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The Milky Way
Today we know that the Milky Way is our home galaxy - a vast rotating spiral of gas, dust, and hundreds of billions of stars. The Sun and its planetary system formed in the outer reaches of the Milky Way about 4.5 billion years ago.
In the center of the galaxy is the bar-shaped galactic bulge which harbors a supermassive black hole with a mass equal to that of about 3 million suns. Surrounding the central bulge is a relatively thin disk of stars about two thousand light years thick and roughly 100,000 light years across. Almost all the stars seen by the human eye are in the thin disk, which accounts for about 90% of the visible light in the Milky Way.
Face on illustration of the Milky Way galaxy, showing the prominent spiral arms, the central galactic bulge and the location of the Sun. Rollover image to view objects discussed in further detail. (Illustration: CXC/M.Weiss)
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