How do we know the shape, mass, and center of Milky Way if we are in it?

Even though we are located within it, we can still know the shape, mass, and center of the Milky Way Galaxy. Astronomers observe the shape of the band of stars that form the Milky Way in the night sky. This tells us the shape of the galaxy and our location within it. They also see where globular clusters of stars are located to see where the galaxy's center is. This can then be compared to other distant galaxies that we can see.

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Considering that we are located within the Milky Way Galaxy, scientists must use various creative methods to get an idea of its actual structure and our location within it. We know that it is a spiral galaxy and that we are located about three-fifths from the center in one of...

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Considering that we are located within the Milky Way Galaxy, scientists must use various creative methods to get an idea of its actual structure and our location within it. We know that it is a spiral galaxy and that we are located about three-fifths from the center in one of the galaxy's arms.

Simply by looking at the band of stars commonly called the Milky Way in the night sky, astronomers can surmise that the galaxy is shaped more or less like a disk. Because the band appears rather narrow, we can be rather certain that our solar system is located near the plane of the disk rather than near the top or bottom. Because the galaxy completely encircles Earth, we know that we are within the galaxy rather than on its outside edge.

Astronomers also notice where globular clusters of stars are located. The majority of these clusters are found in one part of the night sky where they form a rough sphere. Astronomers surmise that the middle of this sphere is the center of the galaxy. Since this is distant, we can see that we are far from the center.

By taking this information and comparing it to what we can observe of other galaxies, astronomers can be pretty certain as to the size and shape of our own galaxy.

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