MILKYWAY

MILKY WAY

MILKY WAY, the large, disk-shaped aggregation of stars, or galaxy, that includes the sun and its solar system. Its name is derived from its appearance as a faintly luminous band that stretches across earth's sky at night. This band is the disk in which the solar system lies. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. The individual stars that are distinct in the sky are those in the Milky Way galaxy that lie sufficiently close to the solar system to be discerned separately.

From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon. It extends through the constellations Perseus, Cassiopeia, and Cepheus. In the region of the Northern Cross it divides into two streams: the western stream, which is bright as it passes through the Northern Cross, fades near Ophiuchus, or the Serpent Bearer, because of dense dust clouds, and appears again in Scorpio; and the eastern stream, which grows brighter as it passes southward through Scutum and Sagittarius. The brightest part of the Milky Way extends from Scutum to Scorpio, through Sagittarius. The center is in the direction of Sagittarius and is about 26,000 light-years from the sun.

Structure.

The Milky Way has been determined to be a large spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. The stars in the central bulge are closer together than those in the arms, where more interstellar clouds of dust and gas are found. The diameter of the disk is about 100,000 light-years. It is surrounded by a larger cloud of hydrogen gas, warped and scalloped at its edges, and surrounding this in turn is a spheroidal or somewhat flattened halo that contains many separate, globular clusters of stars mainly lying above or below the disk. This halo may be more than twice as wide as the disk itself. In addition, studies of galactic movements suggest that the Milky Way system contains far more matter than is accounted for by the known disk and attendant cluster's upto to 2000 billion times more mass than the sun contains. Astronomers have therefore speculated that the known Milky Way system is in turn surrounded by a much larger corona of undetected matter. Another speculation is that the Milky Way is a barred spiral galaxy. In April 1997 scientists discovered a "fountain" of antimatter erupting from the center of the galaxy, raising new questions about the structure of the Milky Way.

Types of Stars.

The Milky Way contains both the young, brilliant blue, so-called type I stars; and the older, giant red, type II stars. The central Milky Way and the halo are composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Radiation from the central region has been recorded by use of such special devices as photoelectric cells, infrared filters, and radio telescopes. Such studies indicate compact objects near the galactic center, possibly starburst remnants or a massive black hole.

Surrounding the central region is a fairly flat disk comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Imbedded in the disk, and emerging from opposite sides of the central region, are the spiral arms, which contain a majority of the type I population together with much interstellar dust and gas. One arm passes in the vicinity of the sun and includes the Great Nebula in Orion.

Rotation.

The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 270 km/sec (about 170 mi/sec).