ALPHA TUC (Alpha Tucanae). As the Big Dipper and Cassiopeia roll around the north celestial pole opposite each other, circumpolar for much of the northern hemisphere, so in the southern hemisphere do Crux (the Southern Cross) and the modern constellation Tucana, the latter more or less the counterpart of the celestial Queen, but being a toucan, much less regal. Tucana is known for the "Small Magellanic Cloud," a small nearby galaxy that lies only 200,000 light years away, and for the magnificent globular cluster 47 Tuc, both of which far overshadow the constellation's luminary, third magnitude (2.86) Alpha Tucanae. A class K (K3) giant, Alpha Tuc shines from a distance of 199 light years with a luminosity of 424 Suns, its coolish surface of 4300 Kelvin appropriate to the class. These observations combined with theory reveal a 2.5-to-3 solar mass star with a radius 37 times solar whose evolutionary status is somewhat uncertain. A bit too cool for its luminosity to be a common helium-fusing "clump star" (such stars ganging together with similar temperatures and luminosities), it may be in the process of brightening and expanding with a dead helium core in preparation for the onset of helium fusion; it might as well be dimming and shrinking following the onset of helium fusion and starting to stabilize in the "clump;" or it might even be done with that and brightening for the second time with a dead carbon/oxygen core. The last option is the least likely, as the star would be trying to show some instability, which it does not. The hydrogen fusion that takes place around the helium core, combined with stellar circulation (convection), can alter the chemical compositions of giant-star surfaces. The effect is well seen in Alpha Tuc through a depression in carbon (which for stars such as this one serves as a catalyst in making helium from hydrogen) and an elevation in nitrogen (also made by the process). (The Sun's core makes helium through a more direct process of proton fusion). Watching the whole thing is a spectroscopically-detected companion that orbits with a period of 11.5 years and about which nothing else is known. If a low-mass dwarf, it is roughly 7.5 Astronomical Units from Alpha Tuc proper. Only 30 degrees from the South Celestial Pole, Alpha Tucanae is circumpolar from much of the southern hemisphere, for all those to the south of 30 or so degrees south latitude, but never rises for anyone north of 30 degrees north.
Written by Jim Kaler. Return to STARS.