Supernova
A supernova (plural supernovae ) is the rapid onset, bright illumination of a star at the end of its life by an explosion, in which the star itself is destroyed. The luminosity of the star increases thereby millions to billions of times, it is for a short time as bright as a whole galaxy.
The notion of Nova derives from the Latin term " stella nova" ( new star ) and goes back to the coined by Tycho Brahe name of an observation of Tycho's star in the year 1572. It refers to the sudden appearance of previously not visible star-like object in the sky. A nova was until the mid-20th century, any kind of outburst of a star with an increase to the maximum in a period ranging from days to years and a return to calm brightness within weeks to decades. When the astrophysical cause of the eruptions has been detected, the term changed to today's definition, in which a supernova can no longer be the "classical" novae counts.
There are two basic mechanisms by which stars can be a supernova:
Famous Supernovae are the supernova 1987A in the Large Magellanic Cloud and the Kepler Supernova 1604. Especially the latter and the Brahesche Supernova 1572 have inspired astronomy, as this classical view has been definitively refuted by the immutability of the sphere of fixed stars.
Designation
Supernovae are named with the prefix " SN ", their discovery year and an alphabetical supplement. Originally, this addition of a capital letter, which was awarded alphabetically in order of discovery. So SN 1987A was the first supernova discovered in 1987. 1954 more than 26 supernovae have been discovered for the first time in a year. Since then, small double letters (up to "zz " from " aa" ) awarded from the 27th supernova one year. With today's telescopes and search programs each year are discovered hundreds of supernovae: 2005 there were 367 ( to SN 2005nc ), in 2006 there were 551 ( to SN 2006ue ), and in 2007 even 572 (up SN2007uz ).
The supernova rate of a galaxy depends on how many stars there arise new, as stars, which end in supernovae, have one by astronomical time scales short lifetimes of tens of millions of years. For the Milky Way about 20 ± 8 supernovae per millennium are estimated, which were observed in the last millennium six. About two-thirds of the galactic supernovae were hidden by the extinction of the galactic disk; the other observed supernovae were held in other galaxies.
Classification
A distinction is historically roughly two types of supernovae. The classification is according to the criterion of whether the spectral lines of hydrogen are visible in the early stages of the supernova in the light or not. One hand there are the type I, in which no hydrogen lines are visible, with the subgroups Ia, Ib and Ic; and on the other hand, the type II with hydrogen lines ( see table). The coarse type designations were introduced in 1939 by Rudolph Minkowski, since they were refined.
This division into type I and type II, however, does not coincide with the two mentioned in the introductory physical mechanisms that can lead to a supernova. Rather, only the subtype Ia supernovae thermonuclear.
Thermonuclear supernovae of type Ia
A supernova of type Ia arises after the currently preferred model in cataclysmic binary systems consisting of a white dwarf and a companion. The white dwarf accretes over time gas from the extended envelope of its companion, which may be several nova outbursts. In these outbreaks merged the hydrogen of the accreted gas, the fusion products are left behind. This continues until the mass of the white dwarf exceeds the Chandrasekhar limit and he begins to collapse by its own gravity. However, in contrast to the non reactive iron core of a type II progenitor star the white dwarf contains large amounts of carbon fusionsfähigem, so that the collapse begins a sudden carbon fusion and explodes the star. Therefore, this phenomenon is referred to as thermonuclear supernova.
A second route to exceed the Chandrasekhar limit can be the Supersoft X -ray Sources ( super soft X-ray sources ). Here, the rate of mass transfer to the white dwarf is high enough to cause a permanent hydrogen burning.
This standard model but came through observations of the X-ray telescope Chandra in distress. Measurements at six selected galaxies showed that the soft X-rays by a factor of 50 is less than the expected value if novae and supersoft X -ray sources were the dominant sources of supernova Ia explosions. Since then, speculation about other progenitor star:
1 and 2 but are not compatible with the currently accepted theory of stellar evolution.
3 is also referred to as the " double-degenerate " scenario. Here, a close double star system from white dwarfs to exchange matter ( so-called AM Canum - Venaticorum stars) begins. Either one of the star exceeds the Chandrasekhar limit (as in the cataclysmic binary stars ), or the supernova explosion is caused by a merger of two white dwarfs.
