Physics Photo of the Week
Lyra the Harp and Beta Lyrae
Photo by Chandler Jones and Gracie McCarroll
High in the sky throughout the mid-summer months in the northern
hemisphere lies the constellation called Lyra - or "The Harp".
Lyra is dominated by Vega, the brightest star of the
constellation. Stars are often designated with Greek letters with
"alpha" being the brightest, "beta" the second brightest, etc, but
there are many exceptions to this rule. Lyra is most noted by the
parallelogram formed by the fainter stars beta, gamma, delta, and zeta,
which can be seen with the naked eye on clear, moonless nights.
The viewer has to use some imagination to see the "strings" of the harp.
Of a number of interesting objects in Lyra, beta Lyrae has been
extensively studied at Warren Wilson this year over a span of about 145
days - essentially the whole observing season for 2008. Beta
Lyrae is a highly variable star, which has been studied quite
extensively by astronomers during the past 200 years. Beta is not
one single star, but two stars that orbit each other in
proximity. The plane of the orbits of the two stars of Beta
happens to lie in the same plane as the line of sight from the Solar
System to Beta. This means that once a cycle - every 12.94 days - the
brighter star passes behind the fainter star. This eclipse of the
stars causes the brightness to become 2.5 times fainter.
That is one stellar magnitude fainter. Halfway through the cycle
the brighter star eclipses the fainter star and the
brightness dips again, but by a smaller amount. The graph at
right is the light curve for Beta Lyrae photographed with
camera (no telescope) at Warren Wilson College. This plot covers more
than 140 days throughout 2008. The horizontal axis on the
light-curve is the phase (fraction of a cycle). The phase is the
fractional part of the time (in cycles of 12.94 days) since
arbitrary starting time. For example if an observation time
occurs 8.59 cycles after the starting time, the phase is 0.59.
The light curve is plotted for two cycles to make the cycles more
evident. The deep primary eclipses and the shallower secondary
eclipses are quite evident. The bottom trace on the plot is a
nearby star Nu 2 which does not vary in brightness.
In spite of observations of Beta Lyrae for more than 200 years since
its discovery of being a variable star by Goodricke in 1784, much still
remains to be learned about this system. As recently as about 45
years ago it was discovered that there is a thick accretion torus
surrounding the secondary star. (AAVSO -2005, Variable Star of the Season, Summer, 2005:
Beta Lyrae - http://www.aavso.org/vstar/vsots/summer05.pdf)
Matter is drawn from the primary star due to tidal interactions; the angular momentum of the
system causes the streaming material to revolve about the secondary
star in a torus rather than falling directly onto the star. See
the model reproduction at left.
The light curve also shows slight variation from cycle to cycle in the
Beta Lyrae system. The Warren Wilson light curve showed
that the system was brighter at early in 2008 than later. Notice
that several observations are "outliers". The star appeared about
10% brighter than in other cycles. Not much is known about the
cause of the brightness fluctuations from cycle to cycle.
Amateurs with digital SLR cameras can monitor this star for several
years to track the variation of maximum brightness. We can also
monitor the times of maximum eclipse. There may be a phase shift
from cycle to cycle - especially when the stars change in intrinsic
brightness. Perhaps the accretion torus develops instabilities,
trapping energy from the secondary star, then erupts at semi-regular
intervals brightening the system significantly. Maybe there is a
hot spot on the torus that is more pronounced during quiescent
times. More data are needed!
Astronomers with large telescopes and sensitive detectors have recently
ignored bright naked-eye stars. There is too much starlight and
the detectors become saturated. Digital cameras with no telescope
can detect the brightness of stars such as Beta Lyrae with a precision
of about 1%. When the star brightens, large telescopes can
observe the system with spectrographs and learn much about the physics
of the brightening whether the brightening comes from the accretion
torus or the secondary star.
Anyone with a digital SLR camera, tripod, and cash to purchase
moderately-priced software is invited to collaborate with Don Collins
(e-mail address below) and help with this research.
The author is indebted to Sara Bacon
for much data reduction and to Anesh
Prasai for making photometry and linearity tests with the camera.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an 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.
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