BETA TUC (Beta Tucanae). Not one star, but three, no five, maybe six, two of them a fine naked eye pair not unlike Mizar and Alcor, but in reverse, in the far southern hemisphere in modern Tucana (the Toucan) rather than in the far northern and ancient Ursa Major (Tucana the home of the Small Magellanic Cloud, a nearby satellite galaxy). With the naked eye, we see fifth magnitude (5.09) Beta-3 tucked up 10 minutes of arc from fourth magnitude (3.70) combined Beta-1 and Beta-2, which are only 27 seconds of arc apart and separable only by telescope (making Beta the second brightest star in the constellation after Alpha Tuc). Beta-1, the western-most and brighter, comes in at magnitude 4.37, while Beta-2 slips to fifth magnitude (4.54). At a distance of 151 light years, Beta-3 is measured at seven light years farther than Beta-1,2 (the errors allowing a closeness of five light years). Nevertheless, the stars all seem joined by a common motion through space, and almost certainly all belong to one another. Beta-1, a class B (B9) hydrogen-fusing dwarf is the hottest, radiating 40 Suns of energy from a 10,990 Kelvin surface, which gives a radius of 1.74 solar, and from the fast rotation speed of at least 107 kilometers per second, a rotation period of under a day. Stellar structure theory gives a mass of 2.5 times that of the Sun, and reveals extreme youth, the star effectively of "zero age" and just starting its career. Orbiting it some 2.5 seconds of arc away is a 13th (13.5) magnitude red dwarf (class M3?) that, with a minimum separation of at least 110 Astronomical Units, must take at least 700 years to circle its much larger mate. Beta-2 is a different kind of double, consisting of class A (A2 and A7) dwarfs of magnitude 4.8 and 6.0 only half a second of arc apart. With temperatures of 9100 and 8000 Kelvin, Beta-2 A and Beta-2 B respectively shine with the light of 17 and 8 Suns with radii of 1.7 and 1.5 solar, their masses 2 and 1.7 times that of the Sun. Consistently, theory shows them as young as Beta-1, as expected. Orbiting with a period of 43.43 years, the two are measured to average 16.5 AU apart, a high eccentricity taking them from 30 to 3.3 AU from each other. However, the orbit then gives a combined mass of just 2.4 solar, well under that found from luminosity and temperature, suggesting that the orbit, or distance, is in some error (a mean separation of 19.5 AU reconciling the two). In between, at class A0, is Beta-3. With a temperature around 10,000 Kelvin and luminosity of 20 solar, it weighs in with a mass 2.2 solar, and again a very youthful age. It too may be a close double, consisting of rather similar stars. However, then the luminosities and temperatures are inconsistent with theory, suggesting that Beta-3 is really single. With a projected distance of 1160 AU, Beta-1 and Beta-2 would orbit with a period of at least 155,000 years, while Beta-3, with its fragile bond, would take tens of millions of years to orbit the inner quadruple -- if it does at all. Both Beta-1 and Beta-3 are surrounded by what appear to be cool disks, as might be expected from the very young. The complex system seems to belong to the Toucan-Horologium moving group, a loose association of the just-born.
Written by Jim Kaler. Return to STARS.