RA Abbreviation for ➤ right ascension.

radar astronomy The use of radar for astronomy. Radar can be used to detect ➤ meteor showers, to measure distances to bodies in the solar system and determine their size and shape, and to map the surfaces of planets and moons. Radar signals transmitted by the 305-m (1000-foot) radio telescope at ➤ Arecibo Observatory have been used to map Venus and to characterize the size, shape and structure of ➤ asteroids. The ➤ Magellan spacecraft, placed in orbit around Venus, used ➤ synthetic aperture radar to map the planet's surface, which is concealed by opaque cloud, and the ➤ Cassini spacecraft used radar to study the surface of ➤ Titan. Radar is of fundamental importance for making precise measurements of distance within the solar system.

radial velocity The velocity of an object relative to an observer along the line of sight.

radiant The point on the celestial sphere from which the trails of meteors belonging to a particular ➤ meteor shower appear to radiate. Meteors entering Earth's atmosphere from a stream create trails that are almost parallel but perspective makes them seem to diverge from a point in the sky.

radiation belt A ring-shaped region around a planet where electrically charged particles (electrons and protons) are trapped and spiral around the direction of the magnetic field of the planet. The radiation belts surrounding the Earth are known as the ➤ Van Allen belts. Similar regions exist around other planets with magnetic fields, such as Jupiter.

Sagan, Carl Edward (1934–1996) The American planetary scientist Carl Sagan was one of the most well-known and influential figures in astronomy in the USA during his lifetime. He was particularly known for his interest in extraterrestial life and he strongly advocated searching for extraterrestrial intelligence. As an advisor to NASA, he contributed to many of NASA's planetary exploration space missions. A gifted popularizer, he wrote several bestselling books. His television series Cosmos, first shown in 1980, was an enormous success. In it Sagan examined a wide range of issues in science, particularly the origin of life in the universe. From 1968 he worked at Cornell University and directed the Laboratory for Planetary Studies there.

Sagitta (The Arrow) The third-smallest constellation, but nevertheless a distinctive little group of stars. The two brightest stars are third magnitude. It lies in a rich part of the Milky Way, next to Aquila.

Sagittarius (The Archer) The southernmost constellation of the zodiac. The center of the Galaxy (the Milky Way) lies behind the star clouds in Sagittarius. It is a large constellation, with many bright stars. It also contains a large number of star clusters and nebulae. The ➤ Messier Catalogue lists 15 objects in Sagittarius, more than in any other individual constellation. They include the ➤ Lagoon Nebula, the ➤ Trifid Nebula, the ➤ Omega Nebula and the third brightest ➤ globular cluster in the sky, M22.

Sagittarius A The strongest radio source in the constellation Sagittarius.

Baade, (Wilhelm Heinrich) Walter (1893–1960) Walter Baade was born in Germany and began his astronomical career at the Bergedorf Observatory of the University of Hamburg. While there, he discovered the unusual asteroid ➤ Hidalgo. In 1931 he moved to the ➤ Mount Wilson Observatory in California where he worked until his retirement. He discovered a total of 10 asteroids, including ➤ Icarus.

In the 1940s, he used the 100-inch telescope at Mount Wilson to resolve individual stars in the ➤ Andromeda Galaxy and two of its small companion galaxies. This led him to divide stars into two broad groups, ➤ Population I and ➤ Population II. He realized that Population I had the characteristics of young stars while population II was older. He also found that each population had its own kind of ➤ Cepheid variable star. Cepheids had been used incorrectly to judge the distance of the Andromeda Galaxy and Baade showed that it was twice as far away as previously thought. In the 1950s he worked on identifying radio sources, including ➤ Cygnus A.

Baade's Window An area of sky around the globular cluster NGC 6522 in the constellation Sagittarius, which is particularly rich in stars. The astronomer Walter ➤ Baade drew attention to it. He realized that very distant stars are visible in that direction because there is relatively little interstellar material to hide them.

Baikonur The Russian manned space-flight center, established by the former Soviet Union. It is situated north-east of the Aral Sea in Kazakhstan.

M Abbreviation for the ➤ Messier catalog of galaxies, nebulae and star clusters.

Mab A small inner satellite of Uranus, discovered in 2003. It is about 25 km (16 miles) across. Mab orbits Uranus at the same distance as one of Uranus's rings and may be the source of the material for that ring.

Magellan A US spacecraft placed in orbit around ➤ Venus to map the surface by means of ➤ synthetic aperture radar. It was launched from the Space Shuttle Atlantis on May 4, 1989. The use of radar was essential because Venus is perpetually covered by opaque cloud. Magellan arrived at Venus on August 10, 1990, and completed its first phase of operations in May 1991, having mapped 84 percent of the surface. The next phase of observation involved filling in gaps and making more detailed observations. Magellan burned up in the venusian atmosphere in 1994.

Magellanic Clouds Two small, irregular galaxies, which are satellites of our own ➤ Galaxy. They are visible as hazy patches in the southern sky. The Large Magellanic Cloud (LMC) is in the constellation Dorado and is about 170 000 light years away. The Small Magellanic Cloud (SMC), in Tucana, is about 210 000 light years distant.

Magellanic Stream A long streamer of neutral hydrogen gas apparently spanning the 200 000 light years between the ➤ Magellanic Clouds and our own ➤ Galaxy. It forms an arc 150° long in the southern sky.

There is always something new in astronomy. Exciting discoveries follow one after another at a dizzying pace, thanks to the batteries of giant telescopes perched on mountain tops and equipped with the latest technological innovations, observatories orbiting high above the troublesome atmosphere, and spacecraft exploring the worlds of the solar system from close quarters. Keeping abreast of it all can be a challenge!

For this illustrated A-to-Z, I have made an up-to-date selection of 1300 entries covering hundreds of named astronomical objects as well as the terms and abbreviations most commonly encountered in astronomy. I have also included biographical entries on 70 people who have made significant contributions to the development of astronomy. Three hundred entries are illustrated, nearly all in color.

The idea for an illustrated dictionary grew from the dictionary I originally compiled in 1988–90, the most recent edition of which was published by Cambridge University Press in 2001. But this is a new book with a fresh style, which I hope will appeal to a wide range of readers young and old – not just as a reference source in which to look things up, but also as a book full of fascinating facts and beautiful pictures to dip into anytime.

Using the book

The alphabetical order takes no account of word breaks or hyphens. Entries beginning with a Greek letter are treated as if the letter were spelled out.