In 1977 the physicists Kip Thorne and Anna Żytkow asked a strange question: what would happen if a neutron star ended up inside another star? Their answer was a new kind of object, now named after them. From the outside it would look like a red giant or red supergiant, but its heart would be a neutron star, the collapsed core of a star that had already died.
How one could form
Most massive stars are born in pairs. When the heavier one explodes as a supernova, it leaves behind a neutron star, and the explosion can give that neutron star a kick. If the kick sends it plunging into its companion, the neutron star could sink to the centre of the other star. Collisions in very crowded star clusters, or a neutron star merging with a white dwarf, are other possible routes.
Once inside, the neutron star would take over. Its surface would be hotter than a billion degrees, and gas falling onto it would be processed by unusual nuclear reactions not found in ordinary stars. A Thorne–Żytkow object might last for 100 000 to a million years, and perhaps 20 to 200 exist in the Milky Way at any time.
Looking for the fingerprint
Because they look like normal red supergiants, the only way to spot one is by its chemistry. The unusual reactions at the core should bring up elements such as rubidium, molybdenum and lithium in unusual amounts.
In 2014 Emily Levesque, Philip Massey, Anna Żytkow and Nidia Morrell reported a candidate in the Small Magellanic Cloud, the star HV 2112, whose spectrum showed strong lines of these elements. It was the first serious candidate.
In 2018 Emma Beasor and colleagues re-examined HV 2112 and found no excess of rubidium, only lithium. They concluded it was probably an ordinary, if rather luminous, aging star of about five solar masses. They proposed another star, HV 11417, which does show extra rubidium, as a new candidate. No Thorne–Żytkow object has yet been confirmed.