109 VIR (109 Virginis). Bright fourth magnitude (3.73), part of the
outline of the constellation Virgo, we
might still tend to roll over 109 Virginis (best known by its Flamsteed number) as just
another white class A hydrogen-fusing dwarf (A0 at that, same as Vega), its distance 134.5 light years,
accurate to a mere one light year. Just two degrees north of the
celestial equator and the
second-most-easterly numbered star in Virgo, the star is beaten (as
expected) only by 110 Vir, a dimmer (magnitude 4.40) ordinary class
K (K0.5) orange giant 195
light years away. We tend to disparage such stars as overly common,
but without them many of the constellation
figures would dissolve; cut out the class B dwarfs too and there would
not be much left at all. 109 Vir is also notable as an old photometric
standard, one used to calibrate the spectral brightness of other stars. But
there is much more. With a temperature of 9760 Kelvin, which tells
of a bit of ultraviolet
radiation, the star shines with a luminosity of 52 times that of the
Sun, from which we derive a radius of 2.5
times solar and a mass of 2.5 Suns. With an age of 360 million years,
it's just over 60 percent of the way through its 585 million year
hydrogen fusing lifetime. The star stands out for its rapid
equatorial rotation of at least 325 kilometers per second, which
gives it a rotation period of under 9.3 hours. The fast rotation
somewhat compromises the temperature, as it flattens the star, which
makes it hotter at the poles, cooler at the equator. Some have
thought it to be a metallic-line star, in which the metallic spectral
absorption lines are enhanced, as seen in Zeta Lyrae, Subra
(Omicron Leonis), Sirius, and so many
other class A stars. But it's hard to see how that could be true
given the high rotation velocity, which stirs up the atmosphere, so
it's an unlikely possibility. There is also a claim of metal
deficiency, the iron content 40 percent that of the Sun. There
appears to be no significant debris disk left over from 109's birth
like those surround Vega or Fomalhaut
and that would suggest planets. However the star may have a real
companion. Nineteenth and
twentieth magnitude 109 Vir D and C are respectively 9.3 and 5.7
seconds from bright "A." But they have been observed only once, so
there is no way of knowing if they track along with the main star.
They are most likely line of sight coincidences. But then there is
109 Vir B. At magnitude 9.7 it's seen just 0.6 seconds from "A,"
which would give it a minimum separation of 25 Astronomical Units.
From the star's true brightness it would be a K4 dwarf with a mass
of around 0.7 times that of the Sun. From Kepler's laws, the orbital
period would be least 69 years. Confirmation awaits.
Written byJim Kaler 5/02/14. Return to STARS.