Radio astronomy is a way of looking at extraterrestrial objects using the radio waves that they emit or reflect.
Radio waves are part of what is known as the electromagnetic spectrum. The electromagnetic spectrum contains all the different frequencies of electromagnetic radiation, of which visible light is a small part, specifically 4 *10^14 to 7*10^14 hertz (7.5*10^-07 to 4.3*10^-7 meters). As you dip below the range of visible light you get into the range of infrared light. Infrared light is the range of the electromagnetic spectrum between 7*10^14 hertz ( 4.3*10^-7 meters) and 10^13 hertz (3*10^-5 meters). Once below infrared, there is microwave radiation. Microwaves continue from 10^13 to 3*10^11 hertz (3*10^-5 to 1*10^-3 meters). Any electromagnetic radiation with a frequency below 3*10^11 hertz (1*10^-3 meters) is considered radio waves. They have a wavelength of at least 1 millimeter(“The EM Spectrum”).
The first form of electromagnetic radiation outside of the visible spectrum was discovered in the year 1800 by Sir William Herschel. He performed an experiment in which he separated light with a prism. He then placed thermometers in each distinct color of light, including one above and below the visible light. The thermometer that was outside of the visible light, on the red end, measured the highest temperature, though it seemed to be wholly outside of the light. It was this experiment that first showed the existence of infrared radiation(NASA).
The theory of electromagnetism was proposed in 1873, by the Scottish physicist, James Clerk Maxwell. He determined that there were four main electromagnetic interactions. The first is that the force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them, the second is that magnetic poles come in pairs that attract the opposite pole and repel like poles, the third is that an electric current in a conductor produces a magnetic field, the poles of the field being determined by the direction of the current, and the fourth is that moving an electric field relative to a conductor produces a magnetic field, and vice versa (Lucas, “Electromagnetic Radiation”).
The theory of electromagnetism by Maxwell predicted the existence of radio waves, and in 1886, Heinrich Hertz, a German physicist, used Maxwell’s theories to produce and receive radio waves. Maxwell used an induction coil and a Leyden Jar to produce the radio waves he detected. Hertz was the first person to transmit and receive radio waves, and the basic SI unit of frequency, the hertz, was named in his honor, as one electromagnetic cycle per second(Lucas, “Radio Waves”).
Sir Oliver Lodge was the first person to attempt to detect radio waves from an extraterrestrial source. He was conducting experiments on the propagation and reception of electromagnetic waves. He went on to create the “trembler”. It was a device based on the work of the French physicist Édouard Branly, who showed that loose iron filings react to electromagnetic waves inside of a glass tube. Lodge’s trembler used the shaking of iron filings to detect Morse code carried by electromagnetic radiation(Britannica). During his experiments with electromagnetic radiation, Lodge viewed the Sun as a possible source for radio waves. He attempted to detect them in the centimeter wavelengths, but though he failed, he was still the first one to try.
Many attempts were made to detect radio waves from extraterrestrial sources, but none were successful until 1931. Karl Jansky, while working for Bell Laboratories, tried to determine the source of radio interference that was present around 20 MHz. Jansky built a large steerable antenna that would receive in wavelengths of 5 to 30 meters, or 15 to 30 MHz frequency. The antenna allowed him to locate the sources of static. He found three distinct sources, local thunderstorms, distant thunderstorms, and a steady hiss of static that was coming from the center of the Milky Way galaxy, as he showed using the source’s position in the sky (“UREI-Radio Astronomy Tutorial-Sec.4”).
Jansky’s findings were largely ignored for quite a while. It wasn’t until 1937, when Grote Reber read Jansky’s work that there were any major advancements. Reber was an electronics engineer, and an avid radio amateur. After reading Jansky’s work, Reber built the first real radio telescope, to which most telescopes today are very similar. It was a 9.5 meter parabolic reflector dish that he built alone in his backyard. He spent years studying radio emissions of varying wavelengths until he detected celestial emissions at a wavelength of 2 meters. Reber confirmed that the emissions were from the galactic plane, and continued to observe various radio sources until in 1944 he published the first radio frequency sky maps. His telescope is still on display today at the Green-Bank Observatory, in Green-Bank, West Virginia(“UREI-Radio Astronomy Tutorial-Sec.4”).
