The Milky Way: Bigger, Faster, Better Understood

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Roger Ressmeyer / Corbis

The center of the Milky Way galaxy, as seen from Earth

It turns out that our solar system is moving nearly 100,000 m.p.h. faster than previously thought — revolving around the center of the Milky Way at 568,000 m.p.h., announced Mark Reid of the Harvard-Smithsonian Center for Astrophysics on Monday at the American Astronomical Society's conference in Long Beach, Calif. Since velocity is related to mass, the 15% increase in solar-system speed translates into a near doubling of mass of the Milky Way, according to Reid's group — and all of that newfound bulk is composed of dark matter.

Original estimates of the solar system's speed were based on what Reid calls "one-dimensional velocity" obtained solely from Doppler shifts. "Now," he says, "we have three-dimensional velocity and more exact measurements" — a huge advancement in the field. The findings debunk the notion that the Milky Way is a little-sister galaxy to her neighbor Andromeda. "They're more like fraternal twins," Reid says. And the fact that they are of equal size increases the likelihood that the two will someday collide. (See the Top 50 space moments since Sputnik.)

But humans needn't flee the galaxy anytime soon. First, there's so much room between stars that Earth likely wouldn't feel any effects of a galactic collision, though our constellations would certainly change. And second, a crash is still about 3 billion to 5 billion years away, by which time our sun will have transformed into a red giant and turned the Earth into a smidgen of charred dust.

A flurry of recent findings has provided a clearer understanding of the Milky Way than ever before. Just a few weeks before Reid's announcement, Martin Pohl, an associate professor of physics and astronomy at Iowa State University, revealed the most detailed map to date of the galaxy's spiral arms. Pohl's map establishes that there are two symmetric arms in the inner part of the galaxy that branch off into four — answering a question that astrophysicists have grappled with for 50 years. (According to Pohl, our solar system is located near one of the branching-off points, about 28,000 light-years — or 168 quadrillion miles — from the galactic center.)

Although it's our home galaxy, the Milky Way is not an easy entity to peg. Drawing a map of it has been likened to describing the town you're in when you've never ventured beyond the confines of your bedroom. In 1958, Jan Oort became the first to map the galaxy, by assessing the strength of neutral atomic hydrogen gas, a widespread component of the Milky Way. Pohl created his map using a kinetic model of the galaxy's gas flow, which was developed by Peter Engelmeier of the University of Zurich and Nicolai Bissantz of Ruhr University in Bochum, Germany, based on infrared data collected by a NASA satellite. Unlike visible light, infrared light penetrates dust clouds, so it provided Engelmeier and Bissantz with more precise measurements, in turn allowing Pohl to produce a more accurate, if not perfect, map. "Our method works over much of the Milky Way but not everywhere," he says.

Pohl's map is the first to take into account the entirety of the Milky Way, including its elusive inner galaxy, which is "really complicated and weird," Pohl says, because of the gravitational pull of a large bar of matter located at the center; the map will be particularly useful to a research group in France that has been studying dark matter, which, as Reid's team has established, the Milky Way has a lot more of than previous studies had revealed. Dark matter, so called because it gives off little or no light, is one of the most abundant elements in the universe (it constitutes 23% of the universe's total density, whereas normal atoms make up only 4.6%), but because of its relatively recent discovery and our inability so far to gather direct evidence of its existence, it's also the least understood. With tools such as Pohl's map, scientists can achieve a better grasp of the mysterious matter. "The better we can understand [the structure of the galaxy], the better the chance for finding more evidence of dark matter," says Pohl.

See pictures of Earth from space.

See the top 10 scientific discoveries of 2008.