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Imaging a Spinning Star

Photonics Spectra
Aug 2007
Interferometric technique with multiple telescopes helps view Altair’s bulging middle.

Hank Hogan

To borrow from an old saying, four telescopes are better than one. Thanks to long-baseline optical interferometry, using a quartet of the devices provides 100 times better performance when it comes to resolving distant objects. Astronomers demonstrated that improvement when they imaged the surface of Altair, a bright star 17 light-years away.

TWAltair_PR_image6.jpg

Because it spins so fast, the bright star Altair bulges at the equator and, therefore, experiences gravity darkening -- a dimming of the equator. With long-baseline optical interferometry, astronomers were able to see this darkening well enough to compare it with models. Courtesy of Ming Zhao, University of Michigan.


The group — with members from the University of Michigan in Ann Arbor; from Georgia State University in Atlanta; from the University of St. Andrews and the University of Cambridge, both in the UK; from California Institute of Technology in Pasadena; from Cornell University in Ithaca, N.Y.; from Laboratoire d’Astrophysique de Grenoble in France; and from the National Optical Astronomy Observatory in Tucson, Ariz. — used an array of telescopes located atop Mount Wilson in California. They created a high-resolution interferometric image with which they could determine that the degree of a phenomenon known as gravity darkening deviated from that forecast by current stellar models.

“Quantitatively, it is way off from the prediction,” said John D. Monnier, assistant professor of astronomy at the University of Michigan.

Altair is part of a class of stars that rotate rapidly and that tend to be hot and bright, to die young and to have an effect on galactic evolution out of proportion to their small numbers. Because they spin so fast, they are oblate, with equatorial regions that bulge outward. Altair, for example, is about 22 percent wider than it is tall. One predicted consequence of oblateness is gravity darkening — a dimming of the equatorial region because it is farther away from the hot core than other regions of the star.

No one, however, has been able to verify gravity darkening predictions with precision. Imaging the surface of a star demands an angular resolution of a milliarc sec, the equivalent of distinguishing a coin the size of a quarter 3000 miles away. That performance would be an order of magnitude better than is possible with the Hubble Space Telescope.

By combining the light from widely spaced independent telescopes, long-baseline interferometry provides more than the needed resolution. If enough telescopes are used, the collection has a resolving power equivalent to that of a single telescope with a diameter equal to that of the array.

For their investigation, the researchers used Georgia State University’s Center for High Angular Resolution Astronomy telescope array, which consists of six 1-m telescopes and has a diameter of 1000 ft. They combined the light from wavelengths of 1.50 to 1.74 μm with the Michigan Infrared Combiner, which uses polarization-maintaining infrared fibers and a variety of commercial and custom optics to manipulate light.

The combiner comprises fiber positioners from Luminos Industries Ltd. of Ottawa, lenslets from Suss-MicroOptics SA of Neuchöatel, Switzerland, custom lenses from ISP Optics Corp. of Irvington, N.Y., and custom fiber injectors made by Nu-tek Precision Optical Corp. of Aberdeen, Md. The current implementation of the combiner is limited to six telescopes, although the technology can be scaled up.

Monnier noted that the device was miniaturized, an important attribute because of the number of instruments involved. “We are interfering every combination of the four telescopes.”

Thanks to an angular resolution 30 to 100 times that of Hubble, the investigators saw the expected gravity darkening: The equatorial band was 60 to 70 percent the brightness of the poles. However, the results did not agree with predictions, a finding in accord with other experiments that will require changes in the models.

While that is being sorted out, long-baseline optical interferometry is being applied to imaging planet-forming disks and extrasolar planets that orbit near their parent stars. “We are focusing on those to try to see if we can image the star and the planet at the same time,” Monnier said.

ScienceExpress, May 31, 2007, doi: 10.1126/science.1143205.


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