The first radio emissions from the Sun were discovered in 1942 by J.S. Hey, who was working with the British Army Operational Research Group to analyze occurrences of radio jamming of Army radar sets. There was a system for observing and recording potential jamming signals, which led Hey to conclude that the sun was emitting intense radio waves. It was later in the same year that G.C. Southworth made the first successful observations of thermal radio emission from the sun at centimeter wavelengths, a frequency of less than 30 GHz(“UREI-Radio Astronomy Tutorial-Sec.4”).
The next major discovery was in 1963 when Bell Laboratories assigned Arno Penzias and Robert Wilson the task of tracing a radio noise that was interfering with the development of communication satellites. They discovered that no matter what direction they pointed the antenna, it would always receive the interference even where the sky was visibly empty. They had discovered cosmic background radiation, an ambient radiation that is always present. It is now widely believed that cosmic background radiation is energy that remains from the creation of the universe. Wilson and Penzias went on to win the Nobel Prize for their discovery in 1978 (“UREI-Radio Astronomy Tutorial-Sec.4”).
A typical radio telescope consists of several different parts. They are the antenna, the reflector dish, the amplifier, and the computer that receives, records and processes the data. The dish is used to reflect and focus the radio waves onto the antenna. The antenna of a radio telescope, at the focal point of the dish, receives the radio waves and converts them to electricity. The resulting electrical signals are sent to the amplifier, which as the name implies, amplifies the signal. Once the signal has been amplified it is usually sent to a computer where it is processed and recorded. Many telescopes are also mounted on a base that allows for movement, so they can be aimed at different parts of the sky(“Design of a Radio Telescope”).
The structure of radio telescopes haven’t changed much since the one that Reber built. Perhaps the largest advancement is telescope arrays. An array is many smaller radio telescopes that have been linked together. The multiple telescopes create an effect similar to having a single telescope with a dish radius as large as the radius of the array. This is due to the different positions from which each telescope views the sky, causing them to receive the same signals that a single dish would if it extended as far as the array. There are many problems with building a large single dish telescope. It would be very hard to build, as it would require an immense amount of materials and have difficulty supporting its own weight. An array has some drawbacks of its own, such as the light gathering abilities of an array are worse than what they would be if it was a single dish telescope of the same size, as light gathering abilities are determined by the total area covered. Another advantage of arrays is that many smaller telescopes are much easier to direct than a large one.
One example of a telescope array is the Very Large Array, or VLA, in New Mexico. It is composed of 28 separate dishes and antennas. Each dish is 82 feet wide and has 8 receivers. They are all steerable. They are arranged in a “Y” shape that varies in size, because the telescope are all on rails, making them mobile to adjust the size of the array. The array is half a mile to 23 miles across, depending on where the telescopes are on the rails(“Very Large Array”). It operates in frequencies from 1 to 50 GHz. The surfaces of the dishes are made of aluminum panels. The VLA has made and contributed to many interesting discoveries, such as ice on Mercury, study of black-holes, and the center of our galaxy. It also observed an effect predicted by Einstein, Einstein rings. These are rings of electromagnetic radiation that have been distorted through intense gravity of massive objects such as black-holes.
One of the largest single dish telescopes is the Arecibo Observatory, in Puerto Rico. It is called the National Astronomy and Ionosphere Center (NAIC) and was built in 1960, in Arecibo, Puerto Rico. It is built into a volcanic crater and has a 305 meter wide reflector dish. The antenna is suspended above it by cables and can be moves to track different parts of the sky. The antenna of the telescope is also unusual. It is a dome that contains multiple reflecting dishes that further focus the radio waves. This is done so that the antenna can be mobile, to aim the telescope, as the dish is stationary. The Arecibo telescope discovered the first extra-solar planets. It was also used to produce detailed radar maps of the surface of Venus and Mercury, and discovered the first binary pulsar.
The EHT (Event Horizon Telescope) array covers the largest distance of any array. It is composed of 10 different telescopes and arrays around the world. Due to the spread of its components it has an effective aperture the size of the earth, which gives it the highest angular resolution possible on the surface of the earth. The Atacama Pathfinder Experiment (APEX), an array that is a collaboration between the Max Planck Institut für Radioastronomie (MPIfR), the Onsala Space Observatory (OSO), and the European Southern Observatory (ESO) is one part of the EHT array. The 30-meter telescope on Pico Veleta in the Spanish Sierra Nevada, the James Clark Maxwell Telescope operated by the East Asian Observatory, the Large Millimeter Telescope Alfonso Serrano built on the summit of Volcán Sierra Negra, the Sub-millimeter Array (SMA) located near the summit of Mauna Kea in Hawaii, the Atacama Large Millimeter Array (ALMA) in Chile, and the South Pole Telescope (SPT) all make up the worldwide EHT array (“Array”).
