The Why Files The Why Files --

Milky Way Bar (bar sign)
4 AUGUST 2005

Giant spiral of stars and dust, with bright, angled bar at centerThe Milky Way Galaxy shows definite signs of a bar. For years, astronomers suspected our galaxy was concealing a central drinking establishment, but gobs of dust and gas obscure our view of the center, so it was impossible to know for sure. New measurements of the location of 30 million stars, taken with an infrared detector aboard the Spitzer Space Telescope, are the surest evidence of a massive watering hole deep inside our galactic home.

Artist's rendering shows the Milky Way is no plain spiral galaxy. According to a massive new survey of stars, a stylish, angled bar across the center distinguishes the Milky Way from economy-model spiral galaxies. Image: NASA/JPL-Caltech /R. Hurt (SSC/Caltech)

These scientists haven't been drinking, but they have found a larger-than-expected bar in the center of our galaxy, one that measures about 27,000 light years in length -- 7,000 light years longer than previous estimates. The bar is made of old, red stars, and it's oriented at about 45° to a line between our sun and the center of the galaxy.

None of these facts was clear before the new sky survey. In fact, it's tough making a picture of the home place, if we may be so familiar with a galaxy that contains very roughly 99,999,999,999 other stars. There's the problem of perspective: Can a fish snap a picture of its ocean?

And then there's that pesky dust and gas, which block the view of the center of the galaxy. Gas and dust are what transform the galaxy's bright, pinpoint stars into that gauzy light-emitting blanket we call the Milky Way. The dust and gas are so dense that we can't even see that big, bad black hole that most astronomers think is clawing up matter at the center of the Milky Way.

2 views of same spiral galaxy; no bar appears at center
Hardly your average spiral galaxy, the Milky Way has a huge bar at the center. These are two views of a regular spiral galaxy: Left in visible light, right in infrared light, taken by Spitzer. Photos: NASA/JPL-Caltech /R. Kennicutt (Univ. of Arizona)/DSS

A matter of perspective
Overcoming the second problem becomes rather easy, if you can get your hands on a top-notch infrared observatory like Spitzer. Infrared light goes through gas and dust quite nicely, so stars that would look foggy to our eyes can distinguish themselves as unique individuals.

Overcoming the first problem -- perspective -- is a mite more complicated. Once you can see a star, it's easy to tell its direction from Earth. But unless you can also tell its distance -- which Spitzer isn't equipped to learn -- how do you tell what kind of structure you are looking at? The star could be next door, or across the galaxy, and you wouldn't necessarily know the difference.

Left-side strip is brighter than right-side strip.
These two bands from Spitzer Space Telescope, shot to each side of the galactic center, show more stars on the side that was left of center. Why? Because the bar at the center of the Milky Way angles toward us on the left, so we see more stars there. Graphic: Robert Benjamin et al (see "First GLIMPSE..." in the bibliography).

The answer comes by comparing what we find to what we know, says Robert Benjamin, an assistant professor of physics at University of Wisconsin-Whitewater, and lead author of the new study. Scientists long ago concluded that we live in a spiral galaxy, and we know how that should affect star distribution. But when Benjamin and his crew counted bands of stars on infrared photos, the distribution didn't match what they should have seen from a spiral galaxy.

In fact, stars are much more common on the left side of the galaxy. "When you look at the number of stars versus the angle from the galactic center, you see an asymmetry from left to right," says Benjamin. "There are 25 percent more stars to the left, from our position."

If you've long suspected that the Milky Way was hiding a bar of stars at the center, Benjamin says, that data can only mean one thing. "The near side of the bar is closer, so it occupies a bigger angle," and appears to have more stars.

Lines angle away from Earth in view of galaxy, showing where telescope took data.The survey was taken between each red line and the nearby yellow line; red area was not imaged. Notice that more bar stars were surveyed on the left? Courtesy Robert Benjamin

Making the bar
Galaxies are giant whirling disks of stars and other matter, held together by gravity as they orbit around a common center of gravity. So how does that bar shape form? The question, "is a very active area of research," Benjamin says. Somehow the bar-stars moved from circular orbits into "coherent, long, skinny, elliptical orbits." But what could have been powerful enough to disturb the orbits of a hundred million (to use a round number) bar-stars in the first place?

One proposal, says Benjamin, "is that when a galaxy interacts with another, fairly substantial galaxy, it can trigger instability. By banging on its neighbor, the other galaxy triggers bar formation." And who were those thuggish neighbors? Two prime suspects lurking in the foreground are galaxies called the Small and Large Magellanic Clouds, says Benjamin.

The bar-stars, like others in the galaxy, must be orbiting -- otherwise gravity would clump them together. Edward Churchwell, a professor of astronomy at University of Wisconsin-Madison and long-time star surveyor who leads the GLIMPSE study, says the shape of the Milky Way bar suggests how its stars must be orbiting. Diagram shows 3 possible orbits for bar stars."It looks like the bar in our galaxy might, like those in a small fraction of other galaxies, be fairly long and thin. This means the orbits of the stars that produce the bar must be very eccentric, and have motions almost radially toward the center of the Milky Way."

Bar-stars in the Milky Way Galaxy may orbit in paths that look like this. Most stars follow a circular path. Courtesy Robert Benjamin

The highly eccentric orbits provide a natural mechanism to transport matter from the disk of the Milky Way into the center of the galaxy, Churchwell adds, and could even be feeding gas or dust into the central black hole.

Gobble, gobble.

Slurp, slurp.

-- David Tenenbaum

First GLIMPSE Results on the Stellar Structure of the Galaxy, Robert Benjamin et al, Astrophysical Journal Letters, in press, 2005.


Meet the galaxy.

Benjamin home.

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