A photographic atlas of stars down to sixteenth magnitude, compiled by John Franklin Adams 1843-1912 and published in 1914.

The dark absorption lines in the spectrum of the Sun and, by extension, in the spectrum of any star. Many of the stronger ones were first mapped by Joseph von Fraunhofer 1787-1826, who also labelled some of the most prominent with letters of the alphabet. Some of these identifying letters are still commonly used in physics and astronomy, notably the sodium D lines and the calcium H and K lines.

Fraunhofer’s original 1817 designations of absorption lines in the solar spectrum
Letter	Wavelength nm	Chemical origin
A	759.37	Atmospheric O2
B	686.72	Atmospheric O2
C	656.28	Hydrogen α
D1	589.59	Neutral sodium
D2	589.00	Neutral sodium
D3	587.56	Neutral helium
E	526.96	Neutral iron
F	486.13	Hydrogen β
G	431.42	CH molecule
H	396.85	Ionized calcium
K	393.37	Ionized calcium

Note: Fraunhofer’s original observations did not resolve the components of his D line.

An observatory on Mount Hopkins in Arizona, operated by the Harvard-Smithsonian Center for Astrophysics. The instruments located there, at an altitude of 2,600 metres 8,500 feet, are a 10-metre 32-foot optical reflector for gamma-ray astronomy, 0.61-metre 24-inch and 1.5-metre 60-inch optical telescopes for planetary, stellar and extragalactic astronomy, automatic photoelectric telescopes, and the reconfigured Multiple Mirror Telescope MMT, operated jointly with the University of Arizona for optical and infrared observations. It is named in honour of a former Director of the Smithsonian Astrophysical Observatory who is particularly well known and distinguished for his work on comets.

The number of times a repetitive phenomenon occurs in a unit of time. In the case of waves, the frequency is the number of waves passing a fixed point in a second. The basic unit of frequency is the hertz Hz.

A model of the universe that can collapse in on itself. Alexander Friedmann 1888-1925 discovered an error in Einstein’s work on cosmology, from which he showed that expanding universes and oscillating universes were possible in General Relativity. This work 1922 and 1924 was completely ignored until rediscovered and presented as the Lema”tre universe. The Friedmann universe can be closed if the density of matter is sufficient to reverse the expansion, and this has led to searches for the so-called missing mass. See also: open universe, closed universe, Robertson-Walker metric.

A type of variable star characterized by a brightening over a period of the order of a year by as much as five or six magnitudes, followed by a long period decades of constancy or slow decline. These stars are supergiants of spectral types F and G, and are surrounded by dust and nebulosity.

The phase of the Moon when its celestial longitude is 180° greater than the Sun’s and its disc thus appears fully illuminated.

A catalogue of star positions which have been determined absolutely and with the highest possible precision from a compilation of many observations. Absolute positions are determined by recording the times of transit across the meridian. Combination of the results from several observatories helps to reduce the effects of both systematic and random errors. A series of fundamental catalogues has been produced and published in Heidelberg, Germany. The current version, known as FK5, contains improved data on the 1,535 stars of magnitude 7 and brighter also listed in FK3 and FK4. Work is in progress on an Extension to FK5 that will cover about a thousand stars in the magnitude range 5-7 selected from the FK4 Supplement, plus around two thousand others in the magnitude range 6.5-9.5.

The zero point of a system of time measurement, from which times may be measured forwards or backwards.

A plane that forms the basis of a particular coordinate system.

English name for the constellation Fornax.

Abbreviation for Far Ultraviolet Spectroscopic Explorer.