Jimmy Westlake's Celestial News column appears monthly in the Steamboat Today.
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In the far northern constellation of Auriga the Charioteer, not far from the bright star Capella, an astronomical mystery is about to play out, as it has every 27 years for the past century. Professional astronomers and citizen astronomers worldwide are watching and waiting for the star Epsilon Aurigae to begin to fade during August. Epsilon Aurigae is an eclipsing binary star, not in and of itself very unusual, but the object that is creating the eclipse remains one of the most puzzling of astronomical mysteries.
For an orbiting pair of stars to eclipse each other as seen from Earth, their orbital plane must coincidentally lie almost directly in our line of sight. Only then will the objects pass in front of each other and cause the periodic dimming characteristic of an eclipsing binary star. Take, for example, the famous eclipsing binary star Algol. Every 2 days, 20 hours and 49 minutes, Algol fades to one-third its normal brightness for two hours as the fainter star eclipses the brighter star and then it returns to its normal brightness.
At a distance of 2,000 light years from Earth, the eclipsing binary system Epsilon Aurigae is enigmatic for a number of reasons. It has the longest orbital period of any known eclipsing binary system, 27.1 years, and, whatever it is that eclipses the normal star is far from normal. With its long orbital period, one would expect the eclipse of the primary star to last for several days, if the eclipser were a normal star. Instead, astronomers have observed that the eclipse lasts for two years. Whatever it is that eclipses the primary star must be huge, and yet, it contributes very little light to the system. Exotic explanations have included black holes and semi-transparent stars, but most of these ideas have been discredited. Instead, astronomers hypothesize that the eclipser is a pair of small, hot stars in a tight orbit, hidden within an enormous torus, or donut, of opaque gas and dust. Midway through the two-year eclipse, there is a slight brightening, as if a little bit of the primary star's light is peeping through the hole in the middle of the dusty donut. So far, there has been no direct observation of the hypothesized pair of small stars holding the dusty disk together and sweeping out the hole in the middle. The last eclipse of Epsilon Aurigae occurred in the 1980s, before there was a Hubble Space Telescope. Now, our refurbished eye-in-the-sky and telescopes all over the world will watch with great anticipation as Epsilon Aurigae's dark behemoth begins its next eclipse.
The International Year of Astronomy 2009 has declared the upcoming eclipse of Epsilon Aurigae their primary observing target for the year, encouraging people all over the world to watch the star and contribute to the unraveling of its mystery.
You can spot the constellation Auriga and its mystery star Epsilon rising in the east before sunrise during August. Epsilon is usually the brightest of the three stars forming the tiny triangle nicknamed "The Kids" just south of the first magnitude star Capella, but when eclipsed, it will become the faintest of the three Kids. Catch it now, before the eclipse begins, and then watch it fade over the coming months, knowing that fellow skywatchers around the globe are doing the same.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. His "Celestial News" column appears weekly in the Steamboat Pilot & Today, and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out Jimmy's astrophotography Web site at www.jwestlake.com.