A neutron star is the gravitationally collapsed core of a massive supergiant star. It results 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. Surpassed only by black holes, neutron stars are the second-smallest and second-densest known class of stellar objects. Neutron stars have a radius on the order of and a mass of about 1.4 solar masses (). Stars that collapse into neutron stars typically have an initial total mass between or possibly more for those that are especially rich in elements heavier than hydrogen and helium.
There are thought to be around one billion neutron stars in the Milky Way, and at a minimum several hundred million, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, many of them have existed for a long period of time and have cooled down considerably. Originally it was thought that neutron stars would be difficult to detect due to low thermal emissions. However, it was discovered that spinning stars emit radiation. Most neutron stars that have been detected are pulsars or a part of a binary system.
Neutron stars in a binary system with a main sequence star can pull in large amounts of gas from its companion, a process called accretion. These binary systems continue to evolve, with many companions eventually becoming compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or collision.
The study of neutron star systems is central to gravitational wave astronomy. The merger of binary neutron stars produces gravitational waves and is associated with kilonovae and short gamma-ray bursts. In 2017, the LIGO and Virgo interferometer sites observed GW170817, the first direct detection of gravitational waves from such an event. Prior to this, indirect evidence for gravitational waves was inferred by studying the gravity radiated from the orbital decay of a different type of (unmerged) binary neutron system, the Hulse–Taylor pulsar.
Formation
Any main-sequence star with an initial mass of greater than (eight times the mass of the Sun) has the potential to become a neutron star. As the star evolves away from the main sequence, stellar nucleosynthesis produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of mass from shell burning cause the core to exceed the Chandrasekhar limit. Electron-degeneracy pressure is overcome, and the core collapses further, causing temperatures to rise to over . At these temperatures, photodisintegration (the breakdown of iron nuclei into alpha particles due to high-energy gamma rays) occurs. As the temperature of the core continues to rise, electrons and protons combine to form neutrons via electron capture, releasing a flood of neutrinos. When densities reach a nuclear density of , a combination of strong force repulsion and neutron degeneracy pressure halts the contraction. The contracting outer envelope of the star is halted and rapidly flung outwards by a flux of neutrinos produced in the creation of the neutrons, resulting in a supernova and leaving behind a neutron star. However, if the remnant has a mass greater than about , it instead becomes a black hole. Observations of gravitational waves from neutron star merger GW170817, which is thought to have generated a black hole shortly afterward, have refined the mass limit estimate for a non-rotating neutron star to ~. Primary sources:
As the core of a massive star is compressed during a Type II supernova or a Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum. Because it has only a tiny fraction of its parent's radius (sharply reducing its moment of inertia), a neutron star is formed with very high rotation speed and then, over a very long period, it slows. Neutron stars are known that have rotation periods from about 1.4 ms to 30 s. The neutron star's density also gives it very high surface gravity, with typical values ranging from to (more than times that of Earth). One measure of such immense gravity is the fact that neutron stars have an escape velocity of over half the speed of light. The neutron star's gravity accelerates infalling matter to tremendous speed, and tidal forces near the surface can cause spaghettification.
Properties
Once formed, neutron stars no longer actively generate heat and cool over time, but they may still evolve further through collisions or accretion. Most of the basic models for these objects imply that they are composed almost entirely of neutrons, as the extreme pressure causes the electrons and protons present in normal matter to combine into additional neutrons. These stars are partially supported against further collapse by neutron degeneracy pressure, just as white dwarfs are supported against collapse by electron degeneracy pressure. However, this is not by itself sufficient to hold up an object beyond and repulsive nuclear forces increasingly contribute to supporting more massive neutron stars. If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit, approximately , the combination of degeneracy pressure and nuclear forces is insufficient to support the neutron star, causing it to collapse and form a black hole. The most massive neutron star detected so far, PSR J0952–0607, is estimated to be .
Newly formed neutron stars may have surface temperatures of ten million kelvins or more. However, since neutron stars generate no new heat through fusion, they inexorably cool down after their formation. Still, surface temperatures will be around one million kelvins after one thousand to one million years,"Q&A: Supernova Remnants and Neutron Stars", Chandra.harvard.edu (September 5, 2008) and older, even cooler neutron stars are still easy to discover. For example, the well-studied neutron star, , has an average surface temperature of about .. The authors calculated what they considered to be "a more realistic model, which accounts for magnetic field and temperature variations over the neutron star surface as well as general relativistic effects", which yielded an average surface temperature of at a confidence level of 2𝜎 (95%); see §4, Fig. 6 in their paper for details. By comparison, the effective surface temperature of the Sun is only .
