The University Record, August 13, 1996

Stargazers can see the Milky Way shining
over Michigan in August

 



By Sally Pobojewski
News and Information Services

Star watchers observing the giant planet Jupiter and the constellation Sagittarius in Michigan's evening skies this month are actually looking into the center of our galaxy---25,000 light-years away, according to U-M astronomer Richard Teske.

"This region of the sky contains the brightest part of the band of the Milky Way," Teske says. "Vast numbers of faint and distant stars are located in this area, but obscuring clouds of dusty material floating in space between the stars prevent us from looking at the galaxy's core directly."

The spectacle of the Milky Way in the night sky hints at our location within the spiral galaxy that is our home, according to Teske, who explains that our sun is an average-sized member of this frisbee-shaped assembly of around two hundred billion stars. The sun is located far from the galaxy's central hub, but almost exactly at the disk's mid-plane.

"We see the bright river of the Milky Way flowing completely around the sky, because we look out from the mid-plane," Teske says. "Since the sun is off center, more of the distant stars congregate toward the galaxy's middle, making the Milky Way appear brighter in one place along its band."

The Milky Way is prominent on August evenings. This year, it is easy to locate the galaxy's center by using the planet Jupiter as a guide, Teske explains.

"Choose a moonless evening and a location with a clear and flat southern horizon, away from city lights and shopping center illumination," he says. "An hour after sunset, Jupiter is the brightest `star' hanging low in the south. It is poised just above the constellation of Sagittarius, the Archer, which to some looks like a teapot. Follow the course of the Milky Way as it stretches from overhead in the constellation Cygnus the Swan southward through Aquila the Eagle and down into Sagittarius. From Cygnus on downward, the Milky Way brightens and widens until our eyes come to the galaxy's central bulge, the fattest part of the Milky Way in Sagittarius.

"If you follow the course of the Milky Way southward from Cygnus, you will see the `Great Rift,' a dark cleft dividing the Milky Way into two parallel streams all the way into Sagittarius," he adds. The dark band signifies the presence of prominent clouds of "dust" along the Milky Way---dust whose individual motes are the size of particles of tobacco smoke each separated from the others by distances of a foot or two. Despite the dust's sparseness, the clouds' thickness of hundreds of light years absorbs starlight, causing dark patches against the Milky Way's distant stars.

"Many dust clouds congregate between our part of space and the true center of the Milky Way galaxy," Teske explains. "Their combined presence totally cuts off a direct view of the nucleus. What we seem to see of the galaxy's center with our eyes, and with our photographs, are great gatherings of stars between us and the center---not the nucleus itself."

Even though ordinary light cannot penetrate the galaxy's dust clouds, radio waves and infrared light navigate the dust easily. Radio views of the galaxy's core show a fiercely energetic environment, with circling masses of hot, magnetized gases. Images made with infrared light depict the presence of huge numbers of stars there, packed so densely that each is separated from its neighbors by only a fraction of one light year.

"Images made with infrared light convey information as understandably as ordinary pictures do, giving access to knowledge of conditions in a region of space humans can never look upon directly," Teske says.