Alert Notice 398: Eclipse of epsilon Aurigae

July 10, 2009

The bright, long-period, eclipsing binary star epsilon Aurigae is predicted to begin its next eclipse late July or early August of 2009. The AAVSO is participating in a global campaign to record this eclipse as part of the International Year of Astronomy 2009 celebrations, organized by the Citizen Sky project ( Epsilon Aurigae is now past solar conjunction and has reappeared as a morning object. All observers -- both visual and instrumental -- are encouraged to contribute observations of the eclipse during the next two years, beginning immediately for morning observers.  Observations are urgently requested right now because it is less likely to be observed in the morning, and the eclipse will begin within the next month.

Epsilon Aurigae is a bright (V ~ 3.0 out of eclipse), northern eclipsing variable with a known period of 27.12 years.  Eclipses have a duration of several hundred days.  It is not currently clear what the eclipsing object is, although the leading theory is that it is a massive, opaque dust disk surrounding either a single massive secondary star, or a pair of stars in a tight orbit.  Observations obtained during this eclipse will help us to refine the properties of this enigmatic system.

Visual observers are particularly encouraged to observe epsilon Aurigae. For experienced observers, please observe this star on a weekly basis, using charts available via VSP from the AAVSO website.  For novice observers, we recommend participating in this observing program by following the Citizen Sky 10-Star tutorial program, which provides a simple training experience in variable star observing.  The 10-star tutorial is available at the following website:

Instrumental observers of all kinds are also requested to take part. Photoelectric observers belonging to the AAVSO PEP-V program may submit data as usual via the WebObs feature of the Blue&Gold section of the AAVSO website. Photoelectric observers may also contribute reduced observations in all filters (including infrared J- and H-bands) directly to the AAVSO via WebObs, and we note that Dr. Robert Stencel (Denver U.) and Jeff Hopkins (HP Observatory) are co-leading the precision photometry efforts in collaboration with Citizen Sky.

Observers using wide-field CCD and DSLR systems are also encouraged to participate. Epsilon Aurigae is a bright object, and care must be taken not to saturate the star. For those with narrower-field systems (D < 2 degrees), we recommend taking a large number (10-100) of very short exposures and then stack the resulting images. This will allow you to observe the variable without saturating, but also to detect and photometer the fainter comparison stars (V < 9) in the near field. For those with wide-field systems or DSLR cameras, we recommend using lambda Aurigae for the comparison star, and eta Aurigae as the check star.

More information about epsilon Aurigae photometric techniques is available here:

Please post questions to the Citizen Sky Forums:

Dr. Arne Henden will be on hand to take your questions about this project via a live web chat on the AAVSO/Citizen Sky web sites July 10 at 1pm EST (-4UT). A transcript will be posted after the chat is concluded.

The following observations of epsilon Aurigae were submitted to the AAVSO during the past seven days:

JD 2455015.5785 (2009 July 3.0785), m(vis)=2.9 (S. Baroni, Italy);
JD 2455016.42361 (July 3.9236), m(vis)=2.9 (V. Makela, Finland);
JD 2455018.42014 (July 5.9201), m(vis)=3.0 (Makela);
JD 2455019.8757 (July 7.3757), m(vis)=3.0 (M. Bradbury, United States);
JD 2455021.5653 (July 9.0653), m(vis)=3.0 (Baroni);
JD 2455022.54167 (July 10.0417), m(vis)=3.05 (S. Swierczynski, Poland).

The J2000 equatorial coordinates of epsilon, lambda, and eta Aurigae are as follows:

        epsilon -- RA: 05:01:58.13 , Dec: +43:49:23.9
                V = 2.98 , (B-V) = +0.54 , (U-B) = +0.32

        lambda  -- RA: 05:19:08.48 , Dec: +40:05:56.6
                V = 4.71 , (B-V) = +0.62 , (U-B) = +0.12

        eta     -- RA: 05:06:30.89 , Dec: +41:14:04.1
                V = 3.17 , (B-V) = -0.18 , (U-B) = -0.67 

Charts for epsilon Aurigae may be obtained from the following links:

Advanced visual observers: Please use VSP as your source for charts --

Beginning and naked-eye observers: Please download and print the following chart from Citizen Sky:

[broken link]

Instrumental observers: A chart that includes both epsilon Aurigae and its closest comparison star is available at this link:

[broken link]

This AAVSO Alert Notice was prepared by M. Templeton on behalf of A. Price, R. Stencel, and J. Hopkins of Citizen Sky.



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