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A spiral galaxy, of which the solar system is a small part. It is the second largest
in our Local Group of galaxies. The Milky Way is a disk-shaped system, with a diameter
of between 80,000 and 100,000 light-years and a thickness of about 2,000 light-years,
containing more than 10^11 stars. The stars are divided into two main categories,
Population II stars and Population I stars. The core, or nucleus, of the Galaxy is
surrounded by an ellipsoidal central bulge that measures some 15,000 light-years in
diameter and about 6,000 light-years in the direction perpendicular to the plane of
the disk. Surrounding the bulge and extending in a near spherical distribution above
and below the Galactic plane is the Galactic halo. The halo contains about 200 globular
clusters and an extremely thinly scattered population of individual stars. The Sun
is located just over half way out from the center to the edge of the disk at a distance
of about 25,000 light-years. In common with other stars, the Sun revolves around the
Galactic Center. Its orbital velocity is about 220 km s^-1 and its orbital period
is about 225 million years. Overall, the Galaxy exhibits differential rotation, that
is stars and gas clouds closer to the center have shorter orbital periods than those
that are located further out. The spiral arms of the Milky Way lie within its disk,
where bright young stars, H II regions, and molecular clouds of gas and dust are concentrated
into curved 'arms' that appear to radiate from the central bulge in a spiral pattern.
The Galaxy's spiral pattern consists of several major arms and a number of shorter
segments, one of which, the Orion arm, contains the Sun and the Orion star-forming
region. Near-infrared observations have shown that the stars in the central bulge
are arranged in an elongated galactic bar, about twice as long as it is wide, that
is seen nearly end on from the present location of the solar system. The exact center,
or nucleus, of the Galaxy coincides with a strong source of radio emission, called
Sagittarius A, that is less than 15 astronomical units in diameter. Observations of
the speeds at which clouds of ionized gas are revolving round the Galactic center
imply that several million solar masses of material are concentrated within a region
of about one light-year in radius. Since only about half of this mass can be accounted
for by stars, it seems likely that the balance (about 2.5 million solar masses) is
contained in a central black hole and that accretion onto this black hole is the underlying
source of the energy radiated by Sagittarius A. The Milky Way also has a dark matter
component. The Galactic rotation curve indicates that there is a large amount of invisible
non-baryonic surrounding the whole Galaxy.
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