Different theoretical models show that nuclear fusion can take place both as a detonation as well as deflagration. According to recent works, which are hotly debated among experts, the most likely scenario is an initial deflagration turns into a detonation. Other theories speak of magnetic fields, from which the explosive energy is removed.
The energy released in a supernova explosion is within defined limits, since the bandwidth of the critical mass and the composition of white dwarfs is known. This property is referred to as a standard candle and it is suitable for distance determination ( see below).
In a supernova explosion of type Ia no compact object remains - all the matter of the white dwarf is thrown as a supernova remnant in the outer space. The companion star is a so-called "Runaway " star (English " fugitive " ), as it with the - fly away orbital speed with which he has orbits its companion star so far - usually high.
Core-collapse supernovae or hydrodynamic
Progenitor star
After the now generally accepted theory of gravitational collapse, which was erected in 1938 by Fritz Zwicky first, a supernova of this type occurs at the end of the " life" of a massive star when he has his nuclear fuel consumed completely. Stars with initial masses between 8 to 10 and about 30 solar masses end their existence as a star in a type II explosion, massive stars explode as Type Ib / c. All these stars go through in their core the various energy -releasing fusion chains to synthesizing iron. Supernovae of type Ib or Ic through before the explosion, a Wolf- Rayet star phase in which they repel their outer nor hydrogen- rich layers in the form of a stellar wind.
After the hydrogen is fused into helium in the core of the star ( hydrogen burning ), decreases the generated by the energy released in the fusion internal pressure of the star, which coincides then under the influence of its gravity. In this case, increase the temperature and density, and there is a further step, a merger, the three- alpha process in which helium via the intermediate beryllium to carbon fused (helium burning). The process ( exhaustion of nuclear fuel, contraction, merger next step ) is repeated, and by carbon burning oxygen is produced. More merger stages ( Neonbrennen and silicon burning) let the shrinking star always merge new elements. However, each merger stage releases less energy is released than its predecessor and is faster. While a massive star of about eight solar masses spends tens of millions of years stage of hydrogen burning, the following helium burning requires "only" a few million years. The final merger stage of the silicon burning can be measured in hours to days.
When iron, the 26th element stops the fusion chain, since iron nuclei have the highest binding energy of all atomic nuclei, and mergers to form heavier elements consume energy instead would release.
The speed at which the reacting a rating fuel in its interior, is dependent on temperature and density, and thus indirectly from the gravitational pressure exerted on its core. An important consequence of this relationship is that a star of layers, in which the conversion rate decreases outwards. Even if the core already using the helium burning occurs in the layers above or hydrogen burning. The absolute rate of fusion in the core increases rapidly with increasing stellar mass. While a star with a solar mass about 10 billion years needed to go through the chain fusion in its core to a halt, the life of extremely massive stars is about 100 solar masses only on the order of a few million years. See late stages of stellar evolution for a detailed look.
Core collapse
The iron, the " ashes" of the nuclear burning, remains at the core of the star. Since no further fusion occurs, the nucleus can not build up outward pressure more that would counteract the gravitation. Two other processes enhance this effect: First, iron nuclei are destroyed by photodisintegration by photons of high energy gamma radiation. Here, α - particles and neutrons arise; the α - particles can in turn be broken down by those photons into its core building blocks of protons and neutrons. Due to the high stability of the iron cores of energy has to be spent for this process. Second, free electrons are captured by protons in the so-called inverse β -decay. This produces more neutrons, and neutrinos are released (J. Cooperstein and EA Baron, 1990). Both of the loss of energy by the photo- disintegration and the loss of free electrons cause a further reduction of the gravity of the opposing core pressure.
Finally, the core exceeds the Chandrasekhar limit and collapses.
