ANN ARBOR—The Milky Way's hazy light stretches high across the sky on December evenings, giving Michigan starwatchers a view of vast clouds of distant stars—all part of the immense galaxy in which our sun is located.
The Milky Way's faint, glowing band owes its visibility to the combined luminosity of uncounted stars lying thousands of light-years from our sun, according to University of Michigan astronomer Richard Teske.
"For the best naked-eye views of the Milky Way, you need a dark location away from city lights, shopping centers and security installations," Teske said. " During December its misty swath of light spans the sky from east to west and passes nearly overhead. Pick a clear, cloudless night and look for the Milky Way between 7 and 9 p.m.
"When you see the Milky Way, you are actually seeing our home galaxy from the inside. It is a flattened, rotating pinwheel of stars about 80,000 light-years across and roughly 2,000 light-years thick," Teske said. " Were it delivered to your door as a pizza, you would think it disappointingly thin. Nevertheless, within its skinny disk are several hundred billion stars. "
According to Teske, the Milky Way pinwheel has a fat center made brilliant by the fires of countless stars. Outside the central bulge are bright spiral arms winding their way around the center with groupings of stars. Between the bright arms are fainter stars.
"We are located about 25,000 light-years from the central hub, just a few hundred light-years from the inside edge of one of the spiral arms," Teske said. " Here, where the galaxy shines less brilliantly, is the home territory of our sun and its nine planets. "
In December, Milky Way observers on Earth are looking away from the galaxy's center and out toward its rim. Almost overhead, near the constellation Perseus and between the constellations of Cassiopeia and Auriga, the Milky Way seems faint because in that region there are only a few stars beyond our position in the galaxy. The galaxy's fat central hub lies in the opposite direction, towards the constellation of Sagittarius, which has already set below the horizon on December evenings and cannot be seen. The best time of year to see the glittering central region is on September evenings, according to Teske.
"Each of the galaxy's many stars circles the center like a car speeding along a race track. Like race cars, nearly all the stars follow much the same traffic pattern," Teske said. " As a consequence of the orbiting motions of all its stars, the galaxy seems to spin like a pinwheel. Our sun, along with its entire planetary family, participates in this swirling rotation, taking about 220 million years to complete one lap around the center. "
Most of the Milky Way's light in the December sky comes from stars in the spiral arm adjacent to our sun. Observers looking at the winter Milky Way see this spiral arm extending from Cygnus in the west to Orion just rising in the east. The brightest and most distant star clouds in the arm are seen toward Cygnus where they are 5,000 light-years away. " When we look toward Cygnus, we are looking right along the inner edge of the arm in the direction toward which all nearby stars are moving," Teske said.