A luminous triangular-shaped nebula in the constellation Monoceros. From photographs taken between 1900 and 1916, Edwin Hubble discovered that the nebula varied in shape and brightness. An irregularly variable star, R Monocerotis, is embedded in the nebula. R Mon is a strong source of infrared radiation and is probably a very young star surrounded by a circumstellar disc and ejecting a bipolar outflow. The nebula is now thought to be an example of a Herbig-Haro object. Light from the star reflected from interstellar dust is also seen.

superluminous variable stars noted by Edwin Hubble and Allan Sandage in their studies of the galaxies M31 and M33. They have been identified with the class of variables called P Cygni stars, or S Doradus stars.

Comet Humason.

A large lunar crater, 207 kilometres 128 miles in diameter, on the extreme south-eastern limb of the Moon.

Asteroid 434, diameter 11.4 km, discovered by M. Wolf in 1898.

A group of asteroids at the inner edge of the asteroid belt, 1.95 AU from the Sun, with orbits inclined at 24° to the plane of the solar system. The group is separated from the main belt by a Kirkwood gap, and is not a true family with a common origin.

English name for the constellation Orion.

English name for the constellation Canes Venatici.

One of the simplest designs of telescopic eyepiece, consisting of two planoconvex lenses. It works well on long-focus refractors, but image distortion by spherical aberration becomes apparent at short focal ratios

Cassini mission.

An open star cluster in the constellation Taurus. Its members appear to be scattered around the star Aldebaran which does not itself belong to the cluster but is in the foreground, over an area 8° in diameter. It is the nearest star cluster, lying at a distance of about 150 light years. Because it appears so scattered, it was not listed in either the Messier Catalogue or the New General Catalogue. See also: open cluster.

The largest constellation in the sky by area, but a difficult one to identify since it contains only one moderately bright star, the second magnitude Alphard. It was included by Ptolemy in his list of 48 constellations c. AD 140.

The brightest radio source in the constellation Hydra. It is associated with a large elliptical galaxy at the centre of a small cluster of galaxies about one billion light years away.

The most prominent line in the visible part of the spectrum of hydrogen. It is the leading line of the Balmer series, with a wavelength of 656.28 nanometres, and is red in colour. It arises from transitions between the second and third energy levels in the hydrogen atom. See also: Balmer lines.

The condition of stability that exists when gravitational forces are exactly balanced by counteracting gas and radiation pressure.