Neutron star
A neutron star is an astronomical object with an extremely high density and a typical diameter of about 20 km at a mass of about 1.44 to 3 solar masses. He stands at the end of his stellar evolution and thus represents the final stage of a star of given mass class dar. It consists of a special form of matter by neutrons cm ³ has a density of about 1011 kg / cm ³ to 2.5 x 1012 kg / in the center. That is, one cubic centimeter of this type of material is about the mass of an iron cube of 700 m side length. Another comparison, the density corresponds to that of a A380 aircraft, which is packed into the size of a small grain of sand. This corresponds to the order of magnitude of density of atomic nuclei. In addition to this neutron matter could be made of a quark -gluon plasma in the center of a core. Such a hypothetical structure is called quark star. Neutron stars are not only because of their density, but also because of other physical quantities such as the strength of their magnetic or its temperature to the most interesting known cosmic objects.
- 4.1 pulsars
- 4.2 magnetars
- 5.1 documents
- 5.2 Literature
- 5.3 See also
- 5.4 External links
Discovery history
In 1932, Sir James Chadwick discovered the neutron as an elementary particle, and received the 1935 Nobel Prize in Physics.
In 1931, a year before Chadwicks discovery, Lev Davidovich Landau suggested theoretically before the existence of neutron stars. 1933 also examined Walter Baade and Fritz Zwicky neutron stars. Described in the theoretical explanation of the operations of a supernova the neutron star as a possible end product of stellar evolution. Robert Oppenheimer and Volkoff George Michael in 1939 calculated a theoretical model of a neutron star and gave the maximum mass of 0.7 M ☉ (see also Tolman -Oppenheimer - Volkoff limit).
Discovered in 1967, astronomers Jocelyn Bell, Antony Hewish and Martin Ryle radio pulses from a pulsar, which was later interpreted as an isolated, rotating neutron star. The energy source of these impulses is the rotational energy of the neutron star. Most neutron stars discovered so far belong to this type.
1971 observed Riccardo Giacconi, Herbert Gursky, Ed Kellogg, R. Levinson, E. Schreier and Harvey Tananbaum pulses with a period of 4.8 seconds in an X-ray source in the constellation Centaurus, referred to as Cen X - third They interpret this observation as a rotating, hot neutron star in orbit around another star. The energy of these pulses is from the released gravitational energy that comes from the incoming to the neutron star, the gaseous matter of the star.
Formation
Neutron stars are formed according to current theories in a core- collapse supernovae ( types II, Ib, Ic), which takes place at the end of the evolution of massive stars. In the context of current models must be to the core mass of the progenitor star between 1.4 solar masses ( the Chandrasekhar limit) and about 3 solar masses ( Tolman -Oppenheimer - Volkoff limit). If the mass about a black hole, it is lower, there is no supernova explosion, but it develops a white dwarf. However, astronomical observations show deviations from the exact limits of this model, because neutron stars were less than 1.44 solar masses found.
The collapse occurs when the fusion processes come to a halt at the end of its development in the interior of the star. In the central region of massive star becomes hydrogen - helium nuclei produces a number of other heavy elements after the merger of. Once iron and nickel have accumulated in the nucleus, no more energy on nuclear fusion is possible. Iron and nickel are the elements with the highest binding energy per nucleon, allowing for a more fusion energy would be required and would not be free. Thus, the radiation pressure decreases, which counteracts the gravitational and stabilizes the star.
The star collapses, the core is highly compressed. This occurs extremely strong forces that cause the electrons are forced into the nuclei and protons and electrons to form neutrons ( and electron neutrinos ) connect. Even after this process, the core shrinks even further, until the neutron build a so-called " degeneracy pressure ", which stops the further contraction abruptly. This is a big part of the liberated during the collapse gravitational energy (ie potential energy) by the emission of neutrinos free. These Neutrinos are produced in large numbers, and the enormous number density in their environment means that they have a significant influence on the further course of the supernova explosion. This is one of the few known situations in which neutrinos interact much with normal matter.
In addition, the nucleus emits an intense neutron showers. This heats the surrounding layers to such an extent that a " cover fire " is created, the more energy supplies and it flings the remaining outer layers of the star in an explosion. The neutron showers also ensures also for the formation of heavy elements beyond the highest binding energy. All elements of our universe that are heavier than iron, either in supernovae (r- process ), or burning in the helium layers of red giants and AGB stars (s- process ) were generated.
