Essay: Stars in Exquisite Accuracy

There’s only one star that astronomers have a firm grasp on: the Sun. It’s 149,600,000 km away, but that’s hardly a daunting distance, on a cosmic scale, across which to photograph and inspect its physical attributes, cycles, and magnetic makeup.

But such fundamental facts about other stars, even the ones in our immediate neighborhood (within a thousand light-years), remain elusive. Stars beyond the Sun are simply too far away to image in detail using conventional telescopes. “I think one common misconception, even among astronomers, is that stars are a solved problem,” says California Institute of Technology astrophysicist Gerard van Belle. “And they're not. We still don't know a lot about these basic building blocks of galaxies. And this is where interferometry comes in.”

To measure the Milky Way’s brightest stars with uncommon accuracy, CHARA (Georgia State University’s Center for High Angular Resolution Astronomy) debuted a powerful interferometer atop Mount Wilson in Southern California in 2004. Data are now rolling in for stars such as Vega, Regulus, and Alderamin. CHARA is working to demystify faraway stars, including those that may harbor planets.

A Single-Scope Substitute

From 16,000 km away, CHARA’s interferometer can distinguish details as small as a nickel. To achieve this superior level of sharpness—or “high angular resolution”—it uses several telescopes in the place of one. Traditional reflecting telescopes use a single mirror to collect light rays from a target object. The larger the mirror diameter, the more resolution, or sharpness, its images have. High resolution telescopes can distinguish between two space objects that may otherwise appear as one blurred blob.

CHARA array_2_crop.jpg

The six telescopes of the CHARA array take the place of a single mirror, which would have to be an impossible 330 m wide to match the distance between the array's two farthest telescopes.

Arlene Ducao for AMNH

On Mount Wilson, an array of six 1 m telescopes inhabit the area of a single mammoth mirror, and the light waves they collect “interfere,” or are combined. “An interferometer basically cuts out the middle man,” says van Belle. “You slice out the middle bit of the telescope, leaving just the edge bits, which you can stretch as far apart as you want to.“ The CHARA array’s effective mirror diameter, or the baseline between its two farthest telescopes, is 330 m. That dwarfs the diameter of the world’s largest optical reflecting telescope—the 10 m telescope at Hawaii’s Keck Observatory. The CHARA array has more baseline—and thus more angular resolution—than any optical interferometer yet built.

Managing Multiple Telescopes

Still, working with six small mirrors instead of one big one can be unwieldy and less complete. With a traditional telescope, the curved mirror ensures that each light ray that hits has traveled the same distance from the source. The peaks and troughs of every light wave arrive in unison, producing an optical image.

CHARA has more variables to consider. The rotation of the Earth, and the fact that each telescope sits at a different elevation on Mount Wilson’s craggy summit, mean that light rays from a star may arrive at one telescope sooner than they arrive at another. This is why CHARA has an entire building—the Beam Combining Lab—containing instruments to synchronize the wavelengths.

The waves are matched to a fraction of a micron, smaller than the width of a visible light wave. “We have to measure distances to exquisite accuracy,” says Hal McAlister, CHARA’s director. “To be an interferometrist, you have to love this level of precision.”

Furthermore, with the “middle bit” of the ostensible large telescope missing, interferometrists can’t collect enough light rays to create complete astronomical photographs of target stars. The sum of all the starlight along one baseline emerges as numbers—mathematical data. Computers analyze this data to work out the star’s physical properties such as size and shape. A computer model of the data fills in the gaps with educated guesses, creating not a photograph but a 3-D visualization of the star.

The more telescopes that are employed in the array, the more an authentic image appears. Most of the 3-D star models CHARA has made so far used only two out of the six telescopes. But a team from the University of Michigan has just created CHARA’s first “real image,” a binary star, using four telescopes. As the instrumentation is perfected, all six telescopes will eventually be integrated.


The shape of the star Regulus (blue) versus the Sun. Regulus' rapid rotation causes its flying-saucer shape.

Courtesy Gerard van Belle

Getting Results

CHARA measures one or two dozen stars in the Milky Way per night. Astronomers like van Belle are using the data to fill gaps in our stellar knowledge. Van Belle observed that both Regulus (in the constellation Leo) and Alderamin (in Cepheus) are rotating so rapidly that their centripetal acceleration bloats their equators, making the stars look more like flying saucers than spheres. As far as scientists know, only about 2 percent of stars look like this. They stand in stark contrast to the sluggish, spherical Sun, which rotates once a month.

CHARA is also being used to distinguish luminous objects orbiting very close together, such as binary stars. Much like spotting a dimly glowing firefly near a blinding street lamp, astronomers can use CHARA to “cut through the glare” to produce a map of the objects’ orbits.

A promise of interferometry’s resolving power is to detect stars with extrasolar planets. By calculating the relative masses of the two objects, astronomers could determine which is star and which is planet. No interferometer has yet found an extrasolar planet, but hopes are high. “My big dream for interferometry would be the detection of a planet the size of Earth around a nearby star that shows signs of life,” says van Belle. “And yes, I think we're going to do it.”

Related Links

GSU Center for High Angular Resolution Astronomy

Mount Wilson Observatory

CalTech: Gerard van Belle