The collapse of the central area happens so quickly - within milliseconds - that the incidence rate already at 20 to 50 km Distance to the center exceeds the local sound velocity of the medium. The inner layers can transport the print information quickly enough only because of their high density. The outer layers fall as a shock wave in the center. When the inner part of the core densities reached in the nuclear level, it is almost completely of neutrons. Neutron collections also have an upper limit mass ( Tolman -Oppenheimer - Volkoff limit, depending on the model about 2.7 to 3 solar masses ). In order that a supernova can occur, this mass limit must not be exceeded by the nascent neutron core. The core is due to quantum mechanical rules ( degeneracy pressure) incompressible, and the collapse is stopped almost abruptly. This causes a huge pressure and density increase in the center, so that even the neutrinos can not escape freely. This pressure information is reflected on the neutron core and running now turn to the outside. The pressure wave rapidly reaches areas with too low speed of sound, which are still in the incident. The result is a further shock wave, but now travels outward. The distance traveled by the shock front material is highly compressed, causing the material to very high temperatures attained ( Bethe, 1990). Much of their energy is consumed by more photodisintegration during passage through the outer iron core. Since the nuclear binding energy of the total iron is approximately equal to the energy of the shock wave, this would not break without a renewal of the star and produce no explosion. The neutrinos still be considered as an additional source of energy and momentum as the correction. Normally neutrinos interact with matter as well as not. However, in the shock front there are such high densities that the interaction of neutrinos can not be neglected with the matter. Because of the total energy of the supernova the vast majority goes to the neutrinos enough a relatively low absorption to revive the shock and to have to break out of the collapsing iron core. After leaving the iron core when its temperature has dropped sufficiently, the pressure wave again gains additional energy from onset of fusion reactions.
The extremely highly heated gas layers that snap neutron- rich material from the outer regions of the central area with it, erbrüten here in the so-called r-process (r of Engl. Rapid, "fast" ) heavy elements beyond iron, such as copper, germanium, silver, gold or uranium. About half of the existing on planet elements beyond iron comes from such a supernova explosion, while the other half was in the s - process erbrütet of less massive stars and released into their giant phase into space.
Behind the shock front, the heated gas masses are expanding rapidly. The gas gains outward directed velocity. A few hours after the collapse of the central region, the surface of the star is reached and the gas masses are blasted into the now visible supernova explosion. The shell of the supernova it reaches speeds of millions of kilometers per hour. In addition to the votes as radiant energy, the majority of 99 % of the energy released during the collapse in the form of neutrinos is emitted. This left the star immediately after the density of the initially impenetrable shock front has become sufficiently small. As they move almost with the speed of light, they can be measured by earthly detectors few hours before the optical supernova, such as in the supernova 1987A.
Another " early warning signal " for the illumination of a core-collapse supernova is a so-called X-ray Outburst. This occurs when the waves of the shock front to reach the stellar surface and erupt into the interstellar medium - days before the outburst is observed in visible light. For the first time such an X-ray signal in January 2008 with NASA's Swift satellite at the supernova SN 2008D was observed.
Supernovae of type II, since they are caused by the collapse of the central region, also referred to as hydrodynamic supernovae. The outlined scenario is based on a broad consensus in science that supernova explosions of massive stars run in principle so. However, there is no closed -functioning, physical model of a supernova explosion, which all thus employing scientists would agree.
A supernova near busier planet ( radius of about 50 light years ) would have a devastating impact on life there due to the radiation.
Supernova types II -L and II- P
Supernovae of type II are differentiated according to the criterion of whether the brightness of the supernova decreases with time rather linear ( SN II-L ) or during the decay of a plateau phase passes ( Type SN II -P). The peak values of absolute magnitudes for SN II -P show a wide scatter, while most SN II - L have almost the same maximum brightness. The brightness in the blue spectral range of SN II -P reached on average -17.0 like I do with a standard deviation of 1.1, while SN II-L usually at -17.6 ± 0.4 may be due. The existence of the plateau phase is explained by the fact that the ejected mass and the velocity of the shell of the supernova is very large. The decrease in brightness due to the cooling is compensated for by the rapid expansion of the casing due to the resulting increased surface area and the light curve is described by a plateau. The maximum brightnesses depend on the radius of the progenitor star, causing the large scatter is explained in the maximum brightness of SN II -P. Supernovae of type II-L have lower expansion velocities, so that its brightness is determined at early stages of radioactive processes. This occurs less dispersion of the maximum brightness on (Young, Branch, 1989). The supernova SN 1979C is an example of the type II -L. Here, however, decreased only the brightness in the visible light; in X-rays the supernova radiates still as bright as when they were discovered in 1979. Which mechanism causes these persistent brightness, is to not fully explored now.
