Astronomers Measure Distance to Milky Way's Spiral Arm
"Plan view" of the Milky Way as seen from its north pole. Estimated locations of spiral arms are indicated by the large number of dots and labeled by a prominent constellation onto which they are projected.
Credit: ©Science

Astronomers can provide detailed images of beautifully swirling galaxies millions of miles away. It's our own galaxy they haven't been able to get their arms around.

"It's clear that the Milky Way has a structure like those, but it's hard to tell being in it," said Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

Now, a team of astronomers has taken an important step toward mapping the Milky Way by accurately measuring the distance to the star-forming region W3OH in the Perseus spiral arm, the nearest arm to us. This long strand of stars streaks out of the Milky Way's disk in the same manner as others seen in galaxies across the universe.

Until now scientists had difficulties figuring just how far away spiral arms are, and various measurements and techniques had discrepancies ranging by a factor of two.

The new results are from a telescope nearly the size of Earth. The astronomers used the Very Long Baseline Array, taking observations from several telescopes stretching from Hawaii to the Virgin Islands, to create the resolution of a telescope nearly 5,000 miles in diameter.

"We have established that the radio telescope we used, the Very Long Baseline Array, can measure distances with unprecedented accuracy--nearly a factor of 100 times better than previously accomplished," Reid said.

In doing so, they determined that W3OH is 1.95?0.04 kiloparsecs away. That's about 36,000,000,000,000,000 miles.

Although the VLBA has extremely high resolution, the source objects need to be very bright and compact. Spiral arms are just that--bright clouds of star-forming gas. In particular, this team focused in on a cloud of methyl alcohol surrounding a newly formed star.

"We measured distance by the simplest and most direct method in astronomy--essentially the technique used by surveyors called triangulation," said team member Xu Ye.

This technique, also known as parallax, comes straight out of a high school geometry textbook. In this case, the researchers used the changing vantage point from the Earth as it orbits the Sun to form one leg of the triangle. Then, they calculated the star's distance by measuring the change in its position as Earth orbits the Sun.

The astronomers found that this young star is actually moving around the Milky Way in an orbit that is about 10 percent off from being circular. It appears to rotate slower than other stars in circular orbits and is falling toward the center of the Milky Way.

Although scientists don't really understand the details of spiral arm formation, these observations support the "spiral density-wave theory." This theory suggests that a combination of gravitational instabilities and shear forces--a result of the outer edge of the galaxy moving slower than the inner--cause material to cluster and eventually shoot off in an arm.

The researchers are now shifting the VLBA's gaze toward three other spiral arms where they will look at about a dozen star-forming areas similar to W3OH.

This research is detailed today in the online version of the journal Science.