Free Astronomy Magazine September-October 2014

STELLAR EVOLUTION giants, with the only difference being the presence of anomalous abundances of heavy chemical elements and lithium in their spec- trum. The problem is therefore that of being able to measure these abundances and find matches in the Thorne- Ż ytkow model. That T Ż Os really exist is not only suggested by mathematical arguments, but also by purely qualitative reasoning. In fact, there are environments and situations in the cos- mos that can surely facilitate the encounter and merging of stars, and consequently the possibility that a degenerate star such as a neutron star could sink into a red super- giant and merge with its core is largely well-founded. This can happen in a partic- ularly crowded star cluster, or even more easily in a close binary system formed by two stars sometimes more massive than the Sun. The bigger one of the two evolves quicker and explodes as supernova, leaving behind as collapsed remnant a neutron star, i.e. an ultra-dense object of only 20-30 km in diameter and heavy as 1.5-3 solar- type stars, compared to the 15-20 solar masses of the ancestor. Since the supernova explosion is usually not symmetrical in re- lation to the star’s geometric centre, the shattered remnants end up on a different orbit from its original one, and this can make it closely interact with the companion star to the point of making the neutron star fall into it. This happens more eas- ily the bigger the com- panion star is, and since red supergiants are in terms of volume the biggest stars of the uni- verse, they are also the most suitable to become T Ż Os. It must be con- sidered that the diame- ters of this kind of stars might exceed one bil- lion kilometres (even though their envelope is in large part extremely rarefied). The size ratio with neutron stars is sim- ilar to that between blue whales and viruses, and thus it may take some A bove, the arrow in- dicates the position of the variable star HV 2112, the most likely Thorne- Ż ytkow object discovered so far. [Digital Sky Survey/Cen- tre de Données astronomiques de Strasbourg] On the left, the fully-resolved disk of one of the largest red super- giants, Betel- geuse. In this case it is excluded that a neutron star may be hiding in its core. [X. Hau- bois et al.]