GALAXY, THE (Gr. gala, galaktox, milk), or the Milky-Way, is the great luminous band which nightly stretches across the heavens from horizon to horizon, and which is ' found, when carefully traced, to form a zone, completely encircling the whole 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. The reader. will find its course mapped out on any celestial globe, and a verbal account of it in sir John Herschel's Outlines of Astronomy, by which we may test the accuracy of the chart. That course, as traced by the naked eye, following the line of its greatest brightness, conforms nearly to that of a great circle, called the Galactic circle, inclined at an angle of about 63' to the equinoctial, and cutting that circle in 0 hours 47 minutes, and 12 hours 47 minutes right ascension. Throughout the space where, as above stated, it is divided into two Vranches, this great circle is intermediate to the two, lying nearer that which is the and more con tinuous. The most casual survey of the Galaxy shows that it is wanting in regularity of outline. Besides the two great branches into which it divides, 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 abruptly, 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 described by sir John Herschel as " irregular, patchy, and winding;" 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 navigators as the "coal sack," is a singular pear-shaped vacancy of about 8° in length, and 5° broad, occurring in the center of a bright area overlying portions of the constellations of the Cross and Centaur. The `1oal-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 G. was examined by sir William Herschel with his powerful telescope, and found to be composed entirely of stars, How a collection of stars can assume such appearances as are presented in the G., is explained in the article stars (q.v.).