A prefix given to objects in the Messier catalogue.

A star of spectral type M. M stars have surface temperatures in the range 2,400-3,480 K and are red in colour. Molecular bands are prominent in their spectra, particularly those of titanium oxide TiO. Examples of M-type stars include the nearest star, Proxima Centauri, which is a dwarf, and the supergiant Antares.

A fairly common type of asteroid with moderate albedo, presumed to have a metallic composition similar to that of iron meteorites.

A philosophical idea propounded by Ernst Mach 1838-1916 to the effect that the inertial properties of an individual piece of matter are determined by the distribution of distant matter in the universe. Einstein was influenced by this idea, though he and others have failed to incorporate it into his General Relativity theory.

Invisible stellar-sized objects, such as dim stars or brown dwarfs, which may account for the difference in mass between the luminous components of the Galaxy and the apparent total mass of the Galaxy implied by its gravitational behaviour. The term is said by those who invented it to stand for Massive Astrophysical Compact Halo Objects, but in part was contrived to contrast with another form of invisible matter, so-called WIMPs. See also: microlensing.

A dark spot on the surface of a planet.

Two galaxies discovered by Paolo Maffei in 1968. They are visible only in the red and infrared. Maffei I is a giant elliptical galaxy about four million light years away, and might be an outlying member of the Local Group. It is a large galaxy with a mass 200 billion times that of the Sun. Its position in the sky means that it is viewed through the dust clouds of the Milky Way, which dims its light by a factor of one hundred. Maffei II, an average spiral galaxy, lies beyond the Local Group, five times further away than Maffei I.

A US orbiting probe of Venus, launched from the Space Shuttle Atlantis on 4 May 1989. Its objective was to map by means of synthetic aperture radar at least 70 per cent of the surface of Venus to a resolution of several hundred metres. The radar technique is essential since Venus is perpetually covered by opaque cloud. Magellan arrived at Venus on 10 August 1990 and its first phase of operation, over a period of 243 days, was successfully completed in May 1991, 84 per cent of the surface having been mapped. The next phase of observation involved filling in gaps and making more detailed observations. Previous studies had shown that volcanic lava flows cover about 80 per cent of Venus. The Magellan images have made it possible to examine these features in much closer detail. A number of large shield volcanoes have been identified, along with impact craters, one of the largest of which is 275 kilometres 170 miles in diameter, together with a variety of features unique to Venus. The wealth of data returned by Magellan makes a very significant addition to our knowledge of the surface of Venus.

A project involving a consortium led by the Carnegie Institution of Washington to construct two 6.5-metre 21-foot telescopes at the Las Campanas Observatory.

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.

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. A possible explanation is that it is gas dragged from the Magellanic Clouds in a tidal interaction with the Galaxy.

A postulated defect in the fabric of spacetime that behaves like an isolated north or south pole of a magnet and has a mass 1016 times that of the proton. None has ever been detected. The potential existence of magnetic monopoles is a serious problem that arises in Grand Unified Theories of the fundamental physical forces. It they exist in significant numbers, they would drastically slow the expansion of the universe, which is not what is observed. The problem of defects is avoided in the inflationary universe.

A star with an exceptionally strong magnetic field. Magnetic fields more than a thousand times stronger than the Sun’s general field have been measured for a group of A stars, which also have peculiar spectra and so are classified as Ap stars. In the presence of a magnetic field, the lines produced in the stellar spectrum are split into polarized components the Zeeman effect. Although the lines are usually too broad for the split components to be resolved, the change in polarization across the broadened spectral lines can be measured and interpreted in terms of a magnetic field strength. In almost all cases, the fields and spectral line strengths vary regularly. This observation can be accounted for if the rotation and magnetic axes of the stars do not coincide. See also: peculiar star.

geomagnetic storm.

An instrument used in solar astronomy for mapping the strength, direction and distribution of magnetic field across the surface of the Sun.