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> History > Milky Way Galaxy
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Milky Way, the large, disc-shaped galaxy, or aggregation of stars, that includes the Sun and its solar system. Its name is derived from its appearance as a faintly luminous band that can be seen stretching across the sky at night. 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 we see 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 north-eastern to the south-eastern horizon. It extends through the constellations Perseus, Cassiopeia, and Cepheus. In the region of the Northern Cross, which is part of Cygnus, 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 Scorpius; 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 Scorpius, through Sagittarius. The galactic centre is in the direction of Sagittarius and is about 26,000 light years from the Sun. [Top]
Structure
The Milky Way has been found to be a large barred spiral galaxy, with
spiral arms radiating from the ends of a central bar some 30,000 light
years long, which passes through 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. Recent
observations in X-ray wavelengths indicate that there is a super massive
black hole at the centre of our Galaxy, as indeed there appears to be in
most galaxies. The diameter of the disc 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 globular clusters of stars, mainly lying
above or below the disc. This halo may be more than twice as wide as the
disc itself. In addition, studies of galactic movements suggest that the
Milky Way system contains up to 2 million million times as much mass as
the Sun contains—far more matter than is accounted for by the known disc
and attendant clusters. Astronomers have therefore speculated that the
known Milky Way system is surrounded by a much larger corona of undetected
matter.[Top] Types of Stars
The Milky Way contains both the so-called type I stars, which are
brilliant, blue stars; and type II stars, giant red 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 in the
radio, infrared, X-ray, and gamma-ray wavelength ranges. Observations from
the Compton Gamma-Ray Observatory indicated a large cloud of antimatter in
the central area by the characteristic radiation resulting from its
annihilation. This may be caused by supernovae or activity related to the
supermassive black hole at the centre.
Surrounding the central region is a fairly flat disc comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Embedded in the disc, 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. [Top] RotationThe Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, with the spiral arms trailing. The period of rotation increases as the distance from the centre of the galactic system increases. 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/s (170 mi/s).
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