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Since we live within the Galaxy, we see it in profile. The Milky Way, viewed at this angle, is shaped rather like a flying saucer—that is, it resembles a disk that bulges toward its center and thins out at the periphery. The thickest part we call the bulge, and the thin part, the disk. (Astronomers aren't always in love with highly technical terms!) We will see later how we figured it out, but we know now that the center of our own Galaxy is in the Galactic bulge, in the direction of the constellation Sagittarius.

Our solar system is located in part of the thinned-out area, the so-called Galactic disk. Our location explains what we see when we look at the Milky Way on a clear summer night in the country, far from city lights and smog. The wispy band of light arcing across the sky is our view into this disk. The band of light is the merged glow of the stars close enough to be seen optically—so many that they are not differentiated as separate points of light without a telescope. A dark band of obscuration runs the length of the arc. This light is blocked by the presence of large amounts of dust in the disk.

When we look away from the arc in the sky, we don't see much of the Milky Way, because we are looking out of the disk. Looking into the disk is like looking at the horizon on the earth: There's lots of stuff there to block our view, including houses, trees, and cars. But look straight up into the sky, and you can see much farther—you might even see a plane flying overhead. When we look more or less perpendicular to the disk of the Galaxy, we can see much farther, and this is the direction to look to see other galaxies.

The Milky Way, as seen by the Compton Gamma Ray Observatory. This black-and-white reproduction of a false-color image of our home clearly shows the galactic disk. Transient sources called gamma-ray bursters (GRBs) are seen to flare on average once a day.

(Image from NASA, Compton Gamma Ray Observatory)

Star Words

The Galactic disk is the thinnest part of our Galaxy. The disk surrounds the nuclear bulge, and contains a mixture of old and young stars, gas, and dust. It extends out some 50,000 light-years from the Galactic center, but is only about 1000 light-years thick. It is the dust in the disk (a few hundred light-years thick) that creates the dark ribbon that runs the length of the Milky Way and limits the view of our own Galaxy.

Star Words

The Galactic disk is the thinnest part of our Galaxy. The disk surrounds the nuclear bulge, and contains a mixture of old and young stars, gas, and dust. It extends out some 50,000 light-years from the Galactic center, but is only about 1000 light-years thick. It is the dust in the disk (a few hundred light-years thick) that creates the dark ribbon that runs the length of the Milky Way and limits the view of our own Galaxy.

Star Words

The Galactic halo is a large (50,000 light-year radius) sphere of old stars surrounding the Galaxy.

Star Words

The Galactic halo is a large (50,000 light-year radius) sphere of old stars surrounding the Galaxy.

It was looking in this direction—up in the air, as it were—that astronomers discovered another component of the Galaxy, the globular clusters. Globular clusters are collections of several hundred thousand older stars held together by their mutual gravitational attraction, that are generally found well above and below the disk of the Galaxy. Reasoning that globular clusters should cluster around the gravitational center of the galaxy, Harlow Shapley used these collections of stars in the early twentieth century to determine where in the Galaxy we were located.

Along with these collections of stars in globular clusters are single stars.

Simple diagram of our view of the Milky Way, showing the principal parts. Studies have shown that the Milky Way is an ordinary spiral galaxy.

(Image from authors' collection)

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