The Milky Way: Bigger, Faster, Better Understood
By Danielle
Dowling
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.
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.