Lyra the Harp

November 14, 2008

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 somewhat close 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 a digital 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 an 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.