Physics Photo of the Week

March 1, 2013

Digital Camera Astronomy and b Persei
Modern digital cameras are proving valuable contributions to astronomical data.  With only a tripod mount (no clock drive as with telescopes) and an inexpensive consumer grade zoom lens the camera shown here has recently detected a rare predicted eclipse of b Persei - a relatively bright variable star in the constellation Perseus.

Dr. Bob Zavala, an astronomer at the Naval Observatory Flagstaff Station (NOFS) had solicited through the American Association of Variable Star Observers (AAVSO) for observations of b Persei near the January 28, 2013 date for the predicted eclipse which had never before been observed in any concerted campaign.  This simple camera was the first instrument to detect the start of the eclipse in the current AAVSO campaign.  Bright stars are cumbersome to photograph with large telescopes - they saturate the electronic detectors.  Digital cameras and amateurs fulfill this niche performing bright star photometry.

Collins DSLR data

The graphs shown above show the brightness of b Persei dropping dramatically during this eclipse.  The left graph is with Collins' camera and the right graph shows all the observations that several observers scattered around the world have submitted to AAVSO as a result of Dr. Zavala's campaign to verify this predicted eclipse with many observers.  This is the first ever eclipse of b Persei to be verified by many observers.  A real contribution of Citizen Science!

The star b Persei, too faint for naked-eye visibility, not to be confused with the well-known similar star beta Persei (Algol), is an interesting triple star system.  Two stars are in a very tight orbit and orbiting quite rapidly with a period of about 1.53 days.  The plane of the binary stars' orbit is inclined to our line of sight such that the stars do not eclipse or overlap each other.  The normal light curve shows only a small regular variation (about 6 % of its average intensity) first observed electronically over 90 years ago (Stebbins, J., Astrophysical Journal, 76, pp 1-6, 1923) in the infancy of electronics.  The two stars are so close to each other that they are gravitationally distorted as ellipsoids, and thus appear as larger and smaller areas from Earth's viewpoint.  Extensive spectroscopic observations (Hill, et. al., 1976, Astrophysical Journal, 208, pp 162-164) revealed the existence of a third star orbiting the close binary system at a much larger distance than the separation of the close binaries, and found evidence that this third star is possibly fortuitous in it's orbit to eclipse the other two stars.  A Hungarian astronomer (Hegedus, et. al. IBVS, 4340, 1996) detected a possible eclipse of b Persei.  The stars in b Persei are all too close to each other and to far away from Earth for any direct detection of distinct components by imaging.  However the Navy Precision Optical Interferometer (NPOI) at Flagstaff has recently been able to resolve all three components of b Persei, obtaining enough information to pin down the orbits, so that the eclipses of the inner components by the third outer star would occur about every 700 days and hence predicted an eclipse to occur on January 28, 2013.  The eclipse should be significantly larger than 6 % regular rapid variation from inner stars' orbits.

January 28 came and went - nobody had observed the predicted change.  I kept observing once every clear night with the digital camera (part of the goal was to show that the digital camera would detect the 6% normal variation of the star's light.  Eventually, on the 5th of February, about 1 week overdue, the the camera detected  the eclipse.  I immediately notified Bob Zavala and the AAVSO.  The campaign was renewed and several other observers subsequently detected the eclipse before the eclipse was finished 3 days later.  Observations such as these are important in understanding the nature of multiple star systems, their interactions, orbits, and eventual evolution.  These permit refinement of the orbit of the third component (the 7 day delay in the eclipse) and will better predict the next eclipse in about 2 years when many more observers will be using similar equipment with improved digital cameras. 

The star image shown above is a portion of an image made by the digital camera used for the photometric measurements.  The photo is a stack of 14 exposures of 15 seconds each, and b Persei is marked.  The stars show short trails because the tripod is stationary and not tracking.  When the College View Observatory is completed we'll be able to attach the camera to the telescope and make use of the telescope's clock drive to improve the image quality.  Similar studies of eclipsing stars were done with the DSLR on beta Lyrae (PPOW November 14, 2008)

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

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