Neutron star material is remarkably dense: a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion tonnes, the same weight as a 0.5-cubic-kilometer chunk of the Earth (a cube with edges of about 800 meters) from Earth's surface.
As a star's core collapses, its rotation rate increases due to conservation of angular momentum, so newly formed neutron stars typically rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars, and the discovery of pulsars by Jocelyn Bell Burnell and Antony Hewish in 1967 was the first observational suggestion that neutron stars exist. The fastest-spinning neutron star known is PSR J1748−2446ad, rotating at a rate of 716.35 times per second or 42,981 revolutions per minute, giving a linear (tangential) speed at the surface on the order of 0.24c (i.e., nearly a quarter the speed of light).
Equation of state Equation of state relate thermodynamic quantities in equilibrium conditions. Models predicting the equation of state are based on experimental data from nuclei on Earth. At the extraordinarily high densities of neutron stars, ordinary matter is squeezed to nuclear densities. Specifically, the matter ranges from nuclei embedded in a sea of electrons at low densities in the outer crust, to increasingly neutron-rich structures in the inner crust, to the extremely neutron-rich uniform matter in the outer core, and possibly exotic states of matter at high densities in the inner core.
Understanding the nature of the matter present in the various layers of neutron stars, and the phase transitions that occur at the boundaries of the layers is a major unsolved problem in fundamental physics. A presumptive neutron star equation of state would encode information about the structure of a neutron star and would explain how matter behaves at the extreme densities found inside neutron stars. Constraints on the neutron star equation of state would then provide constraints on how the strong interaction of the Standard Model works, which would have profound implications for nuclear and atomic physics. This would make neutron stars natural laboratories for probing fundamental physics.
For example, the exotic states that may be found at the cores of neutron stars are types of QCD matter. At the extreme densities at the centers of neutron stars, neutrons become disrupted giving rise to a sea of quarks. This matter's equation of state is governed by the laws of quantum chromodynamics and since QCD matter cannot be produced in any laboratory on Earth, most of the current knowledge about it is only hypothetical.
Different equations of state lead to different values of observable quantities. While the equation of state relates directly only to density and pressure, these in turn lead to calculating observables like the speed of sound, mass, radius, and Love numbers. There are many proposed neutron star equations of state, such as FPS, UU, APR, L, and SLy, and it is an active area of research.
Another aspect of the equation of state is whether it is a soft or stiff equation of state. This relates to how much pressure there is at a certain energy density, and often corresponds to phase transitions. When the material is about to go through a phase transition, the pressure will tend to increase until it shifts into a more comfortable state of matter. A soft equation of state would have a gently rising pressure versus energy density while a stiff one would have a sharper rise in pressure. In neutron stars, nuclear physicists are still testing whether the equation of state should be stiff or soft, and sometimes it changes within individual equations of state depending on the phase transitions within the model. This is referred to as the equation of state stiffening or softening, depending on the previous behavior. Since it is unknown what neutron stars are made of, there is room for different phases of matter to be explored within the equation of state.
Density and pressure
Neutron stars have overall densities of to ( to times the density of the Sun), derives from mass / volume of star of radius 12 km; derives from mass per volume of star radius 11.9 km which is comparable to the approximate density of an atomic nucleus of . NB is The density increases with depth, varying from about at the crust to an estimated or deeper inside. Pressure increases accordingly, from about (32 QPa) at the inner crust to in the center.
A neutron star is so dense that one teaspoon (4.929 milliliters) of its material would have a mass over , about 900 times the mass of the Great Pyramid of Giza.The average density of material in a neutron star of radius 10 km is . Therefore, 5 ml of such material has a mass of , or 5.5 billion metric tons. This is about 15 times the total mass of the human world population. Alternatively, 5 ml from a neutron star of radius 20 km (average density ) has a mass of about 400 million metric tons, or about the mass of all humans. The gravitational field is ca. or ca. . Moon weight is calculated at . The entire mass of the Earth at neutron star density would fit into a sphere 305 m in diameter, about the size of the Arecibo Telescope.