A Mysterious Eclipse Begins in Auriga

Something strange is circling a bright star in the constellation Auriga, and over the next many months, you can see its effect for yourself.

And if you’re really keen, you can make your own observations of this star and contribute to the science of astronomy.

The star in question is called epsilon Aurigae, in the constellation Auriga.  Unlike most strange stars, epsilon is easily visible to the unaided eye.  (You’ll find it on page 28 of the sky tour of Auriga in Stargazing for Beginners: A Binocular Tour of the Night Sky).  And you should have no trouble following the eclipse as it begins this month to dim the star by a factor of two over the rest of 2009, from magnitude 3.0 to 3.8.

In a normal eclipsing binary, two stars revolve around each other in an orbital plane that lies along our line of sight.  As the stars make a complete revolution, each star blocks the other, causing dips in the overall brightness of the pair.  Astronomers measure the depth and period of these dips to determine the period, brightness, and the mass of the pair of stars.   Algol, in Perseus, is the most famous eclipsing binary.  Algol winks out like clockwork every 2.87 days: you can see it for yourself in from August through April when Perseus is primed for viewing.

But epsilon Aurigae is different.


An artist’s conception of epsilon Aurigae (from Sky and Telescope)

For one, the eclipse, which recurs every 27 years, takes nearly two years to complete.  And based on observations from past eclipses, whatever passes in front of the main star isn’t just  a star, but an immense dark, flat, warped disk of opaque material that’s 10 astronomical units (A.U.) long and 1 A.U. tall (1 A.U. is the distance of the Earth to the sun).

To hold the disk together, astronomers believe there must be at least one star at its center, yet none is visible.  Stranger still, near the mid-point of the eclipse, the systems appears to suddenly increase in brightness for a short time.  And the length of the eclipse increases with each 27-year cycle.

It’s not clear what sort of strange, dark disk might explain the observations of epsilon Aurigae, or what might keep the whole system together.  But as this eclipse begins, astronomers will turn dozens of modern telescopes on the star for detailed measurements.  Skilled amateurs can get involved, too, by making their own observations with small telescopes and photometers.  If you’re interested in learning more, the American Association of Variable Star Observers will have details over the coming weeks.

And for more detailed information about this star… check out this excellent article at Sky and Telescope …


And if you can, check out Jupiter tonight until August 5th.   It’s passing in front of the star 45 Capricorni.  The 6th-magnitude star will look like a 5th bright moon of Jupiter, along with Io, Ganymede, Europa, and Callisto.   Pretty cool.