Star clusters have played an extremely important role in mapping the Milky Way Galaxy and clarifying its history. They are groups of stars formed at about the same time, so that their H-R diagrams can be used to measure ages. The distribution of stars in the sky is not isotropic, and counts of stars (corrected for interstellar obscuration) can be used to map out the Milky Way Galaxy. Open clusters and associations trace out the flattened structure of the galactic disk. Globular clusters define the spherical galactic halo. There is also a set of old stars concentrated near the center of the Milky Way that form the galactic bulge. The Sun is located in the disk but offset from the center. The Sun is one of billions of stars in the Milky Way Galaxy.
The Sun is located in the disk of the Milky Way Galaxy. The luminous stars and gas clouds we see in the solar neighborhood are signs of continuing star formation. The spiral arm pattern probably emerged after a number of galactic rotations, perhaps within a billion years. In the spiral arms, the densest clouds contracted and spawned associations and open star clusters. Each group broke apart into scattered stars a few hundred million years after its formation, but new star groups continued to form, so that the galaxy kept its present general appearance. Supernovae blew out gas laced with heavy elements created inside stars, so that later generations of stars had more heavy elements than the earlier stars. Perhaps 7 or 8 billion years after the galaxy's formation, in one of the spiral arms, an obscure star formed — our Sun — and in its surrounding dusty nebula, the Earth was born.
Near the galactic center there are older, redder stars in a stellar component called the bulge. In the heart of the galaxy, behind veils of obscuring dust, there is evidence for a dark mass concentration in the form of a supermassive black hole. The outer parts of the galaxy are home to an even more mysterious form of dark matter. The rotation speed of stars in the disk stays high out to the largest distances measured, evidence that most of the mass of the galaxy is in an extended halo. Dark matter does not emit radiation at any wavelength, revealing itself only by its gravity. The nature of the dark matter is an outstanding puzzle in astronomy.
The stars in our night sky reveal our particular position in the Milky Way. But our understanding of the structure of the Milky Way can be used to imagine changing our viewpoint to elsewhere in the galaxy. In a star-forming region, the night sky would be lit up by young stars and delicate filaments of glowing gas. Deep within the bulge or in a globular cluster, our sky would be filled with old, red stars. Close to the galactic center, our sky would be dazzlingly bright with the light of millions of stars. Located next to a lone halo star, we would look down from a nearly starless sky onto the beautiful spiral of the disk.