Free Astronomy Magazine September-October 2014

time before the collapsed star reaches the centre of the red supergiant, also because its trajectory will not be straight but spiral- ling. Once the extremely compact object has finally reached the centre, it merges with the core of the companion star. At that point, if the mass of the latter added to that of the invader does not exceed the critical limit (so-called Tolman-Oppenhei- mer-Volkoff limit) of about 3 solar masses, the core of the star becomes part of the neutron star, which simply increases its weight and in its new location begins to produce energy by triggering thermonu- clear reactions at the base of the gaseous envelope of the red supergiant – i.e., its closest region to the neutron star. A less significant portion of energy is also produced by the staggering gravitational compression exerted on the gas of that same region. It is clear at this point that the Thorne- Ż yt- kow objects represent a completely new class of stars, characterized by a process of energy production unmatched in other star classes. And it is precisely that particular thermonuclear “engine” that today gives researchers the opportunity to verify the existence of T Ż Os. It is in fact believed that the extremely high temperature that builds up in that extraordinary environment could interfere with the rapid acquisition pro- cesses of neutrons by the nuclei of heavy chemical elements being formed, generat- ing isotopes that in normal conditions would not form inside red supergiants, or at least not in the abundances predicted for T Ż Os. In particular, the Thorne- Ż ytkow model predicts that the convective motions present in the envelope of red supergiants in whose centre nestles a neutron star can transport to the surface unusually high quantities of rubidium, strontium, molyb- denum, zirconium, yttrium, and also some of the much lighter lithium. A telltale for identification purposes of a T Ż O is also the I n this sequence of illustrations, we can see the neutron star with its accretion disk get- ting close to the supergiant companion (here shown undersized) and plunging into its rarefied gaseous envelope, before merging with its nucleus. [NASA/GSFC]