Alpha Antliae

At magnitude 4.28, Alpha Antliae it is the brightest in the obscure constellation of Antlia, the Air Pump, invented by Abbe Lacaille.

Properties

Physically, α Ant is as an orange class K4 giant some 365 light years away. It is a bit on the cool side, however, and has been classed as low as M0, showing that it is not just another helium-burning "clump star". There is some indication of a spectroscopic binary companion. But this is also likely explained by motions in the star's atmosphere, as it is somewhat variable, and is also losing mass. How variable is not known, as the star has never been the subject of a particular study - in fact it is hardly noted at all, less than one scientific paper per year mentioning it.

Evolution

This isolated star provides a good example of the problem astronomers face in dealing with stellar evolution. Knowing its current state depends critically on knowing both its distance and temperature. The formal uncertainty in its distance gives a luminosity uncertainty of about 15%. Its K4 spectral class implies a temperature of 4100 K; while the single actual published measurement - probably no more reliable - gives 3990. If the higher temperature is valid, then its luminosity is 480 Suns. If the cooler temperature is correct, then its luminosity is 555 Suns. The real problem is that stars with a significant range of mass - 1.7 to 3 times the Sun's - all look rather alike during the various giant stages.

We can say with some certainty that α Antliae is not at the moment stable. We cannot quite tell, however, if it is brightening with a dead helium core, dimming with helium core burning to carbon and oxygen, or brightening with a dead carbon core. The most likely scenario is that it is a 2.2-solar-mass star that is on its "way up," brightening for the second time with carbon core and an age of about a billion years. It will become a Mira-type variable and then expel its outer envelope, and turn into a white dwarf.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]