Thorne–Żytkow object

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A Thorne–Żytkow object (TŻO or TZO) is a conjectured type of star wherein a red giant or 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. [1] In 2014, it was discovered that the star HV 2112 was a strong candidate [2] but this has since been called into question. [3]

Star An astronomical object consisting of a luminous spheroid of plasma held together by its own gravity

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023) stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

Red giant Stars powered by fusion of hydrogen in a shell around an inactive core of helium

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

Neutron star degenerate stellar remnant

A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars. Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass lower than 2.16 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.



A Thorne–Żytkow object is formed when a neutron star collides with a 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. [4]

Globular cluster spherical collection of stars

A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes, and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin, globulus—a small sphere. A globular cluster is sometimes known, more simply, as a globular.

Binary star star system consisting of two stars

A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems. These systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems.

Supernova Star exploding at the end of its stellar lifespan

A supernova is a transient astronomical event that occurs during the last stellar evolutionary stages of massive star's life, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months or years.

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, resulting in a supernova that disperses the outer layers of the star. Otherwise, the two will coalesce into a single neutron star.[ citation needed ]

In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.

Orbit gravitationally curved path of an object around a point in outer space; circular or elliptical path of one object around another object

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the central mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.

Black hole Astrophysical object from which nothing can escape

A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

If a neutron star and a white dwarf merge, this could form a Thorne–Żytkow object with the properties of an R Coronae Borealis variable. [5]

White dwarf Type of stellar remnant composed mostly of electron-degenerate matter

A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922.

R Coronae Borealis variable class of variable stars

An R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation, and one irregular, unpredictably-sudden fading by 1 to 9 magnitudes. The prototype star R Coronae Borealis was discovered by the English amateur astronomer Edward Pigott in 1795, who first observed the enigmatic fadings of the star. Only about 150 RCB stars are currently known in our Galaxy while up to 1000 were expected, making this class a very rare kind of 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. [6] [7] 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. [8]

The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units (SI).

Nuclear fusion process where atomic nuclei combine and release energy

In nuclear chemistry, nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy. This difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is the process that powers active or "main sequence" stars, or other high magnitude stars.

Hydrogen Chemical element with atomic number 1

Hydrogen is the chemical element with the symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton and no neutrons.

Observationally, a Thorne–Żytkow object may resemble a red supergiant, [9] or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich Wolf–Rayet star (type WN8). [10]

Wolf–Rayet star Stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. Their surface temperatures range from 30,000 K to around 200,000 K, hotter than almost all other stars. They were previously called W-type stars referring to their spectral classification.

A TŻO has an estimated lifespan of 105–106 years. Given this lifespan, it is possible that between 20 and 200 Thorne-Żytkow objects currently exist in the Milky Way. [11]


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 disc. [12] These neutron stars may form the population of isolated pulsars with accretion discs. [12] The massive accretion disc 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. [13]

Observation history

As of 2014, the most recent candidate, star HV 2112, has been observed to have some unusual properties that suggest that it may be a Thorne–Żytkow object. The discovering team, with Emily Levesque being the lead author, noted that HV 2112 displays some chemical characteristics that don't quite match theoretical models, but emphasize that the theoretical predictions for a Thorne–Żytkow object are quite old and theoretical improvements have been made since it was originally conceptualized. [9]

A 2018 paper reappraising the properties of HV 2112, however, has shown that star is unlikely to be a Thorne-Żytkow object, and it is more likely an intermediate mass AGB star. [3]

List of candidate TŻOs

CandidateRight AscensionDeclinationLocationDiscoveryNotesRefs
HV 2112  01h 10m 03.87s−72° 36 52.6 Small Magellanic Cloud 2014This star was previously catalogued as an asymptotic-giant-branch star, but observationally is a better fit for red supergiant status. [9]
U Aquarii  22h 03m 19.69s−16° 37 35.2 Aquarius 1999This star was catalogued as a R Coronae Borealis variable. [5]
VZ Sagittarii  18h 15m 08.58s−29° 42 29.6 Sagittarius 1999This star was catalogued as a R Coronae Borealis variable. [5]

List of candidate former TŻOs

Candidate former TŻORight AscensionDeclinationLocationDiscoveryNotesRefs
GRO J1655-40  16h 54m 00.14s−39° 50 44.9 Scorpius 1995The progenitor for both the companion star and the black hole in this system is hypothesized to have been a TŻO. [13]

See also

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Anna N. Żytkow is a Polish astrophysicist working at the Institute of Astronomy of the University of Cambridge. Żytkow and Kip Thorne proposed a model for what is called the Thorne–Żytkow object, which is a star within another star. Żytkow in 2014 participated in the team lead by Emily M. Levesque which discovered the first candidate for such an object.

HV 2112 star

HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star.

V915 Scorpii star

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Emily Levesque is an American astronomer and Assistant Professor in the Department of Astronomy at the University of Washington. She is renowned for her work on massive stars and using these stars to investigate galaxy formation. In 2014, she received the Annie Jump Cannon award for her innovative work on gamma ray bursts. and the Sloan Fellowship in 2017 In 2015, Levesque, Rachel Bezanson, and Grant R. Tremblay published an influential paper, which critiqued the use of the Physics GRE as an admissions cutoff criterion for astronomy postgraduate programs by showing there was no statistical correlation between applicant's score and later success in their academic careers. Subsequently, the American Astronomical Society adopted the stance that the Physics GRE should not be mandatory for graduate school applications, and many graduate astronomy programs have since removed the Physics GRE as a required part of their graduate school applications.

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