SIGMA LUP (Sigma Lupi). Touring the Milky Way, the observer's eye quickly goes to the striking figure that makes up Scorpius, the celestial scorpion, with its bright red supergiant Antares and long curved stellar tail. By comparison, the next-door neighbor, the loop of bright stars to the southeast of Antares that make Lupus, the Wolf, tends then to be a bit ignored. And too bad, as it's filled with hot massive blue stars that give the Milky Way much of its sparkle. Among the fainter of the stars near the outline of this lush constellation is a fourth magnitude (4.42) class B dwarf, which after some consideration is most likely subclassed B1/B2. Sigma Lupi, which lies toward Lupus's southern end some 3.5 degrees south-southwest of the constellation's second magnitude luminary Kakkab (Alpha Lupi). A temperature of 24,500 Kelvin coupled with a distance of 575 light years (give or take 19) yield a luminosity 3700 times that of the Sun. There is no correction for interstellar dust as the line of sight seems quite clear. In fact the star is slightly bluer than one might expect given the spectral class, which is something of an oddity. Luminosity and temperature conspire to give a radius of 3.4 times that of the Sun, which with a projected equatorial rotation velocity of 75 kilometers per second, gives a rotation period of less than 2.3 days. Theory suggests a whopping mass of 9.5 Suns. A more sophisticated analysis gives a radius of 4.8 times solar and a mass of 9.3 Suns. The star seems quite young, as it is still close to its birth luminosity and temperature.

Sigma Lup The visual light curve of Sigma Lupi is very subtle. The scale on the left is of relative magnitudes against a comparison star, that on the bottom the fraction of the variation period. (H.F.Henrichs, Astronomy and Astrophysics, 545, A119 (2012).

Sigma Lupi is slightly variable, with an amplitude of about a hundredth of a magnitude and a period of 3.02 days. The origin of the variation is unknown. One suggestion is that the star is an "ellipsoidal variable" like Spica. Rotation and tides distort the shape of the star into an ellipsoid and, as it orbits an unseen companion, presents a face of varying angular size to the observer. Unfortunately, such an arrangement has never been confirmed and now seems unlikely. Sigma Lupi has a weak magnetic field roughly 100 times the strength of Earth's. Variations in magnetically-sensitive ultraviolet spectrum lines give a period of 3.02 days, the same as determined visually, which must be the true rotation period of the star, Given the uncertainty of the input parameters, the limit given above is not all that bad. From mid-class F and hotter we commonly see chemical abundance variations that are produced by separation of elements. At the cooler end of class B we find the "helium-weak" stars, which are converted to "He-strong" stars toward B2, which includes Sigma Lup. Sigma Lupi is right at the edge of the limit of 8-10 solar masses above which stars develop iron cores and explode as supernovae. Indeed local supernovae have cleared out the volume of space surrounding the Sun, putting the Sun into what is called the "local bubble." (Some data and discussion taken from a paper by H.F.Henrichs et. al, Astronomy and Astrophysics, vol. 545, A119, 2012.)

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Written before 1997 and revised byJim Kaler 7/14/17. Return to STARS.