کهکشان ِ راه ِ شیری
kahkešân-e râh-e širi (#)
Fr.: Voie lactée
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
1011 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.
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
there is a large amount of invisible → non-baryonic
surrounding the whole Galaxy.
→ Milky Way; → galaxy.