For example, Earth and Luna both revolve around their barycentre, which is 1707 km inside the Earth along the line between them. Alan Kazlev The centre of mass around which a system of objects rotate. In all other respects resembles a normal spiral galaxy. It appears to occur when an instability in the stellar distribution alters the orbits of the inner stars and generates and elongated nucleus. Alan Kazlev A spiral galaxy whose spiral arms attach to a bar-like feature containing the nucleus. They are distinguished by neutral helium lines, with hydrogen lines strengthening in B6 through B9. Alan Kazlev Hot blue-white star that is among the brightest in the sequence of spectral types, second only to the O-type star. Astrography - Text by Anders Sandberg Interstellar cartography, determining one's position in space, naming of interplanetary and interstellar bodies and regions, the science of interplanetary and interstellar map-making.Arity - Text by Steve Bowers The number of stars in a gravitationally-bound system of stars.Alan Kazlev Distributed telescopic network occupying a spherical volume approximately 1000 ly across in the Inner and Middle Regions. Argus Array - Text by Todd Drashner, Peter Kisner, and M.Apparent Brightness of the Local Sun - Text by Steve Bowers To standard Terragen eyes, suns with a different spectral type from Sol do not seem as bright from the surface of a terraformed planet, since less of their radiation falls within the visible range.Apoapsis is a more general term yet again. Thee point in an object's orbit at which is farthest from the world it is orbiting. Alan Kazlev Originally a term used only for objects orbiting Old Earth, but from the Interplanetary Age onward a term used for any object orbiting any planet or moon. Alan Kazlev For an object orbiting a celestial body in general, the apoapsis is the point in its orbit which is farthest from that body. Alan Kazlev For an object (a planet or comet) orbiting the Sun (or any star), the aphelion is the point in its orbit which is farthest from the primary. Icy bodies tend to have a high albedo, sooty bodies a low albedo. A white, completely reflecting object has an albedo of 1.0 a black object with no reflectivity has an albedo of 0.0. The higher the albedo the brighter the object. Alan Kazlev, amended by Steve Bowers The ratio of the amount of stellar/solar radiation or other megascale light source reflected from an object (such as planet, moon, orbital or habitat) to the total amount it receives. Alan Kazlev and Steve Bowers Whirling disk of hot gas and dust around a large compact object such as a white dwarf, neutron star, or black hole, usually material that has been thrown off one star and is being drawn into the companion object. Alan Kazlev A hot white dwarf star near the brighter end of the sequence of spectral types. They move in circular or near-circular orbits. They commonly range in mass from 0.44 Jupiters (for 51 Peg) to 3.64 Jupiters (Tau Bootis). The orbits are all small, with radii less than 0.1 A.U. Included in this category are 51 Peg itself, Tau Bootis, 55 Cancri, and Upsilon Andromedae, which have orbital periods of just 4.2, 3.3, 14.7 and 4.6 days, respectively. These were among the first extrasolar planets discovered by Terragen astronomers, during the pre-singularity period (early Information Age). Alan Kazlev A planet, that shows orbital periods shorter than 15 terran days.
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