The strongest radio source in the constellation Hercules, associated with an elliptical galaxy. Two long jets extend for half a million light years into space from the faint nucleus.

An X-ray pulsar in the constellation Hercules, consisting of a rotating neutron star which is accreting matter from its companion in a binary system. The rotation period of the neutron star is 1.2 seconds and the orbital period of the system 1.7 days.

English name for the constellation Boötes.

Asteroid 1937 UB, discovered by K. Reinmuth in 1937 when it passed within 800,000 km of the Earth, in what was then the closest approach of an asteroid ever recorded. It reached eighth magnitude and travelled across the sky at a rate of 5° per hour. It was observed for a only a few days and was subsequently lost.

The largest impact crater on Mimas. Its diameter is 130 kilometres 80 miles, one-third the diameter of Mimas.

William Herschel Telescope.

A device for safely reducing the intensity of light transmitted through a telescope during observation of the Sun. It consists of a thin prism with unsilvered faces. The first face is inclined at 45° to the optical axis and reflects 5 per cent of the incident radiation. A baffle is used to intercept the reflected beam from the second surface. The remaining radiation, which passes through, should be safely absorbed in a heat trap. An infrared rejection filter should also be used in conjunction with a Herschel wedge.

A type of reflecting telescope designed by William Herschel 1738-1822, in which the paraboloid primary mirror is tilted so that its focus lies outside the main tube of the telescope and can be accessed without obstructing the incoming light. The system has the disadvantage that distortions are introduced, and it was soon superseded by other types of reflector.

A region on the Hertzsprung-Russell diagram, between the giant branch and the main sequence, where very few stars are represented. The gap reflects a general absence of giant stars of spectral types F and G, because the stage in the evolution of a star when it is observable as a giant of this type is very short. Thus at any one time, relatively few such stars exist.

A graph displaying, for any sample of stars, the relationship between their spectral type and luminosity. Colour, temperature or some other comparable quantity may be substituted for spectral type as the quantity plotted along the horizontal axis. Temperature conventionally decreases towards the right. Either magnitude or luminosity relative to the Sun are frequently used for the vertical scale. The resulting graph may also be called a colour-magnitude diagram or colour-luminosity diagram according to the actual quantities used. What is now known as a Hertzsprung-Russell diagram was first plotted by Henry Norris Russell in 1913. It was later recognized that Ejnar Hertzsprung had independently put forward similar ideas at around the same time. Any star whose spectral type and luminosity are known may be plotted as a single point on the HR diagram, but the diagram acquires particular significance when plotted for a related group of stars, such as a star cluster. For any sample of stars, the points are not distributed randomly: most lie on a band running diagonally from the upper left to the lower right, the so-called main sequence. It arises because the most significant factor determining a star’s spectral type and luminosity is its mass - the main sequence is in effect a mass sequence. The idea once held that it is an evolutionary sequence is known to be wrong. Nevertheless, hot stars are still often called early-type stars and cooler ones late-type stars. These misnomers are a legacy from the misinterpretation of the main sequence. The effects of evolution in fact move stars away from the main sequence, which represents stars burning hydrogen in thermonuclear reactions in their cores. When the central hydrogen is exhausted, a sequence of internal changes leads to a great expansion of the star, coupled with a decrease in surface temperature. Such evolved stars are found in the giant and supergiant branches lying above the main sequence. The highly evolved white dwarfs form a group well below the main sequence. The HR diagram for a star cluster immediately makes clear how many stars there are at each stage of evolution. This, coupled with the theoretical knowledge of how evolution rate increases with stellar mass, gives an important key to the ages of clusters. Plotting apparent magnitude on the vertical axis rather than absolute magnitude for a cluster provides a method of measuring a cluster’s distance. HR diagrams are also useful for displaying the sequence of changes in colour and luminosity that take place in an individual star in the course of its evolution - before, on and after the main sequence. The result is an evolutionary track. See also: stellar evolution.

The astrophysics research organization of the National Research Council of Canada. See also: Dominion Astrophysical Observatory, Dominion Radio Astrophysical Observatory.

An instrument used in microwave astronomy to measure the strength of the cosmic background radiation by switching a detector rapidly between a stable reference source and the sky.

A lunar crater, 118 kilometres 73 miles in diameter, on the western border of the Oceanus Procellarum. There is a system of clefts on the crater floor.

A type of iron meteorite containing less than 6 per cent nickel by weight. Hexahedrites contain the form of iron-nickel alloy called kamacite, which has cubic symmetry. The polished surfaces of such meteorites are featureless except for numerous striations, called Neumann lines, which occur in some examples. They are caused by shock deformation.

Abbreviation for Herbig-Haro object.