|
The Milky Way is a large spiral galaxy of about a hundred billion stars arrayed in the form of a disk, with a central bulge (some 30,000 light-years across) of closely packed stars lying in the direction of Sagittarius. It is surrounded by a flat disk marked by spiral arms, which wind out from the nucleus like a giant pinwheel. Our sun is on an outer arm. At first it was thought to be very near the central nucleus, but studies later showed the sun to be some 28,000 light-years distant and lying in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way. The diameter of the disk is c.100,000 light-years; its average thickness is 10,000 light-years, increasing to 30,000 light-years at the nucleus.
Certain features of the region near the sun suggested that our galaxy is similar to the Andromeda Galaxy. In 1951 a group led by William Morgan detected evidence of spiral arms in Orion and Perseus. Another bright arm stretches from Sagittarius to Carina in the southern sky. With the development of radio astronomy, scientists have extended a nearly complete map of the spiral structure of the galaxy by tracing regions of hydrogen that dominate the spiral arms.
Surrounding the galaxy is a large halo of globular star clusters that extends to about 130,000 light-years. Possibly the galaxy has a vast outer region called the corona, which is over 600,000 light years across and includes some distant globular clusters, the two nearby galaxies called the Magellanic clouds, and four smaller galaxies. Studies of their distribution provided the first realistic estimate of the size of the galaxy and of the sun's location in the plane of the disk.
Sections in this article:
|
