Abstract
Through the ages, the human eye alone was the most important means of observation. In the early seventeenth century, the telescope was invented. Gradually, the size of the biggest telescopes became larger, and, complemented with photography, they helped us to see ever further into the universe. In the 1930s, this process was in full swing and hardly anyone could imagine that there would be any other means of extending our vision except by further development of ordinary optical telescopes. It was also not known that there are other important mechanisms of radiation from celestial bodies than hot gas and that there are many celestial bodies emitting other types of radiation than light.
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Notes
- 1.
Kraus was a student of James Cork (1894–1957) at Michigan, a well-known specialist in radioactivity and nuclear physics. One of Kraus’ most influential students was the Finnish engineer Martti Tiuri (1925–2016) who started the Metsähovi Radio astronomy Observatory near Helsinki and specialized, besides radio receiver technology, in following the variability of known radio sources. The significance of this line of research is described below.
- 2.
Such “bremsstrahlung” has a continuous spectrum but its characteristic shape and cut-off point did not match the radio observations. Ryle and Oort were of the (also incorrect) opinion that the radio waves came from stars which were not unlike the Sun except that for some reason radio emission was unusually enhanced.
- 3.
In order that the source can change its brightness in only one day, it must be possible to readjust all radiating surfaces to the new brightness level within this time. The readjustment cannot be carried out with a speed greater than the speed of light. If the readjustment happens more slowly, then the quasars can be even smaller than the Solar System.
- 4.
Heeschen was one of the pioneers of American radio astronomy. He was a student of Dutch astronomer Bart Bok (1906–1983) at Harvard where he graduated in 1955. As the director of NRAO he brought this branch of astronomy to prominence in the USA. Bok was famous for the charting the structure of our Milky Way galaxy; himself he came from Kapteyn’s line of students. Heeschen’s many students include one of the authors of this book (HL).
- 5.
The largest interferometer system where antenna signals are connected right away using optical fibers is MERLIN of Manchester University with a 134-km baseline. It started operation in 1980, and achieves the resolution of 0.04 arcsec. Among its important results was the discovery of the first Einstein ring, a case of a point-like source becoming a ring in the sky due to gravitational lensing (see below). In the near future, Square Kilometer Array, situated in South Africa and Australia, will set new records as to the size of a radio telescope, with its square kilometer of effective signal-collecting area. The construction of this flagship facility of ground-based astronomy is expected to start in 2019.
- 6.
In future, the lenses may even reveal black holes. A lonely black hole is almost truly black, and it shows up only through its gravity. When it happens to lie in front of a distant star or a quasar, the image of the background object is magnified or split in a way which allows us to recognize the lensing action.
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Teerikorpi, P., Valtonen, M., Lehto, K., Lehto, H., Byrd, G., Chernin, A. (2019). Active Galaxies: Messages Through Radio Waves. In: The Evolving Universe and the Origin of Life. Springer, Cham. https://doi.org/10.1007/978-3-030-17921-2_26
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DOI: https://doi.org/10.1007/978-3-030-17921-2_26
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