Alpha Tucanae, at magnitude 2.86, is the brightest star in the modern southern constellation Tucana, the Toucan. Only 30° from the South Celestial Pole, α Tuc is circumpolar from much of the southern hemisphere, but never rises for anyone north of 30° north.
Properties and Evolution
Alpha Tucanae is an orange class K3 III giant at a distance of 199 light years. It has a luminosity of 424 suns, a cool surface temperature of 4300 K, a mass of 2.5 to 3 suns, and a radius 37 times solar. The hydrogen fusion that takes place around the helium core of a giant star can alter the chemical compositions of the star's surface. This effect is well seen in α Tuc, through a depression in carbon (which for serves as a catalyst in making helium from hydrogen) and an elevation in nitrogen (also generated by the process).
The star may be brightening and expanding with a dead helium core in preparation for the onset of helium fusion; or it might be starting to stabilize, dimming and shrinking following the onset of helium fusion. It might even be brightening for the second time with a dead carbon/oxygen core. The last option is the least likely, as the star would be show some instability, which it does not.
Companion
Alpha Tucanae has a spectroscopically-detected companion that orbits with a period of 11.5 years; little else is known. If a low-mass dwarf, it is roughly 7.5 AU from α Tuc A.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]