Urban light pollution has robbed many of us of one of nature's grandest spectacles: the ethereal glow of the Milky Way. But the opening days of September are the perfect time to find a dark sanctuary in the countryside and get reacquainted with our starry home. Nights are still warm, the air is getting drier, and the center of the galaxy— the brightest, thickest part— shimmers low in the south at the end of twilight.
Ancient cultures widely regarded that band of heavenly light as a splash of spilt cream. The Romans dubbed it Via Galactica, the "road made of milk." When Galileo turned his spyglass to the skies in 1610, he was amazed to find that the Milky Way is in fact "a congeries of innumerable stars distributed in clusters." You can dramatically confirm his discovery using a pair of cheap binoculars.
Modern space-based observatories have provided deeper insights into our island universe. Opaque clouds of gas and dust, some of them visible as a dark rift splitting the Milky Way overhead, block the light from the galaxy's inner regions. Conventional telescopes can peer only a few thousand light-years through the murk. But just as amber sunglasses cut through terrestrial haze, recent infrared, radio, and especially X-ray telescopes can penetrate all 26,000 light-years to the center of the galaxy. Last year NASA's orbiting Chandra X-ray Observatory produced a splendid panorama of the galactic core and evidence that it harbors a black hole as massive as 2.6 million suns.
This beast is strangely silent. Astronomers believe that black holes radiate powerfully only when they are scooping material into their gravitational gullet. These days, the black hole at the heart of the Milky Way does not seem to have much to gobble. But as recently as the American Revolution, according to one theory, it feasted on enough infalling matter to energize the surrounding filaments of X-ray-emitting gases that swish like the tails of riled cats.
The black hole is just one of many unexpected galactic features unveiled by sophisticated astronomical tools. Detailed studies of infrared emissions and stellar motions have revealed a cigar-shaped gathering of stars and nebulae—15,000 light-years across and pointed roughly our way—similar to the central bars seen in other spiral galaxies. Star tracking also reveals that our galaxy is immersed in a gigantic, near-invisible shell of matter, like a ship in a bottle. Extending outward at least 150,000 light-years, this vast halo consists of dark matter that accounts for most of the Milky Way's mass. Last year researchers determined that at least part of the invisible stuff consists of white-dwarf stars, the dim, collapsed corpses of middleweight stars similar to the sun.
Dying stars and hot gas illuminate a Chandra X-ray image of our galaxy's center. A giant black hole probably resides within the bright clump in the middle.Photograph courtesy of NASA/UMASS/S. Wang, et al.
Some aspects of the Milky Way are hidden not in space but in time. Over a human lifetime, the structure of the galaxy barely changes, but every star in the sky—including the sun, with Earth in tow—whirls around the center once every 240 million years. Even at a breakneck speed of 137 miles per second, we have completed fewer than 20 revolutions since our planet's birth.
At 11 p.m. on a mid-September eve, the direction in which we are headed is straight up, toward the bright star Deneb in the constellation Cygnus. If you think of our galaxy as a merry-go-round, you gaze at the mirrored axle when you look southward, and the top of your head indicates which way the painted horses are facing. Talk about perspective: The spot where Deneb lies, 1,500 light-years ahead of us on the galactic carousel, is where we will be in 2 million years. When Deneb sat where we are now, Homo habilis was gazing at the sky with uncomprehending eyes.
Learn about the Milky Way from NASA: imagine.gsfc.nasa.gov/docs/features/objects/focus.html. And read about Chandra and the Milky Way's black hole at science.nasa.gov/headlines/y2000/ast29feb_1m.htm.