A New Twist on the Milky Way

Steven Majewski

Title: Assistant Professor of Astronomy

Area of Interest: Understanding the formation and structure of the Milky Way.

Website: http://www.astro.virginia.edu/

Compared to other astronomers, Steven Majewski's methods seem at first glance to be highly traditional. In an age when astronomers prefer electronic detectors for making images of the skies, Majewski, an assistant professor of astronomy, relies on photographs taken with visible light on glass plates. Nonetheless, his work has led to a major advance in our understanding of the formation and structure of the Milky Way.

Scientists believe that the Milky Way condensed out of a cloud of gas 15 billion years ago. As it shrank, any slight predilection for rotation in a particular direction was amplified, and the Milky Way took on its characteristic spiral spin. Majewski decided to test the theory that the Milky Way rotated uniformly in a single direction by measuring the movement of very distant stars. One way to do this precisely, he reasoned, would be to compare old images of the Milky Way taken by his predecessors with new images. Since astronomers for most of the century used photographic glass plates, Majewski used them too. "My work is directly indebted to the scientists who had the foresight to make those early photographic surveys," Majewski declares.

By comparing recent photographs with images taken decades ago, Steven Majewski found that the structure of the Milky Way is much more complex than astronomers had imagined.

Majewski, however, was thoroughly modern when it came to using the latest computer image processing techniques to compare his photographs with photographs taken many decades earlier. The results were startling. He found that although stars near the center of the galaxy moved as predicted, stars at its fringes rotated backward.

"These findings do not necessarily refute the accepted view," Majewski says. "They simply indicate that the processes that formed the Milky Way are not as straightforward and uniform as first imagined." Majewski believes the dozen or so dwarf galaxies orbiting the Milky Way are the source of this erratic motion. There is some evidence that the gravitaitonal force of the Milky Way is tearing these galaxies apart, in the process stripping off large ribbons of stars. "My goal," he says, "is to show that stars are being cannibal satellite systems." In other words, Majewski believes that the stars in the outer halo of the Milky Way were not produced when the Milky Way condensed, but were pulled from the disintegrating neighboring galaxies. Since the largest of these satellites-which include the Magellenic Clouds-can only be seen from the Southern Hemisphere, Majewski spends several weeks a year observing them from the Las Compañas Observatory in Chile.

For his work, Majewski has won a CAREER Award from the National Science Foundation and a Packard Foundation Fellowship. With these grants, he intends systematically to map stellar motions in a vastly larger volume of the outer Milky Way, which should enable him to detect the ribbons of stars that are the debris of disintegrating satellites. "There is a fair amount of archaeology in what I do," Majewski says. "My ultimate goal is to identify the fossil evidence needed to reconstruct the galaxy's past."

This article appeared in the Winter 1998 edition of Explorations, a publication of the Office of the Vice Provost for Research and the University Development Office of Corporate and Foundation Relations. © 1998 The Rector and Visitors of the University of Virginia.