Free Astronomy Magazine September-October 2014

STELLAR EVOLUTION temperature of 3450 kelvin and an initial mass of about 15 solar masses, some of which lost in its current advanced evolution stage. Not all spectral properties of HV 2112 faith- fully reproduce the original model proposed by Thorne and Ż ytkow, even if its most sig- nificant features suggest that in the core of this supergiant there is actually a neutron star. According to the authors of the discov- ery, the small discrepancies found between theory and observational data will disap- pear as soon as more advanced models of convection in large stellar envelopes will become available. Pending further confir- mation, HV 2112 remains the first known probable T Ż O object. How many others may exist in our galaxy (and in others) is hard to say, but certainly not many, since the red supergiants so far discovered in the Milky Way are just a couple of hundred and only a few dozen in the Magellanic Clouds, making HV 2112 a real rarity. T wo images of the Apache Point Observato- ry in New Mexico, where some of the observations that led to the discovery of the first T Ż O were made. The top picture shows in the foreground the dome of the ARC telescope (3.5 metres diam- eter); on the side is instead more noticeable the one of 2.5 metres of the Sloan Digi- tal Sky Survey. [SDSS] n to be the best candidate for the role of the first T Ż O discovered. HV 2112 is a photome- tric and spectroscopic variable star, initially included in the Harvard Variable Star Cata- logue as a simple red giant belonging to our galaxy, only to find out that it has a ra- dial velocity typical of the members of the Small Magellanic Cloud, in addition to spectral characteristics that have led the discoverers of its hidden nature to reclas- sify it as a M3 supergiant, with a surface