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.
Written by Jim Kaler. Return to STARS.