Supernova types Ib and Ic
For supernovae of type Ib, the hydrogen envelope has been repelled, so that when the explosion no spectral lines of hydrogen are observed before the explosion. The explosion type Ic occurs when in addition the helium envelope of the star was repelled, so that no spectral lines of helium occur. These explosions are caused by a collapse of the core and it remains compact object.
Supernova remnants
The ejected in the supernova material forms an emission nebula, the so-called " supernova remnant ", as opposed to any resulting remnant of the core collapse, which is referred to in astrophysics as a " compact object ". The most famous supernova remnant is the Crab Nebula, which was ejected during the explosion of the SN1054. This supernova also left behind a compact object ( a pulsar ).
Compact Objects
The shape of the remnant that remains of the star depends on its mass. Not the entire outer layers are thrown in the supernova explosion. The residual gas accreted on the collapsed core in the center, which consists almost entirely of neutrons. Any following The gas is also decomposed by the processes described above in neutrons, so a neutron star is formed. If the star through the Any following material harder (more than about 3 solar masses ), then the force of gravity overcome the conditional by the Pauli principle back pressure against each delimits the neutrons in a neutron star and this stabilized (see Degenerate matter). The stellar remnant finally collapses and forms a black hole, from whose gravity field no more signals can escape. Recent observations suggest that there is another intermediate form, the so-called quark stars whose matter is made of pure quarks.
Neutron stars rotate due to the effect pirouettes often at very high speed of up to 1000 revolutions per second, since the angular momentum is conserved during the collapse.
The high rotational speed produces a magnetic field that interacts with the particles of exfoliated gas nebula and thus generates from Earth recordable signals. In the case of neutron stars, one refers to pulsars.
Pair instability supernova
A variant of the core-collapse scenario is the pair instability supernova ( pair instability supernova, PISN ), in which the star does not collapse into a compact object, but is completely torn. The progenitor stars are particularly poor in elements heavier than helium. The pressure in the core is not high enough to form heavy elements such as iron can, which is the prerequisite for a core - collapse. In this phase, the star arrives after the end of helium burning in temperature and density fields in which the photon energies lead to the creation of electron -positron pairs. This leads to a reduction of the radiation pressure, and thus to a further rapid increase of density - and therefore the temperature - of the core until it comes to an explosive insertion of the oxygen and silicon firing, which builds a new counter-pressure against the gravitational pressure. Depending on the size of the gravitational pressure - and thus the mass of the nucleus - can prevent further collapse or just slow down this nuclear explosion. In a PISN no compact remnants, but the star is being formed is completely torn. The energy released in this process are up to 1053 erg ( 1046 J ) by about a factor of 100 higher than those of " ordinary " core-collapse supernova.
Model calculations for vanishing metallicity and without taking into account a possible rotation or magnetic fields provide a critical mass of the helium core of 64 solar masses for the onset of pair instability. If the mass of the helium core is larger than 133 solar masses, then the nuclear explosion the further collapse did not prevent the thus further collapse to a black hole. Extrapolation of these helium - core masses to the necessary total mass of a main sequence star (neglecting mass loss ) is high, the result for the PISN a mass range of about 140 to 260 solar masses. For this reason, this scenario is considered unrealistic in today's universe and mainly considered in the first generation of stars into consideration - there however, this mechanism may have played a significant role in enriching the intergalactic medium with heavy elements.
A special case is the supernova SN 2006gy in the galaxy NGC 1260 shows that was discovered on 18 September 2006 as part of the Texas Supernova Search: the absolute magnitude of SN 2006gy was more than a magnitude above that of other supernovae. The discoverers interpret these about 240 million light years distant supernova therefore as a first candidate for whom the pair instability mechanism could explain possible - however, neither the existing data material nor the theoretical models are sufficient here to make a clear decision.
The first probably safe representative of a PISN is the supernova SN 2007bi that occurred in a dwarf galaxy in the constellation Virgo on April 6, 2007. A group of astronomers from the Weizmann Institute of Science, among others, used the two Keck telescopes to observe the spectra and the brightness curve over more than a year. The investigations revealed that the progenitor star of 1.7 billion light years distant star remnant was unusually high mass and metal-poor than Hyper giant with probably 200 solar masses. In an unusually slow pace also large amounts of silicon and radioactive nickel were released.
