Epsilon Aurigae – a strange eclipse

Something strange is happening around a star in the constellation Auriga. Something is likely circling the star, known as an “eclipsing binary”, and over the next many months, you can see observe the effects of this happening.

The star in question is called Epsilon Aurigae, also known as Almaaz, located in the constellation Auriga.  The system lies approximately 2,000 light years from Earth and Almaaz can be quite easily found as it’s close in the sky to the bright Capella. (Some tips on how to find Auriga and Capella can be found here.)


Almaaz is easily visible to the naked eye. Over the course of 2009 an 2010, one should be able to follow the eclipse as it begins this month to dim the star by a factor of two – from magnitude 3.0 to 3.8.

In most eclipsing binary systems, two stars revolve around each other. If the orbital plane of the two stars lies along our line of sight, as the stars orbit each star blocks the other, causing dips in the overall brightness of the pair.  Astronomers can measure these dips in brightness and try to find out the period of the orbit, the brightness, and also the mass of the pair of stars.

A famous eclipsing binary is Algol, in the constellation of Perseus. Algol blinks out, almost like clockwork, every 2.87 days. (In fact, you can also see Algol for yourself from August through to April next year).

However, Epsilon Aurigae is quite different. The eclipse seems to happen every 27 years and takes around two years to complete.

Based on observations from previous eclipses (serious observations started in 1842), whatever it is that’s passing in front of the main star likely is just a star. Instead it could be an massive dark disc of opaque material. To hold the disk together, which would be 10AU long and 1AU tall, there could also likely be another star at it’s centre which has yet to be observed.

To make things more peculiar, the system appears to suddently increase in brightness for a short time during the mid-point of the eclipse.

It’s not entirely clear what sort of dark disc might explain the observations of epsilon Aurigae, or what might keep the whole system together.  As this eclipse begins, astronomers will turn dozens telescopes on to the star for detailed measurements.  Skilled amateurs have a calling here too, by making their own observations using telescopes and cameras. It is a key project for the International Year of Astronomy 2009 and Citizen Sky.

We at the NEAS like to think we’re keen, and so will be making observations of the star over the next year.

If you’re interested in learning more, both the British Astronomical Association (BAA), with which the NEAS is now affiliated, and the American Association of Variable Star Observers (AAVSO) will have updated details over the coming weeks.

And for more detailed information about this star I would recommended this article from Sky & Telescope by Robert Stencel.


Posted on 5 August, 2009, in Astronomy News, IYA2009-Beyond, Popular Science and tagged , , , . Bookmark the permalink. 2 Comments.

  1. The map showing the constellation of Auriga has an
    arrow pointing not at Epsilon Aur, but at Eta Aur.

  2. Yes you are right. That’ll teach me to find pictures on Google Images….have changed it now.


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