A Thorne–Żytkow object (TŻO or TZO), also known as a hybrid star, is a conjectured type of star wherein a red giant or red supergiant contains a neutron star at its core, formed from the collision of the giant with the neutron star. Such objects were hypothesized by Kip Thorne and Anna Żytkow in 1977. In 2014, it was discovered that the star HV 2112 was a strong candidate (SMC). Another possible candidate is the star HV 11417, also located in the SMC. 
A Thorne–Żytkow object is formed when a neutron star collides with another star, typically a red giant or supergiant. The colliding objects can simply be wandering stars. This is only likely to occur in extremely crowded globular clusters. Alternatively, the neutron star could form in a binary system after one of the two stars went supernova. Because no supernova is perfectly symmetric, and because the binding energy of the binary changes with the mass lost in the supernova, the neutron star will be left with some velocity relative to its original orbit. This kick may cause its new orbit to intersect with its companion, or, if its companion is a main-sequence star, it may be engulfed when its companion evolves into a red giant.
Once the neutron star enters the red giant, 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 hundreds of years. When the two finally collide, the neutron star and red giant core will merge. If their combined mass exceeds the Tolman–Oppenheimer–Volkoff limit, then the two will collapse into a black hole. Otherwise, the two will coalesce into a single neutron star.
The surface of the neutron star is very hot, with temperatures exceeding 109 K: hotter than the cores of all but the most massive stars. This heat is dominated either by nuclear fusion in the accreting gas or by compression of the gas by the neutron star's gravity. Because of the high temperature, 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 X-ray bursts also takes place inside Thorne–Żytkow objects.
The only way to unambiguously determine whether or not a star is a TŻO is a multimessenger detection of both the gravitational waves of the inner neutron star and an optical spectrum of the metals atypical of a normal red supergiant. It is possible to detect pre-existing TŻOs with current LIGO detectors; the neutron star core would emit as a continuous wave. 
It has been theorized that mass loss will eventually end the TŻO stage, with the remaining envelope converted to a disk, resulting in the formation of a neutron star with a massive accretion disk. These neutron stars may form the population of isolated pulsars with accretion disks. The massive accretion disk may also result in the collapse of a star, becoming a stellar companion to the neutron star. The neutron star may also accrete sufficient material to collapse into a black hole.
In 2014, a team led by Emily Levesque argued that the star HV 2112 had unusually high abundances of elements such as molybdenum, rubidium, lithium, and calcium, and a high luminosity. Since both are expected characteristics of Thorne–Żytkow objects, this led the team to suggest that HV 2112 might be the first discovery of a TZO. However, this claim was challenged in a 2018 paper by Emma Beasor and collaborators, who argued that there is no evidence for HV 2112 having any unusual abundance patterns beyond a possible enrichment of lithium. They put forth another candidate, HV 11417, based on an apparent over-abundance of rubidium, and a similar luminosity as HV 2112.
List of candidate TŻOs
|HV 2112||01h 10m 03.87s||−72° 36′ 52.6″||Small Magellanic Cloud||2014||Determined to be an Asymptotic giant branch star.|||
|HV 11417||01h 00m 48.2s||−72° 51′ 02.1″||Small Magellanic Cloud||2019|||
|V595 Cassiopeiae||01h 43m 02.72s||+56° 30′ 46.02″||Cassiopeia||2002|||
|IO Persei||03h 06m 47.27s||+55° 43′ 59.35″||Perseus||2002|||
|KN Cassiopeiae||00h 09m 36.37s||+62° 40′ 04.12″||Cassiopeia||2002|||
|U Aquarii||22h 03m 19.69s||−16° 37′ 35.2″||Aquarius||1999||This star was catalogued as a R Coronae Borealis variable.|||
|VZ Sagittarii||18h 15m 08.58s||−29° 42′ 29.6″||Sagittarius||1999||This star was catalogued as a R Coronae Borealis variable.|||
List of candidate former TŻOs
|Candidate former TŻO||Right Ascension||Declination||Location||Discovery||Notes||Refs|
|GRO J1655-40||16h 54m 00.14s||−39° 50′ 44.9″||Scorpius||1995||The progenitor for both the companion star and the black hole in this system is hypothesized to have been a TŻO.|||
- Thorne, Kip S.; Żytkow, Anna N. (15 March 1977). "Stars with degenerate neutron cores. I - Structure of equilibrium models". The Astrophysical Journal. 212 (1): 832–858. Bibcode:1977ApJ...212..832T. doi:10.1086/155109.
