Thorne-Zytkow object
Encyclopedia
A Thorne–Żytkow object or TŻO is a hypothetical type of star
wherein a red giant
or supergiant
contains a neutron star
at its core. Such objects were hypothesized by Kip Thorne
and Anna Żytkow in 1977. Though some objects have been suggested as candidates, none has been confirmed.
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 cluster
s. Alternatively, the neutron star could form in a binary system. Because no supernova is perfectly symmetric, the neutron star will be left with a small 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, be engulfed by its companion when it 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 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.
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 isotope
s 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–Żytkow objects.
Observationally, a Thorne–Żytkow object may resemble a red supergiant
or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich Wolf-Rayet star
(type WN8).
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
wherein a red giant
Red giant
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 immense and the surface temperature low, somewhere from 5,000 K and lower...
or supergiant
Red supergiant
Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...
contains a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
at its core. Such objects were hypothesized by Kip Thorne
Kip Thorne
Kip Stephen Thorne is an American theoretical physicist, known for his prolific contributions in gravitation physics and astrophysics and for having trained a generation of scientists...
and Anna Żytkow in 1977. Though some objects have been suggested as candidates, none has been confirmed.
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 cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits 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...
s. Alternatively, the neutron star could form in a binary system. Because no supernova is perfectly symmetric, the neutron star will be left with a small 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, be engulfed by its companion when it evolves into a red giant.
Once the neutron star enters the red giant, drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
between the neutron star and the outer, diffuse layers of the red giant causes the binary star system's orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
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 Oppenheimer-Volkoff limit then the two will collapse into a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
, resulting in a supernova that disperses the outer layers of the star. Otherwise, the two will coalesce into a single neutron star.
The surface of the neutron star is very hot, with temperatures exceeding 109 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
: hotter than the cores of all but the most massive stars. This heat is dominated either by nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
may fuse to produce a different mixture of isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s than it does in ordinary stellar nucleosynthesis
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
, and some astronomers have proposed that the rapid proton nucleosynthesis
Rp-process
The rp-process consists of consecutive proton captures onto seed nuclei to produce heavier elements. It is a nucleosynthesis process and, along with the s process and the r process, may be responsible for the generation of many of the heavy elements present in the universe...
that occurs in supernovae also takes place inside Thorne–Żytkow objects.
Observationally, a Thorne–Żytkow object may resemble a red supergiant
Red supergiant
Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...
or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
(type WN8).
See also
- Red giantRed giantA 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 immense and the surface temperature low, somewhere from 5,000 K and lower...
- Red supergiantRed supergiantRed supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...
- Quasi-star
- Wolf-Rayet starWolf-Rayet starWolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
- Neutron starNeutron starA neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...