By Jim Kaler

The sky houses nine birds, four dogs, three snakes, two centaurs, two horses, two goats, some navigational tools, but oddly just one musical instrument: Lyra, the Lyre or Harp. A drum or flute is ne'er to be found. It's said that when played by Orpheus even the rocks applauded. So perhaps one is all we need. Even today, it's hard not to applaud the exquisite beauty of the Lyre's constellation, made of a nearly perfect parallelogram of stars capped by sparkling Vega. Applaud too the other treasures Lyra contains, even though among the ancient patterns it's 13th smallest in size (according to modern boundaries). While it does not boast a profusion of attractions, the ones it does have are outstanding, if not defining. And Lyra's position of 30 to 40 degrees north of the celestial equator renders it eminently visible during northern evenings throughout much of the year.

Vega, the fourth brightest star in the sky and just barely (after Arcturus) the second brightest in the northern hemisphere, dominates. A fundamental standard only 25.0 light years away with an apparent magnitude of 0.03 (almost exactly two magnitudes brighter than Polaris), it's also the archetypal white class A (A0) main sequence star, with equal blue and visual magnitudes and powerful hydrogen absorption lines in its spectrum. Vega (whose name from Arabic alludes to a "swooping eagle") was among the first stars found (with infrared instrumentation) to have a dusty circumstellar disk, which implies planets, though none is yet found. The star also presents a curious problem. Astronomers at first thought that it was slowly rotating, whereas in fact it's spinning quickly, 270 kilometers per second at the equator, but with its pole pointed toward us (negating any possibility of detecting a planetary transit). The star is thus oblate, with the temperature of its poles, which are closer to the stellar center, notably higher than that of its equator, a phenomenon called "gravity darkening," which makes determination of luminosity (36 times solar) hence mass (2.3 Suns) more difficult.

While much dimmer, nearly fourth magnitude (3.45), Sheliak (from Arabic referring to the harp), or Beta Lyrae, draws at least as much attention. An obvious variable (3.4-4.1) with a 12.91 day period, Beta Lyr is an eclipsing binary in which a tidally distorted 2.25 solar mass class B7 (13,500 Kelvin) evolving giant transfers mass at a rate a hundred-thousandth of a solar mass per year (a billion times the flow of the solar wind) to a much hotter (30,000 Kelvin) B0.5 dwarf of over 13 solar masses. The incoming matter is accreted through a circulating disk that nearly hides the more massive star and from which shoot opposing perpendicular jets. As a result of mass loss and exchange, the orbital period is increasing at a rate of 19 seconds per year. The system is so complex, with distorted stars and flowing matter, that it's in a constant state of variation, even when no eclipse is going on. High mass stars are supposed to evolve faster than those of lower mass, but here it's the low mass star that has become the giant. The giant used to be the more massive, but now it is being whittled away much like the giant star in the Algol pair.

Skipping a couple Greek letters, we land on fourth magnitude Epsilon, the "Double-Double." Just to the northeast of Vega, a good pair of eyes might see it break into a pair of fifth magnitude stars, Eps-1 and Eps-2, 3.5 minutes of arc apart. At a distance of 161 light years, they are separated by at least 10,000 Astronomical Units. Even a small telescope then reveals that each is resolved into yet two more doubles with components just under 2.5 seconds of arc apart. All four are white mid class A stars. Within each pair, the stars are seen to orbit, Eps-1 with a likely period of 1800 years at an average separation of 235 AU, with the Eps-2 couple at 700 years and 145 AU. Given individual masses (from luminosity, temperature, and theory) of 1.7 to 2.0 times that of the Sun, the two doubles must take more than 400,000 years to orbit each other. The easternmost of the Eps-2 pair may also be double with a faint companion 0.2 seconds or so away, yielding at least five stars in the system. Delta Lyrae, the northeastern star of the parallelogram, at first looks similar, but its stars of contrasting color are unrelated, Delta- 1 a blue-white B2.5 dwarf, Delta- 2 a reddish M4 giant, the two separated by 250 light years.

Lyra harbors the prototype of a whole class of variables, the RR Lyrae stars. RR Lyrae, an evolved class F helium-fusing giant 850 light years away, varies between magnitudes 7.1 and 8.1 over a remarkably short period of 13.604 hours. Found in large numbers in globular clusters, RR Lyrae stars are low metal, low mass, short period (under a day) versions of the famed Cepheids, supergiants whose absolute magnitudes are closely tied to their pulsation periods, which makes them prime indicators for finding distances to other galaxies. The RR Lyrae stars play a lesser but still important role, as their absolute magnitudes are pretty well fixed at -0.7. They are actually related to the low mass, highly evolved, W Virginis stars, whose period-luminosity relation lies about 1.5 magnitudes below that of the classical Cepheids like Delta Cephei. The separation of the two kinds in 1951 led to a doubling of the scale of the Universe.

Drop an "R" and you get to see much brighter R Lyrae, a red giant (one with a dead carbon core) and semi-regular variable that goes between magnitudes 3.9 and 5.0 over a 46 day interval. Semi-regular variables are related to true long period variables, the Mira stars. These well aged stars are in the process of ejecting their outer envelopes as they nearly expose their hot nuclear burning cores. The result is a planetary nebula. And Lyra contains the most prominent of them all, the Ring Nebula, Messier 57.

Just poke your telescope between Beta and Gamma Lyrae to find its ghostly smoke ring. The Ring was found in 1779, a few years before William Herschel announced the discovery of the first planetary nebula, NGC 7009 (the term meaning "disk-like") in Aquarius, and was incorporated into the class later on. At an uncertain distance of 2300 light years, the nebula, almost a light year long, is the inner portion of the ejecta of a dying advanced giant. It's powered by the copious ultraviolet light from a 15th magnitude central star, an ancient stellar core with a surface temperature of 145,000 Kelvin and a total luminosity of perhaps 650 Suns. Expanding at 30 kilometers per second and surrounded by a huge outer envelope twice its size, M 57 will fade away, leaving a white dwarf behind whose destiny is to cool and dim forever.

Lyra's other Messier object and its lone globular cluster (M 56), has a paucity of RR Lyrae stars. Why some globulars have huge numbers, others none at all, is a mystery. Globulars occupy the vast, ancient halo of the Galaxy, which was formed before giant star ejecta and supernovae had had much time to ramp up the metal content of the star-forming interstellar gases. Nominally 8th magnitude, 33,000 light years away and around nine minutes of arc (80 light years) across, this compact cluster contains nearly a third of a million stars, its iron content just one percent that of the Sun. At 12.5 billion years of age, it's also one of the oldest known, allowing a measure of the halo's age.

Though containing far fewer stars, the open clusters of the Galaxy's younger disk are vastly more numerous. And here again we find just one, seemingly obscure NGC 6791, which lies at the fringe of the Milky Way 13,000 light years away. Open clusters as a rule don't last very long as their stars slowly leak away. NGC 6791, however, has held together for perhaps as long as eight billion years, helping us to date the disk. Old open clusters should have lower metal contents than do younger ones (though not as low as globulars like M 56). Perversely, NGC 6791 has a very high iron abundance, more than double that of the Sun, and oddly also presents evidence for multiple stellar generations, Lyra playing yet another unique melody.

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the January-July 2014 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.