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‘Mc’ is treated as if it were spelled ‘Mac’, and certain shortened forms as if spelled out in full (e.g. ‘St’ is treated as ‘Saint’). Entries that have more than one word in the heading are alphabetized as if there were no space between the words. Entries that share the same main heading are in the order of people, places and things. Entries beginning with numerals are treated as if the numerals were spelled out (e.g. 3C follows three-body problem and precedes 3C 273). An exception is made for HI region and HII region, which appear together immediately after Hirayama family. Biographies are alphabetized by surname, with first names following the comma. (Forenames are placed in parentheses if the one by which a person is commonly known is not the first.) Certain lunar and planetary features appear under the main element of names (e.g. Imbrium, Mare rather than Mare Imbrium). Cross-references SMALL CAPITALS in an article indicate a separate entry that defines and explains the word or subject capitalized. ‘See also’ at the end of an article directs the reader to entries that contain additional relevant information. Measurements Measurements are given in metric (usually SI) units, with an imperial conversion (to an appropriate accuracy) following in parentheses where appropriate. In historical contexts this convention is reversed so that, for example, the diameter of an early telescope is given first in inches. Densities, given in grams per cubic centimetre, are not converted, and neither are kilograms or tonnes. Large astronomical distances are usually given in light-years, but parsecs are sometimes used in a cosmological context. Particularly in tables, large numbers may be given in exponential form. Thus 103 is a thousand, 2 106 is two million, and so on. ‘Billion’ always means a thousand million, or 109. As is customary in astronomy, dates are expressed in the order year, month, day. Details of units of measurement, conversion factors and the principal abbreviations used in the book will be found in the tables on this page. Stellar data In almost all cases, data for stars are taken from the HIPPARCOS CATALOGUE. The very few exceptions are for instances where the catalogue contains an error of which the editors have been aware. In tables of constellations and elsewhere, the combined magnitude is given for double stars, and the average magnitude for variable stars. Star Maps pages 447–55 Acknowledgements page 456 FRONTMATTER IMAGES Endpapers: Andromeda Galaxy The largest member of the Local Group, this galaxy is the farthest object that can be seen with the naked eye. Half-title: Crab Nebula This nebula is a remnant of a supernova that exploded in the constellation of Taurus in 1054. Opposite title: M83 Blue young stars and red HII emission nebulae clearly mark out regions of star formation in this face-on spiral galaxy in Hydra. Opposite Foreword: NGC 4945 This classic disk galaxy is at a distance of 13 million l.y. Its stars are mainly confined to a flat, thin, circular region surrounding the nucleus. Opposite page 1: Earth This photograph was obtained by the Apollo 17 crew en route to the Moon in 1972 December. SYMBOLS FOR UNITS, CONSTANTS AND QUANTITIES a semimajor axis Å angstrom unit AU astronomical unit c speed of light d distance e eccentricity E energy eV electron-volt f following F focal length, force g acceleration due to gravity G gauss G gravitational constant h hour h Planck constant Ho Hubble constant Hz hertz i inclination IC Index Catalogue Jy jansky k Boltzmann constant K degrees kelvin L luminosity Ln Lagrangian points (n = 1 to 5) l.y. light-year m metre, minute m apparent magnitude, mass mbol bolometric magnitude mpg photographic magnitude mpv photovisual magnitude mv visual magnitude M absolute magnitude, mass (stellar) N newton p preceding P orbital period pc parsec q perihelion distance qo deceleration parameter Q aphelion distance r radius, distance R Roche limit s second t time T temperature (absolute), epoch (time of perihelion passage) Teff effective temperature v velocity W watt y year z redshift α constant of aberration, right ascension δ declination λ wavelength μ proper motion ν frequency π parallax ω longitude of perihelion Ω observed/critical density ratio, longitude of ascending node ° degree  arcminute arcsecond Distances 1 nm = 10 Å 1 inch = 25.4 mm 1 mm = 0.03937 inch 1ft = 0.3048 m 1 m = 39.37 inches = 3.2808 ft 1 mile = 1.6093 km 1 km = 0.6214 mile 1 km/s = 2237 mile/h 1 pc = 3.0857 × 1013 km = 3.2616 l.y. = 206,265 AU 1 l.y. = 9.4607 × 1012 km = 0.3066 pc = 63,240 AU Temperatures (to the nearest degree) °C to °F : 1.8, 32 °C to K : 273 °F to °C : 32, 1.8 °F to K : 1.8, 255 K to °C : 273 K to °F : 1.8, 460 Note: To convert temperature differences, rather than points on the temperature scale, ignore the additive or subtractive figure and just multiply or divide.