A Thorne-Zytkow object may result from a binary star system, one of whose members has ended its life in a supernova explosion and become a neutron star. As the other member reaches the end of its main sequence lifetime and swells into a red giant, it may engulf the neutron star.
Once this happens, drag between the neutron star and the outer, diffuse layers of the red giant causes the binary star system's orbit to decay, and the neutron star and core of the red giant spiral inward toward one another. Depending on their initial separation, this process may take from 100 - 1000 years. When the two finally collide, one of two things may happen.
If the combined mass of the neutron star and the red giant's core exceeds the Oppenheimer-Volkoff limit (the maximum possible mass of a neutron star, somewhere between two and three times the mass of the sun), then the two collapse together into a black hole, resulting in a supernova that disperses the outer layers of the star. Otherwise, the two merely coalesce into a single neutron star.
The surface of the neutron star is very hot, 109 kelvin, hotter than the cores of all but the most massive stars, so unusual nuclear processes may take place as the envelope of the red giant falls onto the neutron star's surface. Hydrogen may fuse to produce a different mixture of isotopes than it does in ordinary stellar nucleosynthesis, and some astronomers have proposed that the rapid proton nucleosynthesis that occurs in supernovae also takes place inside Thorne-Zytkow objects.
Kip Thorne and Anna Zytkow proposed the existence of such stars in 1977. However, astronomers have yet to show that any particular red giant is actually a Thorne-Zytkow object.