PROPUS (Eta Geminorum). Gemini, the Twins, represented by Castor (Alpha Geminorum) and Pollux (Beta Gem) at its northern end, is anchored at the bottom by Alhena (Gamma) and at the southwest corner by a pair of similar stars, Tejat (Mu Gem) and our Propus, to which Bayer assigned the Greek letter Eta. The name, taken directly from the Greek, means "forward foot," appropriate to its position as the foot of the western twin, Castor. Its proximity to Tejat also gave it the no-longer-used monkier "Tejat Prior." Propus is a third magnitude (3.23) grand red giant star, one of the rather few bright cool (3600 Kelvin) class M (M3) stars in the naked-eye sky, its orange-red color rather obvious through binoculars. Set rather well away at a distance of 350 light years, the star shines brightly with radiance 2400 times that of the Sun. The combination of temperature and luminosity lead to a radius of 130 times solar, or 0.60 Astronomical Units, just short of the size of the orbit of Venus. Direct measures of angular diameter yield 0.63 AU, nearly the same value, though with a complication. Cool stars like this one contain significant dark bands of titanium oxide in their spectra. When observed in the residual "light" of TiO, the star is a bit larger than it is in parts of the spectrum where there is no absorption, the latter leading to a radius of 0.57 AU. The origin of the difference is the higher opacity of the stellar gas in the part of the spectrum where the star absorbs the radiation, showing that "radii" for stars this large and cool are inherently uncertain. Propus is a "semi-regular" variable star that varies between magnitudes (roughly) 3.3 and 3.9 over a period of 234 days.
Propus -- Eta Geminorum -- varies by typically six or so tenths of a magnitude over a 234-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 R Lyrae 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, with an original mass of about three times solar (and at one time class B), is now brightening with a dead helium core, and is said to be on the "early asymptotic giant branch," as it prepares to become a much more unstable "Mira" variable. The star has two companions, one distant, the other close. The inner one, roughly 7 Astronomical Units away, is measured to be some two magnitudes fainter than Propus-A, and is probably on the cool end of class B again with a mass near triple solar, and orbits with period of 8.2 years. (The red giant must have been just a bit more massive to start with, allowing it to be the first to evolve). Curiously, the major star's variation is somewhat linked to the companion's orbital period. Much farther out, 1.4 seconds of arc away, is an 8th magnitude class F or G dwarf not too much more massive than the Sun. At least 150 Astronomical Units from the inner pair, it orbits with a period greater than 700 years. From the outer companion you would see Propus as a disk about the angular size of our Sun as seen from Earth, and the encircling inner companion up to 2 degrees away. Propus is perhaps most famed as the star nearest Uranus when the planet was discovered in 1781 by William Herschel. See the Moon ready to occult Eta Gem. Thanks to Jeff Bryan, who suggested this star.
Written by Jim Kaler. Return to STARS.