TEJAT (Mu Geminorum). The southwestern end of Gemini turns gently in a three-star tail that directs the eye to the Summer Solstice, the position where the Sun will be on the first day of northern summer. Here, at the southwestern corner of the long rectangle that makes the constellation and at the north end of the stellar trio, lies Tejat (soft "j"), an Arabic plural for a term of unknown meaning and one that has been applied collectively to all three of the stars. Though fourth brightest in the constellation (after Pollux, Castor, and Alhena, Bayer unaccountably assigned Tejat the Greek letter Mu. It is seemingly closely paired with the middle star of the trio, Propus (Eta Geminorum), which is also referred to as Tejat Prior, rendering Mu Geminorum Tejat Posterior. The name has now been transferred exclusively to Mu. Bright third magnitude, Tejat is noticeably reddish in color, reflecting its cool 3650 Kelvin degree surface and class M (M3) status. Lying 232 light years away (less than half the distance of Propus), Tejat is a red giant that radiates 1540 times more energy than the Sun (after correction for a large amount of infrared radiation). While not all that much compared with some of the stars of the naked-eye sky, the low temperature leads to a star of great proportions, one large and close enough for an accurate measure of its angular size, 0.0135 seconds of arc. From its distance, Tejat therefore has a radius 104 times that of the Sun, or 0.48 Astronomical Units, about half the size of the Earth's orbit, which agrees nicely with that found from luminosity and temperature. A ninth magnitude "companion" two minutes of arc away (itself a double star) is most likely a line of sight coincidence. Like many large giants and supergiants, Tejat is slightly variable. Classed as a "giant irregular," its apparent brightness erratically wanders around by about 25 percent, between magnitudes roughly 2.8 and 3.0 with a period of about 27 days (with a long-term variation of some 2000 days).
Mu Geminorum varies by about two-tenths of a magnitude over a 27 day period. The scale on the bottom is the "Julian Date" of 2440000 plus the number that appears, where the Julian Date is the number of days since January 1, 4713 BC of the Julian Calendar and is commonly used for variable phenomena in astronomy. JD 2446500 corresponds to March 11, 1986. The left-hand scale expresses the difference between the apparent visual magnitude of Tejat and a nearby comparison star. (From an article in the Publications of the Astronomical Society of the Pacific by J. R. Percy, J. B. Wilson, and G. W. Henry.)
The star's greatest importance to observers seems to be in calibration of the stellar temperature scale. Of greater interest is the star's evolutionary status. Tejat's temperature and luminosity tell of a three solar mass star (one that began life as a hot mid-class B dwarf) that has not only given up hydrogen fusion, but also helium fusion, and now -- with a dead carbon core -- is making a run to a much higher luminosity, at which point it will vary like Mira and will eventually shuck its outer envelope to become a massive white dwarf like Sirius B. Tejat is also distinguished as a high velocity star that is moving at about five times normal speed relative to the Sun.
Written by Jim Kaler. Return to STARS.