The "A to Zee" of Astronomee.

By: Wolf Lange

Sources include: Collins Dictionary of Astronomy 2^nd Edition, Burnhams Celestial Handbook Revised and Enlarged Edition, Patterns in the Sky by Julian DW Staal and the Amateur Astronomers Handbook by JB Sedgwick.

Callisto – the faintest of the four giant Galilean Satellites of Jupiter. Heavily cratered with a radius of 2 400km it has interesting ray systems i.e. craters with radiating bright streaks. There are also some major concentric ring mountains near to the huge Valhalla Basin – slightly north of the equator. Callisto is thought to have a thick crust of ice and rock of at least 200km depth, covering a layer of convecting water or soft ice.

Canals – linear markings on Mars that were first observed by Giovanni Schaparelli in 1877 and later charted by many observers most notable by Percival Lowell who strongly advocated that they were irrigation ditches dug by Martians to distribute the planets rare water resources. For many years these markings confounded observers until both the early Mariner and Viking spacecraft observations show that neither the canals nor the associated plant growth on the assumed embankments, exist.

Cancer (the crab) – faintest of the 12 constellations of the Zodiac between Gemini and Leo. In ancient Egyptian history also seen as a Beetle or Lobster and by the Tibetans as a Frog. The cancer constellation contains many double and variable stars, the beehive open cluster (Praesepe) and the fainter open cluster M67 as well as a strong radio source NGC 2623.

Canis Major and Minor (Sirius and Procyon) – in Greek mythology these are the two hunting dogs of Orion the hunter. Two of the three stars making up the northern hemisphere winter triangle or our southern hemisphere summer triangle. The third star of the triangle is Betelgeuse, the second brightest star in the constellation of ORION. Sirius is shimmering blue-white and the brightest star next to the sun and Procyon is yellow-white (See further information under Sirius and Procyon later on).

Capricornus (sea goat) – a southern hemisphere zodiac constellation following Sagittarius. It contains the globular cluster M30. Also the general area where the minor meteor Capricornids "showers" are observed, maximising on 25 July.

Carbon or C stars – red giant stars of low temperature that have an over abundance of carbon relative to oxygen in their surface layer. These "cool" stars combine carbon and oxygen atoms to form stable carbon monoxide All carbon stars undergo mass loss enriching the interstellar medium with considerable carbon, some nitrogen and oxygen. In earlier Harvard classification, carbon stars were divided into R & N stars. N stars were the classical carbon stars many of which were discovered in the Magellanic clouds and other galaxies. (Carbon, nitrogen, oxygen are all vital building blocks of life as we know it – it makes you wonder what’s out there!)

Cartwheel galaxy – a remnant of a large galaxy that has been distorted by a head-on collision with a small neighbour into a shape resembling a cartwheel. This sounds like a great object to observe with a decent size telescope because……….. the "hub" and "spokes" of the wheel are formed by the old stars of the original galaxy and are surrounded by a "rim" of young blue stars and ionised gas triggered by a rippling outward shock wave. Now next question: Where is this object?!

Cassegrain – Guillaume Cassegrain a Frenchman who designed an excellent innovative telescope with a convex hyperboloid secondary mirror mounted inside the focal plane of the primary mirror. The final image is project through a hole in the centre of the paraboloid primary mirror where the eyepiece is placed just behind the primary normally in a stardiagonal fitting for easy viewing. This type of telescope design has been adopted for most modern large reflectors and it effectively "triples" the focal length within a third of the physical dimensions of the telescope!

Cassiopeia – a conspicuous northern hemisphere constellation named after the queen of Ethiopia. The outline is in the shape of a slightly skewed capital letter W and can also be imagined as the crown adorning the royal head of the queen.

CCD – "Charged Coupled Device". one of many new innovations brought about by the revolutionary advancements of computer technology. It is a light sensitive electronic detector, invented as far back as 1970 (I thought I would never write that the 1970s were far back!!) and now widely used in ground- and space-based astronomy for Imaging, Photometry, Spectroscopy and Astrometry. What makes CCDs special is the ability to have wide-spread sensitivity over a wide range of wavelengths from blue to near-infrared. Further advancements have pushed CCD ability into infrared, ultraviolet & X-ray regions. Compared to photographic plates CCDs are small – normally only a few square centimetres, covering a relative small field of view.

Astronomical CCDs are fabricated as a two-dimensional arrays of tiny pixels on a thin wafer of semiconductor – normally silicon – with literally thousands of rows and columns of pixels. Each pixel responds to the photons falling on it by producing electrons. The values that result from the exposure are digitised and stored in computer storage for further analysis. CCDs can be super cooled with liquid gasses to reduce "noise" when charges are moved out of a CCD into computer storage.

Celestial equator – imagine the terrestrial equator being projected against the sky – in short that would represent the celestial equator. The plane of the celestial equator is perpendicular to the celestial axis and is the reference plane for the equatorial co-ordinates Right Ascension and Declination used for….. correct….. to find objects like stars, clusters and galaxies in the sky using e.g. setting circles. Important Note: the celestial equator is slowly changing – about one zodiac constellation shift per every 2 000 years – as a result of the Precession of the Earth’s axis.

Cepheid – ever wondered how the distances of stars are measured that are too far away to use the diameter of the earth’s orbit around the sun as a baseline? Cepheid variables are used in this process.

Cepheids are a large group of very luminous yellow supergiant stars that are pulsating variables with periods mainly in the range of 1 to 50 days. Over 700 are known in our galaxy and several thousand in our local group. Found in two areas of galaxies, we have Type I or classical Cepheids that are found in spiral arms on the galactic plane and less common type II which are much older and less massive, are found in the galactic centre or halo.

The refurbished Hubble Space Telescope can study classical Cepheids in galaxies as far away as the Virgo cluster (about 15 megaparsecs). By measuring period vs absolute magnitude Baade and Kukarkin demonstrated the two categories. This in turn forms the basis for measuring the distances of stars further than about 300 ly away and galaxies.

We’ll continue with C in the next edition of Canopus.

Wolf Lange