GALAXY (from Lat. galaxias, Gk. yaNaelas, galaxias, milky way, from 7dXa, gala, milk), or MILKY WAY. The luminous band, seen at night, which forms a zone encircling the sphere almost in a great circle. At one part of its course it opens up into two branches, one faint and interrupted, the other bright and continuous, which do not reunite till after remaining distinct for about 150°. This great zone has occupied the same position in the heavens since the earliest ages. Its course, as traced by the naked eye, following the line of its greatest brightness, conforms near ly to that of a great circle, called the 'galactic circle,' inclined at an angle of about 62° 30' to the equinoctial, and cutting that circle in 0 h. 49.1 m. and 12 h. 49.1 m. right ascension. Throughout the space where, as above stated, it is divided into two branches, this great circle is intermediate to those branches lying nearer that which is the brighter and more continuous. The Galaxy is wanting in regularity of outline. Be sides the two great branches into which it di vides, it has many smaller ones which spring out from it. At one point it diffuses itself very broadly, and opens out into a fan-like expanse of interlacing branches nearly 20° in breadth. At the same point, the branches terminating abrupt ly, a wide gap presents itself in the zone, on the opposite side of which it recommences its course with a similar assemblage of branches. At other
points its course is irregular, patchy, and wind ing; while at more than one point, in the midst of its brightest parts, broad dark spaces occur. One of these, known from early times among navi gators as the 'coal-sack,' is a singular pear-shaped vacancy about 8° long and 5° broad, occurring in the centre of a bright area overlying portions of the constellations of the Cross and Centaur. The 'coal-sack' occupies about half the breadth of this bright space, and presents only one star visible to the naked eye, though it contains many telescopic stars. Its blackness, which attracts the most superficial observer, is thus due to the contrast with the brilliant ground by which it is surrounded. The Galaxy was examined by Sir William Herschel with his powerful telescope, and found to be composed entirely of stars. Mod ern photographic researches have added but little to Herschel's observations as to the structure of the Galaxy; but some of his conclusions con cerning the form of the sidereal universe are no longer tenable. See STAR.