THE HARP
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.