Nearly 4 decades after it was first proposed by physicist Kip Thorne and astronomer Anna Zytkow, a paper by E. M. Levesque et al. (2014) reports on the discovery of a possible Throne-Zytkow object in the Small Magellanic Cloud - a satellite galaxy of the Milky Way. A Throne-Zytkow object is an exotic type of hybrid star consisting of a red supergiant with a neutron star tucked inside its core. Basically, it is two stars in one. This possible Throne-Zytkow object is a star called HV 2112 and it is probably the first of its kind to be found.
There are 2 ways through which a Throne-Zytkow object might form. The first involves 2 massive stars in a tight binary system. The more massive star explodes as a supernova and leaves behind a neutron star. Subsequently, the companion stars evolves and expands to form a red supergiant, engulfing the neutron star in the process. The neutron star then spirals into the core of the red supergiant, producing a Throne-Zytkow object. Alternatively, a Throne-Zytkow object can form when a neutron star produced from an asymmetric supernova receives a ‘natal’ kick in the direction of its companion and becomes embedded in the core of its companion.
A Throne-Zytkow object is nearly indistinguishable from a typical red supergiant. However, the extremely high temperature at the surface of the neutron star deep within the core of the Throne-Zytkow object supports nuclear processes that produce various unique heavy elements. The completely convective envelop surrounding the neutron star brings these elements to the surface of the Throne-Zytkow object where they become detectable.
Indeed, observations of the light emitted by HV 2112 show that it is enriched with the elements rubidium, lithium and molybdenum, consistent with theoretical predictions. Although red supergiants produce these elements as well, they do not produce them in the high abundances observed for HV 2112. Nevertheless, more work is still needed to confirm whether HV 2112 is truly a bona fide Throne-Zytkow object.
E. M. Levesque et al. (2014), “Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud”, arXiv:1406.0001 [astro-ph.SR]