AL ATHFAR (Mu Lyrae). Celestial theft is not all that uncommon,
one star's name going to another. Fifth magnitude (5.12) Mu Lyrae,
in western Lyra (the Harp or Lyre) 2.5
degrees west-northwest of Vega and not far
from the border with Hercules, originally
carried the Arabic name "Al Athfar." The name refers to the
"talons of the falling eagle," in line with the Arabic meaning of
"Vega" (from a longer phrase) as the "swooping eagle." In more
modern times, Mu Lyrae's name was confused with somewhat brighter
fourth magnitude Eta Lyrae, which lies
opposite Vega on the other side of the constellation and became
"Aladfar." The small tale gives us a good reason to stick with the
Greek letters. Not that it matters much,
as the star, with a mere 31 professional citations in the past
century, is pretty well neglected. Al Athfar the First, now our Mu
Lyrae, is actually a rather luminous, white class A (A3) subgiant
(a star that has just given up core hydrogen fusion, or will
shortly do so) that lies 439 light years away (with an uncertainty
of just 10). With a roughly-estimated temperature of 9000 Kelvin,
the star turns out to be 137 times more radiant than our Sun (and intrinsically four times brighter than
Vega), which makes it 4.8 times larger than the solar radius,
similar to earlier calculations, the star important as a calibrator
for interferometric radius measures of other stars. Theory then
yields a mass of 2.9 Suns and indeed shows the star to be a true
subgiant with an age of some 350 million years. Mu Lyr's greatest
physical distinction (other than its neglect) is that it is a
fairly rapid rotator, with a projected equatorial speed of 165
kilometers per second (83 times faster than our Sun), giving it a
rotation period of under 1.5 days, which would probably make it
somewhat oblate. An alternative A0 class would raise the
temperature to 10,000 Kelvin, which requires a larger correction
for ultraviolet radiation. Moreover, the star's color would then
suggest a dimming of a quarter magnitude by interstellar dust, not
too unreasonable as the star is not that far off the Milky Way and
fairly distant. These changes would boost the luminosity to 207
Suns and the mass perhaps as high as 3.1 solar masses. We should
probably stick to the first analysis. There is a note about the
star being a spectroscopic
binary, but no follow-up and the suggestion is likely spurious.
At three solar masses, within a few million years the pure helium
core will shrink and heat, while the outer envelope will expand and
cool, and a new red giant will
have entered the sky, its new form stabilized by the fusion of the
core helium into carbon and oxygen. When the helium runs out, Mu
Lyrae will brighten again to become thousands of times more
luminous than the Sun. Shucking off its outer envelope, the star
will eventually die as a white dwarf of some 0.7
solar masses.
Written by Jim Kaler 10/7/11. Return to STARS.