ALPHA ANT (Alpha Antliae). Little attention is paid this star. Even less would be if it were not the remarkably dim luminary of an even more obscure constellation, Antlia, the Air Pump. It's hard to fathom that anyone would invent a figure in which the brightest star is on the faint end of fourth magnitude (4.28), but the nineteenth-century's Abbe Lacaille (Antlia's inventor) did anyway. Physically, Alpha Ant appears as an orange class K (K4) giant placed 365 light years away, which is why it appears so dim. 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" (one of the pack), but is a bit different. There is some indication of the spectroscopic detection of a binary companion, but that is most likely caused 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 science paper a year mentioning it. This isolated star provides a good example astronomers face in dealing with the problem of stellar evolution, the ageing process. Its state of being depends critically on both distance and temperature. The K4 spectral class says 4100 Kelvin, while the single actual measure, which probably no more reliable, gives 3990. The problem is in the allowance for low-energy infrared radiation not seen by the eye, which climbs quickly with dropping temperature. If the higher temperature is valid, then the luminosity is 480 Suns. The formal error in the distance (the degree to which we are uncertain about it) gives a luminosity range of about 15 percent. If the cooler temperature is correct, then the star shines with the brightness of 555 Suns (with the same percentage range). The real problem is that stars with a significant range of mass all rather look alike during the various giant stages, allowing a range of 1.7 to 3 times that of the Sun. One thing is sure, that Alpha Antliae is not at this 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 (supported by its variability). It will turn into a Mira-type variable and then pop its outer envelope to turn into a white dwarf.
Written by Jim Kaler. Return to STARS.