Radioastronomy

In addition to celestial bodies that radiate visible light, the universe contains many objects that emit radiation of various nonvisible wavelengths, such as radio waves. In fact, some astronomical objects emit very little light (or even none at all) and are, therefore, unimpressive when observed visually. They may, however, radiate relatively large amounts of energy at nonvisible wavelengths. The range of such nonvisible radiations is extremely broad, spanning almost all the electromagnetic spectrum (of which visible light constitutes only a small part). Therefore, it is important to study as wide a range of electromagnetic emissions as possible in order to gain the fullest possible understanding of the universe.

Historically, the nonvisible part of the electromagnetic spectrum was first studied in 1931-32, when the American Karl Jansky detected radio emissions (at a frequency of 20MHz—equivalent to a wavelength of 49 feet or 15 meters) emanating from the center of our galaxy. Since then, radioastronomy has developed considerably and has made substantial contributions of our knowledge of the universe.

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The nature of radio waves

Radio waves are low frequency and, therefore, long wavelength electromagnetic radiations. The radio part of the electromagnetic spectrum lies beyond the infrared region and comprises all radiation with a frequency lower than about 300,000 million hertz (300,000MHz) or with a wavelength longer than about 1 mm. Because of their low frequency, radio waves are not absorbed by the earth’s atmosphere. As a result, radio observations of celestial objects can be made from the earth’s surface during the day and at night, both in cloudy and clear weather.