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New interferometer gets Sirius.

New interferometer gets Sirius

Stars do not appear as disks in the images made by terrestrial telescopes, so astronomers cannot directly determine their sizes. Sometimes, however, the technique of interferometry -- combining wave trains, or signals, received from the source at different points and measuring the differences between them -- can yield information about stellar sizes. Now two Australian astronomers, John Davis and William J. Tango of the University of Sydney, report that they have determined the angular diameter of the star Sirius by a new interferometry, that has not previously been used for stars of ordinary size. They say amplitude interferometry is more sensitive and applicable to more stars than techniques previously in use.

Classic interferometry works with differences in phase. Two wave trains received simulatenously from a single source at different observing stations will usually differ in phase, and the phase difference depends on the geometry of the source and the distance between the observing stations. Stars offer two other interferometric possibilities -- intensity and amplitude. A star is a collection of billions of individual emitters, and the sum of their activity yields a signal that shows a complicated pattern of minute fluctuations in both intensity and amplitude of the waves. This pattern depends on the size of the star, so an interferometer built to work with intensity or amplitude could determine stellar sizes. Intensity interferometry has been pioneered in Australia, and the intensity interferometer at Narrabri, New South Wales, has measured a number of stars.

Amplitude interferometry is more delicate, and there were fears that atmospheric turbelence would destroy the correlations that make it work. But it does work, Davis and Tango report in the Sept. 18 NATURE. They measured the angular diameter of Sirius to be about 5.63 milliseconds of arc, agreeing well with the Narrabri measurement of 5.60. They say this is the first independent check of any of the Narrabri measurements, and it took only 2 percent of the observing time that the measurement of Sirius did with the Narrabri instrument. The angular diameter, the angle between lines of sight from the earth to opposite ends of the star's diameter, can be converted to kilometers if astronomers know the distance to the star.

Encouraged by this success, Sydney astronomers are planning to build a large amplitude interferometer, which will have 100 times the sensitivity of the intensity interferometer at Narrabri.
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Title Annotation:amplitude interferometry used to measure star
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Oct 4, 1986
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