Download THE MILKY WAY GALAXY

The Milky Way
Our Home Galaxy
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The Milky Way (MW) is the name given to the faint band of light visible in
the night sky.
This light is the sum of billions of stars comprising our home galaxy.
Today, we understand that the Milky Way is a flattened, rotating disk of
stars, gas and dust, about 100,000 light years in diameter. The density of
stars increases towards the center of the Galaxy.
The Milky Way appears as a band of light because stars are concentrated
within the flattened disk. Since the solar system is also located within the
disk, from our perspective more stars are visible looking through the disk then
are seen looking away from the disk.
The nature and size of the Galaxy, as well as our location within this stellar
system were finally appreciated in the early 20th century when:
 The locations of the globular clusters were mapped out, indicating that
the galactic center was located thousands of light years distant, towards
the constellation Sagittarius
 Distances to globular clusters and the Andromeda nebula were
determined using the newly discovered period-luminosity relation for
certain variable stars. The derived distances proved that the Milky Way
was an “island universe” of stars, similar to other nebulae seen all
around the sky.
The MW is classified as a spiral galaxy, containing about 200 billion stars.
Our solar system is located about 24,000 light years (~8 kiloparsecs) from the
center – NOT at the center of the Galaxy.
Our Galaxy can be subdivided into several major physical components:
 Disk – a thin (a few thousand light years) layer containing stars, most of
the interstellar gas, and dust, rotating around the central regions. Star
formation occurs in the disk, where stars of all ages can be found.
 Spiral arms – spiral-shaped regions within the disk in which star
formation, O and B stars, gas, and dust clouds are especially
concentrated.
 Bulge – a slightly flattened, spherical region surrounding the galactic
nucleus, containing mostly old, cool, red stars. No star formation occurs in
the bulge.
 Nucleus – the inner few hundred light years of the Galaxy, where the star
density peaks, possibly harboring a massive black hole
 Halo – a spherical region encompassing the entire Galaxy, comprised of
very old stars, probably the oldest stars in the Galaxy
 Globular Cluster System – about 150 old star clusters, each containing
up to a million stars. These clusters orbit around the center of the galaxy.
The Galaxy began forming about 10-12 billion years, within a few billion years
of the Big Bang. The bulge and globular clusters formed first, followed by the
disk. Star formation continues within the disk to the present day.