The EHT array has been used to monitor black holes. It has watched the black-holes SgrA at the center of the Milky Way and M87 in the center of the Virgo A galaxy. It has been studying the validity of many theories about the mass of black holes, the event horizon, gravitational lensing, and possible emissions. The silhouettes of the black-holes matched predictions. In 2019, the EHT also managed to take a direct picture of the silhouette of the black-hole in the center of our galaxy, SgrA.
Radio telescopes have made many discoveries and observations. One such observation is the imaging of asteroids. Scott Hudson and Steven Ostro were the first people to image an asteroid. They imaged the peanut-shaped asteroid 4769 Castalia using the Arecibo observatory in 1989.
Another discovery is that of millisecond pulsars. The discovery was made in 1983, by Donald C. Backer, Miller Goss, Michael Davis, Carl Heiles and Shrinivas Kulkarni at the Arecibo telescope. A millisecond pulsar is one with a very fast rotational period. The one discovered is known as PSR B1937+21, and spins about 641 times a second. Since this discovery nearly 200 more have been discovered (Maggio).
One of the many applications for radio astronomy has been in the search for extraterrestrial intelligence. The term most commonly used for scientific pursuits of extraterrestrial life is SETI (Search for ExtraTerrestrial Intelligence). SETI has involved many arrays and telescopes around the world. There are also several projects that allow people to help observe the sky and donate idle processing power to analyzing the radio signals. SETI has not found any definite signs of other intelligent life, but it has detected signs that may indicate other intelligent life, such as the WOW signal. The WOW signal was an unusual burst of electromagnetic radiation, as if another planet was sending out radio bursts in the chance that someone detects them as we are.
Mercury’s rotational period was determined by radio telescopes. Using the Arecibo telescope, in 1964, Pettengill found that Mercury actually completed a full rotation every 59 days. It had long been thought to be tidally locked, maing the rotational period the same as the orbital, at 88 days (Maggio).
Binary pulsars were also discovered through radio astronomy. They were discovered in 1974. A binary pulsar is a pulsar with a white dwarf or neutron star orbiting it (Maggio).
In 2008, Arecibo was used to detect organic molecules in a starburst, an unusually fast developing star system that consumes available gases much more quickly than normal, 250 million light-years from Earth. Methanimine., a carbon based molecule (CH3N), and hydrogen cyanide (HCN) were discovered in the starburst Apr 220, which lies in the constellation Serpens. The discovery of organic molecules is very important for the possibility of life in other solar systems (Maggio).
Another very important discovery of radio telescopes is that of exoplanets. An exoplanet is any planet that exists outside of our solar system. On January 9, 1992, the astronomers Alex Wolszczan and Dale Frail, using the Arecibo telescope, discovered exoplanets orbiting a pulsar named PSR 1257+12. It is 2,300 light-years away in the constellation Virgo (“Top Astronomical Discoveries”).
Radio astronomy has made many interesting discoveries, and given rise to many theories. It has been used for many different purposes, from photographing the event horizon of black-holes to searching for extraterrestrial life. It will likely continue to be enlightening and interesting into the future as it has been for the past 90 years
NASA, NASA, imagine.gsfc.nasa.gov/science/toolbox/history_multiwavelength1.html.
The EM Spectrum, labman.phys.utk.edu/phys222core/modules/m6/The EM spectrum.html.
A Brief History of Radio Astronomy, www.astronomytoday.com/astronomy/radioastro2.html.
UREI-Radio Astronomy Tutorial-Sec.4, www.haystack.mit.edu/edu/undergrad/materials/tut4.html.
Design of a Radio Telescope, www.sas.upenn.edu/~patann/RadioTelescopeDesign.htm.
“Array.” Event Horizon Telescope, eventhorizontelescope.org/array.
Britannica, The Editors of Encyclopaedia. “Sir Oliver Joseph Lodge.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 8 June 2019,
Lucas, Jim. “What Are Radio Waves?” LiveScience, Purch, 27 Feb. 2019,
Lucas, Jim. “What Is Electromagnetic Radiation?” LiveScience, Purch, 12 Mar. 2015,
Maggio, Patricia K. “The Top Five Discoveries Made by Radio Telescopes.” Sciencing, 2 Mar. 2019, sciencing.com/top-discoveries-made- radio-telescopes-7566858.html.
“Top Astronomical Discoveries Made by Radio Telescopes.” Home Page -, 7 Nov. 2018,