SIGMA GEM (Sigma Geminorum). Tucked right next to Pollux in Gemini
on the line between it and Castor, fourth
magnitude (4.28) Sigma Geminorum is largely ignored. In its own way,
however, it outshines its brighter neighbors. At first seeming like
just another class K (K1) giant,
Sigma Gem is a fine example of a cool and very well studied "RS Canum
Venaticorum" star rather like Epsilon Ursae
Minoris, Lambda Andromedae, and Rana (Delta Eridani). From its distance of 125
light years (plus or minus just one), the star radiates
39 solar luminosities from its 4600
Kelvin surface, from which we derive a radius 10.2 times that of the
Sun. Direct measure of angular diameter
coupled with distance gives 9.3 solar radii, showing that the
parameters are close to the mark. Sigma Gem is most likely a fairly
low mass (perhaps 1.25 solar, though it is very difficult to derive
an accurate number) giant star that is now fusing its internal helium
into carbon and oxygen. The "RS Canum Venaticorum" tag derives from
a close, fainter companion (possibly a class G or K dwarf) that orbits
the bigger star in 19.605 days at a distance no more than about 0.2
Astronomical Units. An equatorial rotation speed of between 22 and
27 kilometers per second shows that the star rotates at the orbital
period, the pair synchronized quite like the rotation of the Moon
(which keeps one face pointed toward us). This tidal locking has spun
up the giant to a higher than normal speed, and has created considerable
magnetic activity (as a result of a natural dynamo caused in part by
the rotation; the Sun does the same thing). Up to 30 percent of the
star can be covered by cool, magnetic (3500 Kelvin) "starspots" (akin
to sunspots) that lie at mid-latitudes north and south of the stellar
equator and cause it to vary in brightness about a tenth of a magnitude
as it rotates. The magnetic activity produces active outer regions
and a hot corona, that make Sigma Gem one of the brightest stellar X-ray sources in the sky and even
make it shine nicely in the radio spectrum. Sigma is even seen to
launch mighty flares that have brightened it in the high energy
ultraviolet part of the spectrum by as much as a factor of 9. As a result
of the close binary interaction, the star is an "antisolar rotator,"
that is, it rotates faster as we proceed from the equator to the poles
rather than slower, the reverse of what we see in the Sun. About three
minutes of arc away lies an eleventh magnitude "companion." The two have been
drawing apart, however, revealing the apparent duplicity to be just
a line-of-sight coincidence, our Sigma clipping along at a goodly pace
of 62 kilometers per second relative to the Sun, about four times
normal.
Written byJim Kaler 3/19/04; revised 9/09/15.
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