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Astronomers capture rare stellar eclipse
Posted: 8 April

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The dusty disc that eclipses the puzzling star epsilon Aurigae has, thanks to the most sophisticated optical interferometer on the planet, been imaged passing in front of the star for the first time.

Every 27 years, the brightness of epsilon Aurigae fades from +2.92 to +3.83 over the course of a year, before brightening once again. These variations were first discovered in 1821, but it has taken until this most recent eclipse, which began in August 2009, to solve the mystery of what was causing the dips in brightness.

An illustration of the dust disc orbiting a companion star that together orbit epsilon Aurigae. AN graphic by Greg Syme-Rumsby.

The leading contender had been a thick, dusty disc around a companion star that is orbiting the primary star of epsilon Aurigae. The observations by American astronomers at the universities of Denver, Michigan and Georgia State University using the array of six optical telescopes at the Center for High Angular Resolution Astronomy (CHARA) at Mount Wilson have confirmed this to be the case. CHARA is an interferometer, combining the light of the six telescopes with great precision so that their resolution is equivalent to a telescope 330-metres across.

The pre-eclipse observations in 2008 (left) show the F-star as a nearly uniformly illuminated 2.27 milliarcsecond circle. The November 2009 observation (centre) shows the shadow of the disc (outlined in a white ellipse) around second contact and the December 2009 observations (right) show that nearly 50 percent of the stellar surface is obscured by the eclipsing object. Image: John D. Monnier, University of Michigan, Nature 8 April 2010.

The images taken over a month show the black silhouette of the disc beginning to edge across the surface of the primary star, which is an F-type star with a mass 3.6 times that of the Sun. The size of the disc has been measured at about 2.4 billion kilometres, twice the size of the orbit of Jupiter, but with a mass less than a tenth that of Earth. Consequently, it is unlikely to form any planets of note, but its behaviour could teach us about the protoplanetary disc that formed the planets of our Solar System 4.6 billion years ago. The disc itself hides a companion B-type star that is hotter, brighter and, at 5.9 solar masses, more massive than the F-type primary, but the disc effectively blocks most of its light.

“It is very satisfying to finally resolve some of the long standing questions associated with this famous star,” says Dr Robert Stencel of the University of Denver, who has been studying epsilon Aurigae since the 1980s. Stencel, along with amateur astronomer Jeff Hopkins, is overseeing a team of observers who make regular measurements of epsilon Aurigae’s brightness to try and track the stages of the eclipse.

Related stories:
5 Jan 2010 What is epsilon Aurigae?
25 Aug 2009 Citizens to help solve stellar mystery

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