The 17th century marked a pivotal era in astronomy, driven by the invention of the telescope by Galileo and the significant scientific achievements of Kepler and Newton regarding gravitational forces. Astronomy made a substantial leap from astrology to become a natural science. Physics began when astronomers deciphered the movements of celestial bodies. They then explored the Sun’s energy and the stars, along with the origins of the Universe.
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The antennas in the BIMA radio telescope array at the University of Berkeley (California, USA). The central region of the Milky Way is in the background (Photo: vietsciences) |
Radio astronomy, along with antennas designed to capture signals emitted by celestial bodies, requires not only the observation of visible radiation but also the collection of radiation across different wavelengths of the electromagnetic spectrum, such as X-rays, ultraviolet, infrared, and radio waves. Each type of radiation is emitted through distinct physical mechanisms. For instance, visible radiation is observed to study stars and ionized gas in galaxies. Dust components emit only infrared radiation. Molecules in dark clouds within interstellar space emit radio waves. The cosmic background radiation at 2.7K, the strongest among the stars, emits radio waves from the Sun without interference from celestial bodies. Moreover, the radio radiation from the Sun does not interfere with celestial observations. Radiation emitted at centimeter wavelengths is not obstructed by rain, making observational work independent of weather conditions.
In 1932, Carl Jansky, working at Bell Telephone Laboratories, accidentally discovered the first radio wave radiation from the Universe. This radiation originated from the Milky Way galaxy. After World War II, radio astronomy began to flourish due to advancements in radar antenna technology and radio signal receivers. The intensity of radio waves received from celestial bodies via telescopes is only about one trillionth of a watt (10-18 W), extremely small compared to the intensity of a typical 60W light bulb. Therefore, antennas used in astrophysics must be quite large, and the receiving components must be extremely sensitive to “catch” every single photon. Astronomers have successfully captured radio emissions from our Solar System, stars, nebulae in the Milky Way, distant galaxies, and quasars.
Thanks to radio wavelength observation tools, unexpected discoveries have arisen, such as pulsars, a type of star that rotates and emits signals like a lighthouse. A major achievement in radio astronomy was the discovery of cosmic background radiation in 1965, a crucial factor reinforcing the Big Bang theory that explains the Universe’s origin. A few years later, radio astronomers found numerous molecules within the Milky Way, including complex organic molecules. Notably, these organic molecules contain amino acid samples found in proteins, the building blocks of life. The discovery of molecules in the Universe marked a significant event in the study of dark clouds that had previously gone unobserved.
Telescopes used in radio astronomy function as antennas, similar to radar, capable of tracking celestial bodies across the sky. The signals emitted from celestial bodies captured by radio telescopes are akin to the noise heard on a radio receiver. Specialized receivers and signal amplifiers using superconducting diodes, which produce minimal noise, are developed to enhance the signals from celestial bodies. Signal receiving components are often housed in a Helium-cooled chamber to reduce noise generated by the equipment. Data is processed using computers, and the radio signal is transformed into images displayed on screens. To achieve a clear image of a celestial body, astronomers must “capture” the object for an extended duration by tracking it for hours. Computers are also utilized to control the automated rotation of telescopes towards the observation target.
The VLA antenna system (Photo: public)
Short-wavelength radio emissions, particularly millimeter wavelengths, are absorbed by water vapor in the atmosphere. Millimeter telescopes are often placed at high altitudes in dry locations, similar to optical observation telescopes. Observations at longer wavelengths, such as centimeter and meter waves, are frequently affected by artificial interference from radar and satellite TV transmission systems. Radio observatories are typically located in remote areas away from industrial centers. An International Committee has been established to allocate specific frequency bands in the electromagnetic spectrum for radio astronomers. These frequency bands contain numerous molecular lines that astronomers often observe. In principle, no one is allowed to transmit signals within these protected frequency bands. However, in reality, there is often interference, partly due to the increasing sensitivity of cosmic radiation receivers.
Radio telescopes possess diameters ranging from 10 to 300 meters. Currently, there are approximately 40 radio telescopes worldwide. Since the surface of the antenna does not need to be as smooth as the mirror surface of optical telescopes, constructing large antennas is more feasible than making large mirrors. Furthermore, the resolution limit (the ability to distinguish fine details) at radio wavelengths is inferior to that at optical wavelengths. To achieve a resolution limit of a radio telescope comparable to that of a 10-meter optical mirror, the antenna size would need to be 10 kilometers. With modern technology, constructing such a gigantic antenna is not feasible. However, excellent resolution limits akin to those of a 10-kilometer giant antenna can be attained by using two smaller antennas spaced 10 kilometers apart, which operate in correlation by mixing the signals received from each antenna. According to the principles of optics, mixing signals enhances the resolution capability of the telescope.
In practice, an array of antennas is often utilized to enhance the sensitivity of the interferometer system. The VLA (Very Large Array) consists of 27 antennas, each with a diameter of 25 meters, located in New Mexico, USA, operating at centimeter and millimeter wavelengths. The antennas in the VLA system are arranged in a Y-shaped configuration, with a maximum distance of about 35 kilometers between them. The VLA’s interferometer system has a resolution limit equivalent to that of a 35-kilometer antenna, capable of distinguishing details as small as a candle flame from a distance of at least 4 kilometers. The greater the distance between the antennas, the clearer the details of the celestial object. Each year, more than 500 astrophysicists worldwide utilize the VLA antennas. Currently, there is an international interferometer network comprising around ten antennas spaced thousands of kilometers apart on different continents to achieve extremely fine resolution limits. There are proposals from Russia, Japan, and the European community to launch rotating antennas into orbit around Earth to increase the distance between antennas observing distant galaxies and star clusters with extremely small angular sizes. These interferometer systems have the potential to analyze fine details comparable to a candle flame placed on the Moon.
The VLA antenna system (Photo: astro.nmsu.edu)
