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Orion Gets a Bit Closer

11 October 2007 (All day)
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C.R. O'Dell and S.K. Wong (Rice University)/NASA

Zeroing in.
New parallax measurements have established the distances of young stars in the Orion Nebula with unprecedented precision.

Radio astronomers have taken a look at one of the Milky Way's hottest star-forming regions using a new and better yardstick. What they've found figures to refine the search for nearby worlds and solidify theories about how young stars evolve.

For nearly 2 centuries, astronomers have been using a trigonometric device called a parallax to measure the distances between Earth and other objects in our region of the Milky Way galaxy. It works by measuring the angle between the sun and a telescope's line of sight on a celestial object, and then comparing this angle to the angle recorded 6 months later, when Earth is at the opposite side of its orbit. Knowing the distance of the diameter of Earth's orbit, plus the two angles measured, gives a good notion of the distance to, say, another star. The problem is that even today's sophisticated telescopes have technical limitations that can lead to measurement errors. And those errors can be large enough to skew calculations about the ages of stars and how they formed.

Now a team from the University of California, Berkeley, has applied the formidable observing power of the Very Long Baseline Array of radio telescopes to one of the most familiar objects in the night sky: the Orion Nebula. The array comprises dishes in 10 locations from Hawaii to Puerto Rico, simulating a radio telescope with a collecting area thousands of kilometers wide. This allows it to compile parallax measurements that are four times more precise than any other instrument currently available. The result of the measurements, the team reports in an upcoming issue of The Astrophysical Journal, is that the nebula is only 1270 light-years away, or about 20% closer than previous estimates. "We were surprised by the new distance, although in retrospect, we shouldn't be," says astronomer and co-author Geoffrey Bower. The new measurement fits with previous estimates when their margin of error is considered, he says. Because distance measurements are used to calculate the brightness--and hence age--of stars, the new data indicate that Orion's stars are older than previously thought, which makes them consistent with current theory about stellar evolution, Bower notes.

Astronomer Volker Bromm of the University of Texas, Austin, calls the Berkeley team's work "excellent" and says it's "certain to spark a plethora of new theoretical work." Because Orion's stars represent the nearest example of the stellar formation, he explains, they are "at the core of our attempts to unravel the star formation mystery."

Bower says his team next will be taking more precise measurements of nearby low-mass stars to detect the telltale wobble in their motion that reveals orbiting extrasolar planets. That project, called the Radio Interferometric Planet Search, also will rely on the Very Long Baseline Array.

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