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University of Arkansas astrophysicists have taken an important step toward solving the mystery of how disk galaxies maintain the shape of their spiral arms. Their findings support the theory that these arms are created by a wave of denser matter that creates the spiral pattern as it travels across the galaxy.
"The structure of spiral arms in disk galaxies is a mystery," said Ryan Miller, visiting assistant professor of physics. "No one knows what determines the shape of these spirals, or why they have certain numbers of arms. Our research provides a clear answer to part of that mystery."
Disk - Galaxies - Milky - Way - Comprise
Disk galaxies, including the Milky Way, comprise 70 percent of known galaxies. They are characterized by their spiral-shaped arms, but astronomers are not sure how these form and maintain themselves.
The mystery begins with a simple paradox: stars in a disk galaxy orbit a central mass called a "galactic bulge," and the stars closer to the center orbit faster than the stars toward the edge. But, if the spiral arms were composed of a fixed group of stars, the ones on the edges of the pattern would have to cover more distance than the stars in the middle in order to maintain the spiral pattern. Like runners in the outer lane of a circular track, they would need to move faster to keep their position in the group.
1960s - Astronomers - Density - Theory - Paradox
In the 1960s, astronomers proposed the "density wave theory" to explain this paradox. The theory holds that the arms of disk galaxies are not formed from static bundles of stars. Instead, these arms are waves of denser areas that move through the stars. The stars move in accordance to the laws of physics, and as they orbit the center of the galaxy, they encounter these denser areas.
Many astronomers have compared the wave of denser matter to a...
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