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.