group of lines or colored regions with their widest breadth (representing the greatest intensity) in the constellation Sagittarius. Other galaxies have been found that emit radio frequency (rf) energy. Scientists at Cambridge University, in the early days of radio astronomy, identified four different external galaxies as radio sources. One of these is the Great Nebula in Andromeda, approximately 2.2 million light-years from our own galaxy.

Reception From The Solar System

As radio astronomy evolved, scientists turned their attention to several objects in our own Solar System. One of these is the Sun. The radio Sun is somewhat larger than the visible solar disk, and it appears oblate or flattened along the plane of the equator. This is to say, the apparent diameter of the radio Sun is smallest through the poles and largest through the equator.

Visible solar flares are also observed with radio telescopes. Such flares have long been associated with disruption of the ionosphere of our planet, a phenomenon that wreaks havoc with radio broadcasting and communications at some frequencies. There are several different kinds of solar flares at radio wavelengths. Radio outbursts from the Sun usually portend a disturbance in the Earth’s magnetic field a few hours afterward as the high-energy particles arrive and are focused toward the Earth’s north and south magnetic poles. Then, at night, we see the aurora (northern lights and southern lights). We also observe an abrupt change in radio wave propagation at some frequencies.

Radio observations of the Moon and the planets have enabled astronomers to more accurately ascertain the surface temperatures, especially of planets with thick atmospheres such as the “gas giants” Jupiter, Saturn, Uranus, and Neptune.

Jupiter produces exceptionally strong radio emissions and has a fairly high temperature deep within its shroud of gas. At a wavelength of about 15 m, the EM radiation from Jupiter is almost as strong as that from the Sun. Jupiter is also a strong radio source at shorter wavelengths. Some of this radiation can be attributed to the fact that Jupiter generates considerable heat of its own, in addition to reflecting energy from the Sun. However, the internal heat of Jupiter cannot account for all the radio emissions coming from the giant planet. Several theories have been formulated in an attempt to explain the unusual levels of EM radiation coming from Jupiter. According to one idea, numerous heavy thunderstorms rage through the thick atmosphere, and the radio noise is caused by lightning. However, the noise is too intense for this idea to fully explain it. A more plausible theory is that electrons, trapped by the intense magnetic field of Jupiter and accelerated by the high rotational speed of the planet, cause a form of EM emission called synchrotron radiation .

Practice problems of this concept can be found at: Electromagnetic Fields Practice Problems

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