NU VIR (Nu Virginis). Nu Vir, of no proper name, is one of Virgo's extrema, as it lies at the extreme
western end of the classical constellation, almost immediately to
the south of Denebola, the "tail of the
Lion" (Leo). Right at mid-fourth
magnitude (4.03), this class M (M1) red giant is testimony to the
fact that we still cannot always nail down a star's characteristics
and character. At a measured distance of 315 light years and with
a cool measured temperature of 3610 Kelvin, the star (accounting
for a lot of invisible infrared radiation) should shine at us with
a luminosity 1075 times that of the Sun.
Temperature (T) and luminosity (L) then combine to yield a radius
of 84 times solar (0.39 Astronomical Units, about the radius of
Mercury's orbit). The star is big enough, however, that it's
angular diameter is easily measured with interferometers (that make
use of the ability of light waves to interfere with themselves).
Direct measures give about 0.0055 seconds of arc and a radius only
57 solar. Like all stars, however, including the Sun, the semi-
transparent nature of the outer stellar gases make the it look
dimmer at the edge than at the center (a phenomenon called "limb
darkening" caused by our looking deeper into the star at the center
than at the edge, where the gases are on the average hotter).
Accounting for limb darkening gives a diameter of 0.0061 seconds of
arc and a radius that at 63 solar still falls short of the L-T
value. Working backwards, the temperature and observed radius give
a much lower luminosity of 600 solar. Something is clearly wrong,
but what it might be is unknown. The star's evolutionary status in
turn depends on L and T. At the lower luminosity, we have a 1.25
solar mass red giant that
could be brightening with a dead helium core, dimming with a core
fusing helium to carbon, or brightening with a dead carbon core.
At the high luminosity end, Nu Vir becomes a 1.7 solar mass "second ascent" giant that is
brightening with a dead carbon core. That the star has a slight,
0.1 magnitude variation (of unknown period, if any) suggests the
latter to be the case. Even naked eye stars still have their
secrets. See the Moon Nu Virginis.