Distance measurements using supernovae
Since the radiation set ( this theory especially in the later stages of a supernova of type Ia largely through the radioactive decay of 56Ni to 56Co and this is fed to 56Fe, the half -lives are approximately 6 and 77 days first Fred Hoyle and William Alfred Fowler in 1960 ), the shape of the light curve is always approximately the same. The amount of energy released should, due to the mechanism always be about the same, resulting in an ever about the same luminosity because of the approximately the same structure. These properties of a standard candle can be the basis of such supernova explosions relatively accurate distance measurements in space to make, with the timescale of the light curve in addition to the spectral lines to determine the redshift can be used, as at a redshift of, for example, 2 including timing procedure for the observer extended by this factor. The idea goes back to Fritz Zwicky. Due to the distance measurements of supernova explosions that occurred before about 7 billion years (eg Hubble constant or Supernova Cosmology Project see ) one can prove the accelerated expansion of the universe. To really use supernovae as standard candles, the explosion mechanisms still need to be better researched and understood.
Computer simulations of supernova explosions
Since the beginning of the 21st century it is possible to simulate supernova explosions in parts with the help of supercomputers. Until then prepared especially the modeling of thermonuclear explosions problems because it required high firing rate of a few thousand kilometers per second was not achieved. A solution of the problem suggests itself, since one similar to the operations works with the calculation of flame turbulence in a spark ignition engine. Furthermore, difficult is the calculation of the running at the same time in a very large and very small scales processes and the fact that the operations are possible in three dimensions. A major problem of all simulations is, however, up to now (April 2010 ) of the unmatched transition from collapse to the actual explosion. According to astrophysicist Fiona Harrison this indicates inadequate knowledge of the basic physical principles and is subject to the most current research.
First hydrodynamic numerical calculations of supernova explosions led Stirling Colgate and Richard White of the Lawrence Livermore National Laboratory in 1966, also recognized the importance of this neutrinos for the explosion mechanism. Other important progress achieved James R. Wilson in the early 1980s. Other well-known scientists who dealt with supernova simulations, by W. David Arnett, Stanford E. Woosley, Wolfgang Hillebrandt.
The far -consuming simulation was performed in 2004 at the Max Planck Institute for Astrophysics in Garching near Munich. This 512 ³ grid points were calculated in each simulation step, which corresponds to a resolution of a few kilometers. A whole simulation took 15,000 CPU hours. The simulations show that the emergence of turbulent bubble- like structures is likely, however, the results agree with the present observations are not yet satisfactory agreement.
Other computer models shall also include shock front formed by the emitted neutrinos, but here the shortcomings are even greater, which is mainly due to the extremely high number of arithmetic operations.
Effects on the soil
The outbreak of a supernova in the vicinity of our solar system is called near-Earth supernova. It is estimated that at distances from the supernova significantly below 100 light years, notable impact on the biosphere of our planet would be observed. Gamma-ray supernova can trigger chemical reactions in the upper atmosphere, in which nitrogen is converted to nitrogen oxides. This can completely destroy the ozone layer, and the earth would be exposed to hazardous radiation.
The mass extinction in the upper Ordovician, became extinct in which about 50 percent of the oceanic species, taken by some authors with such near-Earth supernova in conjunction. Some researchers suggest that a past near-Earth supernova is still detectable by traces of certain metal isotopes in rock layers. Enrichment of the isotope 60FE were noted, for example, in deep-sea rock of the Pacific Ocean.
Potentially the most dangerous are probably supernovae of type Ia. As they emerge from seemingly inconspicuous, dark white dwarfs, it is conceivable that the progenitor of such a supernova remains undetected even in relative perigee or is studied insufficiently. Some predictions suggest that such a supernova could still affect the earth up to 3,000 light years in distance. As erdnächster known candidate for a future supernova of this type applies IK Pegasi in about 150 light years away.
Supernovae of type II shall be regarded as less dangerous. Recent studies predict that such a supernova should light at a distance of less than 26 light-years to double the biologically effective UV radiation on earth.
Others
In October 2011, the Nobel committee said the three American astrophysicists Saul Perlmutter, Brian Schmidt and Adam Riess the Nobel Prize for physics for their observations of supernovae, which they in the 1990s - contrary to the prevailing school of thought - found out that dark energy the universe apart with increasing speed drives.