THETA GEM (Theta Geminorum). Not part of the classical outline of Gemini (the Twins), fourth magnitude (but at 3.60, almost third) Theta Geminorum lies in lonely splendor of sorts in a corner of the northern boundary of Gemini with Auriga nine degrees to the west-northwest of Castor. Consistent with its offset location, this white class A (A3) giant (so classed; see below) is pretty well ignored. Its temperature of 8165 Kelvin tells that little correction is needed for light outside the normal visual spectrum. The star's distance of 189 light years (with an uncertainty of but two) and a six percent correction for dimming by interstellar dust lead to a luminosity 101 times that of the Sun and a radius of 5.0 times solar, in reasonable agreement with a variety of values calculated from indirect estimates of angular diameter. Luminosity, temperature, and theory give a mass of 2.5 to 2.6 times that of the Sun and show that if the star is not a giant with a dead helium core, one that has just debarked on its journey to real gianthood, it is a subgiant that will shortly do so. Now around 560 million years old, Theta Gem will, after losing its outer layers as an advanced giant with a dead carbon/oxygen core, turn into a white dwarf of about 0.67 solar masses. As stars expand as giants, their rotation speeds diminish as a result of the conservation of angular momentum (the same reason that a skater or dancer slows as she extends her arms). Having just become a giant (or is in the process of becoming one), Theta is still a fast rotator, spinning at the equator with a speed of at least 128 kilometers per second (the axial tilt not known), which gives it a rotation period of under 2.0 days. The rapid movement keeps the atmosphere stirred up so there are no obvious composition anomalies as a result of gravitational settling of some elements, radiative lofting of others (resulting in a "metallic- line star" like Acubens or some other oddball). Theta Gem is an outstanding example of a "non-multiple star." It has three "companions," thirteenth magnitude Theta B and C 81 and 99 seconds of arc away and fourteenth magnitude Theta D at 21 seconds distance. Their motions relative to Theta Gem proper over the years tags them all as "optical" companions, stars of the neighboring Milky Way that are just in the same line of sight.

Written by Jim Kaler 5/03/13. Return to STARS.