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Twinkle, Twinkle little star, how I wonder what shape you are. August 4, 2008

Posted by jcconwell in Astronomy, stars.

One of the most exciting talks at the American Astronomical Society meeting last month was about an old friend, stars. The problem that stellar astronomers have is that stars are so very far away, compared to their size, that they appear as points in photographs. Close galaxies can be as wide as 1 degree of arc in the sky, twice the size of the moon, but a close star may only a few millionths of a degree, or about 5 millisecond of arc ( 5mas).

Now most stars rotate, our sun completes one rotation in about 25 days, but many heavy and brighter stars are rapid rotators. Since stars are gas, when they rotate their equators bulge out as illustrated by Achernar

illustrtion of Achernar

illustration of Achernar

The problem is how do we know it’s real shape? Until recently we could not image the shape of any star except for the sun. Diameters could be measured, but not imaged, using interferometers. They were first used in 1920 at Mt Wilson observatory by Michelson and Pease to measure the size of Betelgeuse. In the last few years one of the first stars to be imaged by the Hubble space telescope was Betelgeuse, an easy target since it is 45 mas , compared to 5 mas. for bright stars.

To form an image you effectively need several pairs of telescopes, along different oriented paths, and different spacings. This was first done at radio wavelengths, and now it has final been done in the infrared. The shorter the wavelength, the better the detail (or resolution), but the harder it is to keep the phases from different telescopes in sync.

Thanks to Dr. John Monnier and associates at the U. of Michigan, and Georgia State U. we have some of the first pictures of Altair in the infrared, using the CHARA interferometer at Mt Wison.

U of Mich.

Credit: U of Mich.

What is seen here is not only the distorted shape, but the distribution of temperature, deep red is cooler, yellow hotter. Rapid rotators were thought to be cooler along the equator compared to the poles, up to 1000 degrees difference. Now, for the first time, the effects of rotation can be observed. One of the modifications that might be needed in the near future is a modification of the old H-R diagram to include modification of spectral class due to rotation.

In the future, this tool can open up new vistas, such as observing differential rotation, the poles rotating differently than the equator, along with convection, star-spots, and how all of these effects can vary in time.


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