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NASA
How far across is the Milky Way?
At a given time how can one locate the center of our galaxy as we orbit the Sun and the Sun orbits the galaxy? Is our solar system moving away from the center of the galaxy? How many years does it take the Sun to orbit the center of the galaxy? Specifically, where in the universe can our solar system be found?
The Milky Way galaxy is the home of our solar system, as well as more than 200 billion other stars. It is a spiral galaxy, with a bulged central section and radial arms that start in the center and form a flat pinwheel shape. The galaxy is roughly 90,000 light-years across, and the Sun is located about two-thirds of the way out from the center in one of the arms (ours is known as the Orion Arm). At our position in the galaxy, the Milky Way is only about 3,000 light-years thick, which makes it about the same proportions (ratio of thickness to width) as a music CD.The Sun (and our solar system) is revolving around the center of the galaxy, and makes a complete revolution every 220 million years or so. As far as we can tell, the Sun and the solar system revolve around the center of the galaxy at a constant radius. The Sun has made less than 25 trips around the galaxy in its lifetime.
The center of our galaxy is located about 28,000 light-years away, beyond the constellation Sagittarius (actually just beyond the border of Sagittarius and Scorpio). We can look down the arm the solar system is located in by looking in the direction of the constellation of Orion, and we can look up the same arm by looking in the direction of the constellation of Cygnus. Looking up or down the arm we are located in doesn't point you in the direction of the center of the galaxy because the arms are curved.
The Milky Way is part of a set of galaxies known as the Local Group, which includes several dozen different galaxies within 3 million light-years. Only one of these, the Andromeda galaxy, is close to the size of the Milky Way--the rest are much smaller (each is less than 5% of the mass of the Milky Way). This Local Group is part of a supercluster, known as the Virgo supercluster, which has at least 5,000 member galaxies and is roughly 100 million light-years across. Beyond this level of organization, not much is known about our position in the universe.
