A tour of the milky way

We've mentioned the Milky Way a number of times without describing it in detail. Now let us take you on a quick tour through the Milky Way as visible from the earth. Imagine that you are on the top of a mountain in the desert.

A warm September day slowly cools as the sun vanishes behind the hills to the west. The brightest stars emerge from the purple sky. Overhead you see the Northern Cross in the constellation Cygnus. As the darkness gathers, you pick out the wispy glow of the Milky Way. Scan down the Milky Way to the south. Before you reach Scorpius, you find that the Milky Way splits into two distinct bands (Fig.23.2). The dark blot between the bands is called the Great Rift. Near it lie the brightest regions of the Milky Way visible from the Northern Hemisphere.

Figure 23.2 The Milky Way in the constellation Aquila. The bright star Altair is at the lower left. Now view this region with binoculars. What do you see? Stars, in countless numbers. The more intently you look, the more you see. Slowly scan around the constellations Scorpius, Scutum, and Sagittarius. You get the impression that in any location where you see more or fainter stars, you are looking deeper into space. Occasional areas appear swept clean of the stars-dark dust clouds here actually block out the starlight.

If you scan a large enough area, you will come across small clusters of stars (Fig.23.3). These are the open or galactic clusters. A few you see immersed in a faint, glowing cloud (Fig.23.4) an emission nebula. You can also pick out a few very dense star clusters (Fig.23.5), the globular clusters. In other regions you come across the loose groups of bright, blue-white stars, the OB associations.

Figure 23.3 As open cluster of stars (NGC 2682) in the constellation Cancer.

Figure 23.4 A bright emission nebula (Messier 16 in the constellation of Serpens) engulfing a cluster of young stars. Stars in loose and tight clusters, irregular clouds of glowing material, dark blotches, individual stars everywhere: these are the contents of the Milky Way viewed with binoculars or a small telescope. Ask these questions: How far out are you seeing stars?

Which are the closest, and which the farthest? What is their layout in three dimensions? Seeing the Milky Way encircling the sky, you get the impression that the sun is situated in a flat system of stars that is thick in the center and thins out toward the edges.

Figure 23.5 A globular cluster (Messier 15). Note how concentrated the stars are compared with the open cluster in Fig.23.3 Earlier astronomers had similar impressions. Before we present the story of their detective work, we'll give away the story's end by sketching a rough model of the Milky Way Galaxy.

Astronomers picture the Galaxy as a vast disk of stars and gas with a bulge in the middle. The disk of stars has a diameter of 40,000 parsecs; its thickness is about 1/130 of its diameter. The stars orbit around the center of the Galaxy bathed in a disk of gas that extends to a diameter of 50,000 to 60,000 pc; its thickness is only 1/200 of its extent. The nuclear bulge is about 4000 pc in diameter and 3000 pc thick.

The sun lies in the Galaxy's suburbs, about 8500 pc from the center but almost directly in the Galaxy's plane. The sun and other stars whirl around the Galaxy at speeds of hundreds of kilometers per second. Such rapid motion accounts for the flatness of the disk of stars.

Figure 23.6 A schematic, contemporary model of the Milky Way Galaxy, showing its main features