- Levesque, Emily M.; Massey, Philip; Zytkow, Anna N.; Morrell, Nidia (2014). "Discovery of a Thorne–Żytkow object candidate in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society: Letters. 443: L94–L98. arXiv:1406.0001. Bibcode:2014MNRAS.443L..94L. doi:10.1093/mnrasl/slu080. S2CID 119192926. Lay summary – PhysOrg (4 June 2014).
- Beasor, Emma; Davies, Ben; Cabrera-Ziri, Ivan; Hurst, Georgia (2 July 2018). "A critical re-evaluation of the Thorne-Żytkow object candidate HV 2112". Monthly Notices of the Royal Astronomical Society. 479 (3): 3101–3105. arXiv:1806.07399. Bibcode:2018MNRAS.479.3101B. doi:10.1093/mnras/sty1744. S2CID 67766043.
- Brandt, W. Niel; Podsiadlowski, Philipp (May 1995). "The effects of high-velocity supernova kicks on the orbital properties and sky distributions of neutron-star binaries". Monthly Notices of the Royal Astronomical Society. 274 (2): 461–484. arXiv:astro-ph/9412023. Bibcode:1995MNRAS.274..461B. doi:10.1093/mnras/274.2.461. S2CID 119408422.
- Vanture, Andrew; Zucker, Daniel; Wallerstein, George (April 1999). "U Aquarii a Thorne–Żytkow Object?". The Astrophysical Journal. 514 (2): 932–938. Bibcode:1999ApJ...514..932V. doi:10.1086/306956.
- Eich, Chris; Zimmerman, Mark; Thorne, Kip; Żytkow, Anna N. (November 1989). "Giant and supergiant stars with degenerate neutron cores". The Astrophysical Journal. 346 (1): 277–283. Bibcode:1989ApJ...346..277E. doi:10.1086/168008.
- Cannon, Robert; Eggleton, Peter; Żytkow, Anna N.; Podsialowsky, Philip (February 1992). "The structure and evolution of Thorne-Zytkow objects". The Astrophysical Journal. 386 (1): 206–214. Bibcode:1992ApJ...386..206C. doi:10.1086/171006.
- Cannon, Robert (August 1993). "Massive Thorne–Żytkow Objects – Structure and Nucleosynthesis". Monthly Notices of the Royal Astronomical Society. 263 (4): 817–838. Bibcode:1993MNRAS.263..817C. doi:10.1093/mnras/263.4.817.
- Foellmi, C.; Moffat, A.F.J. (2002). "Are Peculiar Wolf-Rayet Stars of Type WN8 Thorne-Zytkow Objects?". In Shara, Michael M. (ed.). Stellar Collisions, Mergers and their Consequences. ASP Conference Proceedings. 263. arXiv:astro-ph/0607217. Bibcode:2002ASPC..263..123F. ISBN 1-58381-103-6.
- Podsiadlowski, P.; Cannon, R. C.; Rees, M. J. (May 1995). "The evolution and final fate of massive Thorne-Żytkow objects". Monthly Notices of the Royal Astronomical Society. 274 (2): 485–490. Bibcode:1995MNRAS.274..485P. doi:10.1093/mnras/274.2.485.
- DeMarchi, L.; Sanders, J.; Levesque, E. M. (Apr 2021). "Prospects for Multimessenger Observations of Thorne-Żytkow Objects". The Astrophysical Journal. 911 (2): 101. Bibcode:2021ApJ...911..101D. doi:10.3847/1538-4357/abebe1.
- Mereghetti, Sandro (1995). "A Spin-down Variation in the 6 Second X-Ray Pulsar 1E 1048.1-5937". Astrophysical Journal (published December 1995). 455: 598. Bibcode:1995ApJ...455..598M. doi:10.1086/176607.
- Brandt, W. Niel; Podsiadlowski, Philipp; Sigurðsson, Steinn (1995). "On the high space velocity of X-ray Nova SCO 1994: Implications for the formation of its black hole". Monthly Notices of the Royal Astronomical Society. 277 (2): L35–L40. Bibcode:1995MNRAS.277L..35B. doi:10.1093/mnras/277.1.L35.
- Kuchner, Marc J.; Vakil, David; Smith, Verne V.; Lambert, David L.; Plez, Bertrand; Phinney, E. Sterl (May 30 – June 2, 2000). Shara, Michael M. (ed.). A Spectroscopic Search for Massive Thorne-Żytkow Objects (PDF). Stellar Collisions, Mergers and Their Consequences. ASP Conference Series. 263. American Museum of Natural History, New York. pp. 131–136. Bibcode:2002ASPC..263..131K. ISBN 1-58381-103-6.