Forget what you thought the Milky Way looked like. The galaxy is far from the simple and elegant spiral-armed structure so often portrayed. New observations, presented today at the 212th meeting of the American Astronomical Society in St. Louis, Missouri, reveal, among other things, that the Milky Way is missing two of the four spiral arms it was thought to have. The findings should force a significant rethinking about how the Milky Way evolved and how its stars formed.
Mapping the Milky Way is extremely difficult. William Herschel first tried it in 1765 by counting stars with his small optical telescope. But even with improved modern instruments, astronomers have faced several challenges. For starters, our solar system sits on the Milky Way's outskirts, along a branch of one of the spiral arms, so there isn't a panoramic view of the main structure. In addition, our vantage point is obscured by many stars and large clouds of interstellar dust.
Now two teams have pierced that veil with unprecedented clarity. One team used the Spitzer Space Telescope, which can see through dust, to chart the positions and orbital speeds of more than 110 million stars. They discovered a big surprise: Two of the galaxy's four spiral arms are actually just small side-branches. On the other hand, the central bar of the galaxy turns out to be nearly twice as big as previously thought, Spitzer team member Robert Benjamin of the University of Wisconsin, Madison, said at a teleconference today.
Meanwhile, another team probed the galaxy with the Very Long Baseline Array, which comprises a telescope of such power that you could use it to read a newspaper on the moon. They have learned that many young stars in the spiral arms orbit the galactic center slower than calculations suggest. Analyses of their motions reveal what happened: The stars were born when gravity compressed interstellar gas clouds. The new stars got kicked out of the circular orbit of their parent clouds and into more elliptical paths.
"The new data give a much, much better picture of what's going on," says theoretical astrophysicist Avi Loeb, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not involved in the survey. So theorists should be able to improve their models of the Milky Way significantly and "perhaps gain a better understanding of how galaxies of its type are organized."