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 |
AAVSO observations by RICHARD
BIERNIKOWICZ, DONALD COLLINS, JOHN CENTALA,
HEINZ-BERND EGGENSTEIN, JAMES FOX, ARNE HENDEN, and WOLFGANG VOLLMANN. |
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 dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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