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high-frequency interference. For this reason, radio observatories are best built in remote locations.

The difficulty of achieving high resolutions with single radio telescopes led to radio interferometry, which was developed in 1946 by British radio astronomer Martin Ryle and Australian engineers, radio physicists, and radio astronomers Joseph Lade Pawsey and Ruby Payne-Scott. It is based on superimposing images from two or more spatially distant sources.

Modern radio interferometers consist of widely separated radio telescopes observing the same object, connected by a coaxial cable, waveguide, optical fiber, or other types of transmission lines. Not only does this increase the overall signal, but it can also be used in a process called ‘aperture synthesis’ or ‘synthesis imaging’ to greatly increase resolution. This technique works by superimposing (interfering) the signal waves from the different telescopes on the principle that waves coincident with the same phase add up, while two waves with opposite phases cancel each other out. This creates a combined telescope that is the size of the most widely spaced antennas in the array.

To produce a high-quality image, multiple and different distances between telescopes are required (the projected distance between two telescopes, as seen from the radio source, is called the ‘baseline’). As many different baselines as possible are needed to obtain good quality images. For example, the Very Large Array (VLA) has 27 telescopes that simultaneously provide 351 independent baselines.

Since the 1970s, improvements in the stability of radio telescope receivers have made it possible to combine telescopes from around the world—and even from Earth orbit—to perform interferometry with very long baselines. Instead of physically connecting the antennas, the data received at each antenna is paired with timing information, usually from a local atomic clock, and then stored for later analysis. At that later time, the data is correlated with data from other antennas recorded in a similar manner to produce the resulting image. Using this method, it is possible to produce an antenna that is effectively the size of the Earth. Because of the large distances between telescopes, very high angular resolutions can be achieved, greater than in any other field of astronomy.

Radio astronomy has led to a significant increase in astronomical knowledge, in particular through the discovery of several classes of new objects, including pulsars, quasars, and radio galaxies and, for a different kind of example, Sagittarius A*, the black hole at the center of the Milky Way. This is because radio astronomy allows us to see things that are undetectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.

The cosmic microwave background radiation was first discovered with radio telescopes, and radio telescopes have also been used to study objects much closer to us, such as the sun and its activity, and radar mapping of the planets.

Ultraviolet Astronomy

Ultraviolet astronomy is the observation of electromagnetic radiation in the ultraviolet wavelength range between about 10 and 320 nanometers. Ultraviolet light (infamous for the sunburns it causes on our skin) is not visible to the human eye. Although many a cancer-risking red arm or back would seem to say otherwise, the Earth’s atmosphere absorbs most of the light at these wavelengths, so astronomical observations must be made from the upper atmosphere or space.

Measurements of the ultraviolet light spectrum (UV spectroscopy) reveal the chemical composition, density, and temperature of the interstellar medium, as well as the temperature and composition of young stars. UV observations also provide essential information about the evolution of galaxies. The ultraviolet universe looks very different from the familiar stars and galaxies seen in visible light. Most stars are relatively cool objects that emit much of their electromagnetic radiation in the visible or near-infrared part of the spectrum.

Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. When viewing ultraviolet light emissions from the terrestrial sky, most stars would fade. Instead, the most visible stars would be a few very young and massive stars, and some very old stars and galaxies that are becoming hotter and producing high-energy radiation shortly before they die. However, gas and dust clouds could also block the view in many directions along the Milky Way.

With the help of ultraviolet astronomy, it was possible to learn significantly more about gas flows around hot stars and in binary systems. But researchers also gain new insights within our solar system with data from ultraviolet observations. For example, by studying the gases ionized in the tails of comets by the solar wind, it is possible to determine their composition. In addition, UV light has provided data on the composition of the atmospheres of planets such as Venus.

Infrared astronomy

Infrared astronomy studies objects visible in the infrared (IR) range, but invisible to the human eye. The wavelength of infrared light, also called thermal radiation, ranges from 0.75 to 300 micrometers.

Infrared astronomy had its early beginnings in the 1830s, a few decades after William Herschel discovered infrared light in 1800. Only after radio astronomy provided essential discoveries in the 1950s and 1960s, however, and astronomers then realized the true value of the information available outside the visible wavelength range, was modern infrared astronomy founded.

Practically no new techniques had to be developed for this purpose. Infrared and optical astronomy are often performed with the same telescopes because the same mirrors or lenses are usually effective over a wavelength range that included both visible and infrared light.

However, the water vapor in the Earth’s atmosphere absorbs some of the infrared light. Therefore, like radio telescopes, most infrared telescopes are located at high altitudes in dry locations, i.e., above as much of the atmosphere as possible. There are also infrared observatories in space, including the Spitzer and Herschel space telescopes. The James Webb Space Telescope (JWST), scheduled for launch in late 2021, also observes primarily in the infrared.

Infrared telescopes have helped find just-forming stars, nebulae, and stellar nurseries. They are also useful for observing extremely distant objects such as quasars. This is because quasars are moving away from Earth as the universe