THETA TUC (Theta Tucanae). Tucked (had to be said) into deep southern Tucana only 19 degrees from the South Celestial Pole, Theta Tuc is the most southerly star in the constellation with a Greek letter. It's hard to know what's more remarkable, Theta Tucanae's setting or its character. Dim to the naked eye (if visible at all), only sixth magnitude (6.13), it has a prominent place just to the west of the Small Magellanic Cloud. Easily visible though 200,000 light years away, the SMC is the second brightest of our two prominent satellite galaxies, the other (surprise) the Large Magellanic Cloud, which is somewhat closer (160,000 light years). Equally good, Theta Tuc is but a degree northeast of 47 Tucanae, one of the greatest globular clusters in our Galaxy. With a luminosity close to half a million Suns, containing maybe a million stars, 47 Tuc is exceeded only by Omega Centauri, which seems more to be the remains of a small galaxy that collided with us, rendering 47 Tuc on top. A class A7 subgiant 424 light years away, Theta Tuc has been the focus of intense scrutiny, as it's both a Delta Scuti variable and a close mass-transfer binary. Delta Sct stars are typically class A dwarfs, subgiants, or giants that vary by hundredths to tenths of a magnitude with multiple periods measured in hours. Theta Tuc fits right in, varying by a few hundredths of a magnitude with at least 10 (!) different periods and probably more, the main one at 1.183 hours, the second at 1.193 hours, and so on, none differing by very much. Through straightforward calculation, we find a luminosity for the star of 46 times that of the Sun and a mass of 2.1 to 2.3 Suns depending on its status as a near or true subgiant. But there's more! Theta Tuc has a spectroscopically observed companion that orbits every 7.04 days and seems to have an extraordinarily low mass of about 0.2 times that of the Sun. Not a red dwarf as might first be expected, Theta Tuc B apparently has a class near G0, a temperature of 7000 Kelvin, and a luminosity roughly similar to that of Theta-A, which cuts the luminosity of Theta A down to 23 Suns and the mass to 2.0, in agreement with that derived from oscillations. If not yet a true subgiant, it will be soon. The companion is probably in a "post red giant" state, having already gone through its giant phases. When in its swollen condition, it passed mass onto Theta Tuc A. The binary used to behave like Algol in Perseus and it has become what Algol has to look forward to. With a radius of 2.8 times that of the Sun and an equatorial spin speed of 81 kilometers per second, Theta A rotates every 1.7 days. The two stars are incredibly close together, just 0.09 Astronomical Units apart, 19 solar radii. When "A" begins to expand as a giant, it will pass mass back in the other direction onto "B," allowing weird things to happen. (Thanks to C. Serkin in Astronomy and Astrophysics, 325, 563, 1997, K. De Mey et al., A&A 336, 527, 1998, M. Tempelton et al, Astrophyical Journal, 528, 979, 2000, and M. Paparo and C. Serkin, A&A, 362, 245, 2000. Thanks especially to Cas Liber for suggesting this star.)

Written byJim Kaler 4/04/14. Return to STARS.