Skip to main content

Web Publications

AAVSO Alert Notice 398: Eclipse of epsilon Aurigae AAVSO HOME > publications > alert notice

AAVSO 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

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:

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

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


Information on submitting observations to the AAVSO may be found at:


An Alert Notice archive is available at the following URL:

Subscribing and Unsubscribing may be done at the following URL:

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484