It is noteworthy that the formation of the neutron star first place completely in the core of the star, while the star remains outwardly unremarkable. Only after a few days, the supernova is visible to the outside. Thus neutrino detectors can detect a supernova earlier than optical telescopes.
Properties
Gravity
The gravitational field at the surface of a typical neutron star is about 2.1011 times as strong as Earth's. An object which would fall from 1 m height onto the surface of a neutron star would have a case duration of one microsecond and smite with a case rate of 7.2 million km / h.
The escape velocity of an object must be accelerated so that it can leave the neutron star is of the order of 100,000 km / s, which is about one third the speed of light. The strong gravitational field acts as a gravitational lens and directs the light emitted from the neutron star such that parts of the back of the star come into view, and more than half of its surface is visible.
The gravitational binding energy of a neutron star is twice the mass of the sun according to the law of the equivalence of mass and energy, E = mc ², equivalent to one solar mass. This is the energy that is released during the supernova explosion.
Rotation frequency
At the collapse of the core zone of the progenitor star is its diameter is reduced to less than one hundred thousandth of the original value. Due to the associated pirouettes effect a neutron star rotates initially with about a hundred to a thousand revolutions per second. The highest recorded so far on rotation frequency is 716 Hz ( Pulsar PSR J1748 - 2446ad ). It is not too far below the stability limit due to the centrifugal force of a pure neutron star of about 1 kHz.
Various effects can also change over time, the rotational frequency of a neutron star. If there is a binary star system, in which a material flow takes place from a main sequence star to the neutron star, so angular momentum is transferred, which accelerates the rotation of the neutron star. In this case, values in the range of 1 kHz to adjust. Braking effects can increase to several seconds or even minutes, the rotation period. Cause is the magnetic field of the neutron star.
Construction
From the known properties of the particles involved is obtained for a typical neutron star of 20 km in diameter following shell structure:
There is the pressure to zero at the surface. Because free neutrons are unstable in this environment, there is only iron nuclei and electrons. These nuclei form a crystal lattice. Due to the enormous gravity but the highest elevations on the surface of maximum a few millimeters high. A possible atmosphere of hot plasma would have a maximum thickness of a few centimeters.
The zone of crystalline iron nuclei is made to continue to a depth of about 10 meters. The average density of the crystal lattice increases to about one-thousandth the density of nuclei. Furthermore, the neutron fraction of nuclei increases. They form neutron-rich iron isotopes that are stable only under the local, extreme pressure conditions.
From a depth of 10 meters, the pressure is so high that free neutrons endure. A transition layer having a thickness of 1 to 2 km: there, the so-called inner crust begins. In their areas of crystalline iron nuclei exist alongside those from neutron fluid, with increasing depth of the iron content of 100 % decreases to 0%, while the proportion of neutrons increases accordingly. Furthermore, beyond the average density increases to the atomic nuclei and.
After the inner crust, the star consists mainly of neutrons associated with a low content of protons and electrons in thermal equilibrium. If the temperatures are sufficiently low, the neutrons behave there superfluid and the superconducting protons. For a typical neutron star the corresponding critical temperature is about 1011 Kelvin; Neutron stars are so superfluid already very shortly after their formation.
What forms of matter are present from a depth at which the density at three times the increase of atomic nuclei, is unknown because it does not produce such densities even in collisions of atomic nuclei in particle accelerators on earth, and not cause so study.
Maybe there begins a core zone of pions or kaons. Since these particles are bosons and are not subject to the Pauli principle, they could take all the same energetic ground state, thus forming a so-called Bose -Einstein condensate. They could the enormous external pressure provide little, so that a second collapse into a black hole would be possible.
Another possibility would be the existence of free quarks. Because next up-and down - quarks vorkämen also strange quarks, one calls such an object as a strange star or quark star. Such a form of matter would be stabilized by the strong interaction and therefore could exist without the gravitational external pressure. Since quark stars are denser and therefore smaller, they should be able to rotate more rapidly than pure neutron stars. A pulsar with a rotation period under 0.5 ms would already be an indication of the existence of this type of matter.
In four pulsars a sudden tiny increase of the rotational frequency was observed several times, followed by several days of relaxation phase. This could be a kind of tremor, in which an exchange of angular momentum between the crystalline iron crust and the more inwardly frictionless rotating vortices of super liquid neutron liquid takes place.
