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.

Dark Matter – Matter that probably comprises 75% of the mass of the Universe but is undetectable except by its gravitational effect. The term missing mass was coined amounting to between 10 and 100x the amount of visible matter in the galaxies. In the 1970s research in rotation of spiral galaxies have shown that these galaxies have Dark Halos containing 10x more matter than the visible parts of the galaxies. This topic is still getting a high level of attention from scientists and astronomers. Our own Prof. Bloch has done significant research work in this field.

Dark Nebula – an absorption nebula that is of such density that it obscures partially or completely the light of stars and other objects behind it. Normally these nebulas become visible to the naked eye as dark extrusions because of their positioning in front of emission or reflection nebulae.

Declination - a co-ordinate used with Right Ascension, in the Equatorial Co-ordinate System. The declination of a celestial body is its angular distance between 0 and 90 degrees North or South of the Celestial Equator.

Deep Impact – a NASA mission planned for January 2004, to intercept comet Tempel 1 on July 4, 2005. It is proposed to send a 500kg copper projectile into the comet and study the ejected matter from both the spacecraft and ground based observatories.

Deimos – a satellite of Mars lying further from Mars than the slightly larger Phobos. Irregular in shape (15 x 12 x 11km) it is too small to be called a moon. Like Phobos, it may be a captured asteroid similar to the Carbonaceous Chondrite meteorites.

Delphinius (the Dolphin) – a small constellation in the northern hemisphere near Cygnus. The brightest stars being 3rd magnitude (a) and 4^th magnitude (b). Approx position: RA 20.5h dec +10degrees and an area of 189 sq degrees.

 Deneb – a very luminous white super giant that is also the brightest star in the constellation of Cygnus, lies at one end of the arm of the northern cross. Distance about 500 pc.

Despina – a small satellite of Neptune discovered in 1989.

Dione – a satellite of Saturn discovered in 1684. With a diameter of 1120km it just about qualifies as a moon. It is also slightly larger than Tethys. It has one important characteristic in that it is non-uniform in brightness with the trailing hemisphere being dark and the leading hemisphere being distinctly brighter.

Diurnal – happening in a 24 hour day (daily).

Dobsonian – a telescope with a simple yet very stable altazimuth mounting. Pioneered by John Dobson it is popular for large-aperture, short-focus reflecting telescopes. In addition to providing a simple low cost mount design for telescopes, John Dobson also popularised Astronomy through the means of "pavement" Astronomy i.e. taking the telescope to the ordinary people in cities and suburbs who normally would not have access to either an Observatory or private telescopes.

Donati and De Cheseaux comets – two magnificent comets of the past.

Donati 1858 VI was famous for its Coma with multiple haloes.

De Cheseaux 1744 reached a magnitude of –3 in February. Named after a Swiss astronomer.

Dorado – another small southern hemisphere constellation which contains large parts of the Large Megallanic Cloud with a 3^rd magnitude extremely luminous star S Dorado and the Tarantula Nebula NGC 2070, an extensive luminous emission nebula that is a grouping of H II regions characterised by rapid complex motions. It is a strong radio source and visible to the naked eye away from the city lights. The most luminous object lies in its centre and is designated R136. Three components designated a, b and c have been observed by the Hubble Space Telescope and it found that R136a is an extreme dense cluster of 12 massive young "O" stars within a region only 0.25 parsecs across! (<1ly).

Double star – a pair of stars that appear close together. There are optical and physical double stars. Optical can be described as "in the line of vision" they appear to be double. If we physically moved ourselves to a position far away from earth – even outside of our solar system on either side of the double star, the two stars will no longer appear as a double star. An example is DENEB appearing close because they lie nearly in the same direction if viewed from earth.

Physical simply means the stars are in actually in the same close space vicinity and a good example is: Alpha Centaurus, through a moderate telescope we’ll see two different bright stars near each other.

Dumbell nebula – (M27 NGC 6853) a Planetary nebula in the constellation of Vulpecula. It has an hour-glass shape, a magnitude of 7.6 (not visible to the naked eye) and covers 330 x 900 arc seconds.

Dwarf – normally short for dwarf star but can also refer to dwarf cepheids, dwarf galaxies, dwarf novae and dwarf planets.

Dwarf stars are any stars that lie on the main sequence of the Hertzsprung – Russell diagram.

Dwarf cepheids are a group of pulsating variables that are classed as RR Lyrae stars with a low luminosity and a short period of about 1 to 5 hours.

Dwarf galaxies – a faint galaxy because of small size, very low surface brightness or both. Since galaxies range from giant ellipticals downwards, the dividing line between average and dwarf is somewhat arbitary

Dwarf novae – a group of intrinsically faint stars that are characterised by sudden increase in brightness occuring in intervals of weeks or months, the maximum brightness lasting only a few days.

Wolf Lange