Stability and Pauli exclusion principle
An existing mainly from neutron star is stabilized by forces that are a consequence of the Pauli principle. Thereafter, a maximum of two neutrons of the star can be in the same energy state, where they differ in the orientation of their spins. As a consequence of quantum mechanics, the possible energy states form an energy ladder, which rung spacing increases with a reduction of the star volume. Since the states from the lower end of the conductors are all busy, must be supplied to the neutrons at the top end of the conductor at a compression energy. This phenomenon leads to a counter-pressure, the so-called Fermi pressure which can withstand the gravitational pressure. In this situation, since the pressure is almost independent of temperature, but almost exclusively of the distribution of the quantum mechanically allowed energy states, referred to as a state of matter that degenerated matter. If the mass of the progenitor star is larger than the Tolman -Oppenheimer - Volkoff limit of about three solar masses, so no balance is possible, and the star collapses according to current knowledge on to the black hole.
It is noteworthy that the typical diameter of a neutron star is related in this model directly with the neutron mass, an astronomical size therefore a direct function of a microcosmic nature is constant, apart from factors that arise from the unknown equation of state. The stability of a white dwarf based, moreover, in a manner identical to the Pauli principle, the place of the neutron is in this case with respect to the electrons to fruition.
More
- The temperature in the interior of a neutron star is initially 100 billion Kelvin. However, the emission of neutrinos removes so much thermal energy that it drops to 1 billion Kelvin within a year.
- The equation of state for a neutron star is still unknown. It is assumed that they differs significantly from that of a white dwarf. The equation of state of a white dwarf is that of a degenerate gas, which can be described to a good approximation with the special theory of relativity. In a neutron star, however, the effects of general relativity are no longer negligible. This also results in particular the observed deviations from the predicted boundaries of the mass for a neutron star.
Magnetic field
Neutron stars have an extremely strong magnetic field, which is both for their further development as well as for the astronomical observation of importance. As a consequence of the laws of electrodynamics, the product of star cross section and magnetic field remains constant during the collapse of the progenitor star. For a typical neutron star, this results in an increase of the magnetic field by a factor of 1010 to values in the range of 108 Tesla (1012 Gauss ). The mass density, the mc ² can be associated with such a magnetic field over its energy density in combination with the equivalence of mass and energy according to E =, is in the range of a few dozen g / cm ³. These magnetic fields are so strong that atoms in their sphere of influence would assume an elongated cigar shape, since the interaction of the electrons with the magnetic field dominates over that with the core. Due to the rotation of the neutron star is located between the center and the equator a Hall voltage of the order of 1018 V a. Which corresponds to an electric field strength of a few 1000 V per atomic diameter.
Pulsars
The axis of the magnetic field to the axis of rotation is inclined so a periodic radio wave having a typical range of the power is radiated in the 100,000 -fold of the total radiant power of the Sun. Such radiation sources are known in astronomy as pulsars or radio pulsars. The energy required for this is taken from the rotational energy, which is thus largely absorbed within a few million years. A similar time course is to be expected with regard to the magnetic field and the temperature.
Are located in the vicinity of the pulsar ionized gases (plasma), the electrons are entrained by the magnetic field at the poles and move simultaneously along the axis of the magnetic field outward. At the latest at the point where the axis rotates with the speed of light, they can no longer follow her and stay behind. They emit a portion of their kinetic energy as X-ray and gamma radiation in the direction of this axis. Such objects are called X-ray pulsars.
Typical systems of this type are X-ray binaries from a star that just expands into a red giant and a neutron star, where material flows to the neutron star, an accretion disk forms around him and eventually falls on its surface. In this case, X-ray emitted services which are in the range of 10,000 times of the solar power.
Magnetars
A particular class form neutron stars that arise with an initial rotation period of less than 10 ms. In this case also provides a special dynamo effect for conversion of energy by convection currents in the stellar interior into magnetic energy. Here, the flux density of the magnetic field within a few seconds after the collapse increase to values above 1,011 Tesla. The corresponding energy density equivalent to a bulk density in the range of many kg / cm ³. Objects like this are called magnetars. Due to the larger magnetic field, they are significantly more decelerated, so that their rotation frequency decreases after about 1000 years below 1 Hz. In this initial phase they experience occasional giant X-ray outbursts. In the Milky Way a dozen candidates for such X-ray active magnetars are approximately known.