Novae and Supernovae
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Wikipedia-Article "Novae"
- For other uses, see Nova (disambiguation).
A nova (pl. novae) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star.
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If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the star's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so is largely unresponsive to heat, while the accreted hydrogen is not. Eventually, the pressures and temperatures within the hydrogen layer becomes great enough to trigger a nuclear fusion reaction that rapidly converts a large amount of the hydrogen into helium and other heavier elements. The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf's surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid (as in fast novae) or gradual (as in slow novae); after the peak the brightness declines steadily.[1]
In spite of their violence, the amount of material ejected in novae is usually only about 1/10,000th of a solar mass, quite small relative to the mass of the white dwarf. Nonetheless, the accreted matter is blown off the star at velocities as high as several thousand kilometers per second, with a concurrent rise in luminosity from a few times solar to 50,000-100,000 times solar.(1)
A white dwarf can potentially generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. An example is RS Ophiuchi, which is known to have flared five times (in 1898, 1933, 1958, 1967, and 1985). Eventually, however, either the companion star will run out of material, or the white dwarf will undergo a nova so powerful that it is completely destroyed in the process. This is somewhat similar to a type Ia supernova. Supernovae in general, however, involve different processes as well as much higher energies, and should not be confused with ordinary novae.
Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The most recent example was Nova Cygni 1975. This nova appeared on August 29, 1975 in the constellation Cygnus about five degrees north of Deneb and reached magnitude 2.0 (nearly as bright as Deneb).
Historical Significance
The ancients did not believe that the "fixed stars" could show any changes, and considered these occurrences to be objects close to the earth. The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and described it in his book de stella nova (Latin for "concerning the new star"), giving rise to the name nova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away.
Novae as Distance Indicators
Novae have some promise for use as standard candles. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude -7.5, and a lesser one at -8.8. Novae also have roughly the same absolute magnitude 15 days after their peak (-5.5). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those done with Cepheid variable stars have shown them to be of comparable accuracy.(2)
References
(1) Zeilik, Michael. Conceptual Astronomy New York: John Wiley & Sons, Inc., 1993.
(2) Alloin, D., and W. Gieren, eds. Lecture Notes: Stellar Candles for the Extragalactic Distance Scale. Robert Gilmozzi and Massimo Della Valle, "Novae as Distance Indicators", pp. 229-241. Berlin: Springer, 2003.
Bright novae since 1890
| Year | Nova | Maximum brightness |
| 1891 | T Aurigae | 3.8 mag |
| 1898 | V1059 Sagittarii | 4.5 mag |
| 1899 | V606 Aquilae | 5.5 mag |
| 1901 | GK Persei | 0.2 mag |
| 1903 | Nova Geminorum 1903 | 6 mag |
| 1905 | Nova Aquilae 1905 | 7.3 mag |
| 1910 | Nova Lacertae 1910 | 4.6 mag |
| 1912 | Nova Geminorum 1912 | 3.5 mag |
| 1918 | V603 Aquilae | −1.8 mag |
| 1919 | Nova Lyrae 1919 | 7.4 mag |
| 1919 | Nova Ophiuchi 1919 | 7.4 mag |
| 1920 | Nova Cygni 1920 | 2.0 mag |
| 1925 | RR Pictoris | 1.2 mag |
| 1934 | DQ Herculis | 1.4 mag |
| 1936 | CP Lacertae | 2.1 mag |
| 1939 | BT Monoceretis | 4.5 mag |
| 1942 | CP Puppis | 0.3 mag |
| 1943 | Nova Aquilae 1943 | 6.1 mag |
| 1950 | DK Lacertae | 5.0 mag |
| 1960 | V446 Herculis | 2.8 mag |
| 1963 | V533 Herculis | 3 mag |
| 1970 | FH Serpentis | 4 mag |
| 1975 | V1500 Cygni | 2.0 mag |
| 1975 | V373 Scuti | 6 mag |
| 1976 | NQ Vulpeculae | 6 mag |
| 1978 | V1668 Cygni | 6 mag |
| 1984 | QU Vulpeculae | 5.2 mag |
| 1986 | V842 Centauri | 4.6 mag |
| 1991 | V838 Herculis | 5.0 mag |
| 1992 | V1974 Cygni | 4.2 mag |
| 1999 | V1494 Aquilae | 5.03 mag |
| 1999 | V382 Velorum | 2.6 mag |
Note:- Please add all Novae brighter than 6 mag ( http://www.tsm.toyama.toyama.jp/curators/aroom/var/nova/1600.htm )
Recurrent novae
Notes
- Under the Morgan-Keenan spectral classification scheme, novae are classified as Type-Q.
See also
External links
Wikipedia-Article "Supernovae"
- For other uses, see Supernova (disambiguation).
Supernovae refer to several types of stellar explosions that produce extremely bright objects made of plasma that decline to invisibility over weeks or months. There are two possible routes to this end. A massive star may cease to generate fusion energy from fusing the nuclei of atoms in its core and collapses inward under the force of its own gravity, or a white dwarf star may accumulate material from a companion star until it reaches its Chandrasekhar limit and undergoes a thermonuclear explosion. In either case, the resulting supernova explosion expels much or all of the stellar material with great force.
The explosion drives a blast wave into the surrounding space, forming a supernova remnant. One famous example of this process is the remnant of SN 1604, shown at right.
According to the Big Bang theory, supernova explosions are the main source of all the elements heavier than oxygen, and they are the only source of many important elements. For example, it could be said that all the calcium in bones and all the iron in hemoglobin were synthesized in a supernova explosion, billions of years ago. Supernovae inject these heavy elements into the interstellar medium, thus enriching the molecular clouds that are the sites of stellar formation. It is believed that this enrichment process is what determined the composition of the Solar System 4.5 billion years ago, and ultimately made possible the chemistry of life on Earth.
Supernovae generate tremendous temperatures, and under the right conditions, the fusion reactions that take place during the peak moments of a supernova can produce some of the heaviest elements like californium.
"Nova" (pl. novae) is Latin for "new", referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super" distinguishes this from an ordinary nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. However, it is misleading to consider a supernova as a new star, because it really represents the death of a star (or at least its radical transformation into something else).
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Classification
As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra.
The first element for division is the presence or absence of a line from hydrogen. If a supernova's spectrum contains a hydrogen line, it is classified Type II, otherwise it is Type I.
Among those groups, there are subdivisions according to the presence of other lines and the shape of its light curve.
Spectral classification
- Type I
- No hydrogen Balmer lines
- Type II
- Has hydrogen Balmer lines
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- Type II-P
- Plateau
- Type II-L
- Linear
Type Ia
Type Ia supernovae lack helium and present a silicon absorption line in their spectra near peak light. The most commonly accepted theory of these type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant, until it reaches the Chandrasekhar limit. The increase in pressure raises the temperature near the center, and a period of convection lasting approximately 100 years begins. At some point in this simmering phase, a deflagration flame front powered by fusion is born, although the details of the ignition - the location and number of points where the flame begins - is still unknown. This flame accelerates dramatically, through the Rayleigh-Taylor instability and interactions with turbulence. It is still a matter of considerable debate as to whether this flame transitions from a subsonic deflagration into a supersonic detonation.
The energy release from the thermonuclear burning (~1044 joules) causes the star to explode violently and to release a shock wave in which matter is typically ejected at speeds on the order of 10,000 km/s. The energy released in the explosion also causes an extreme increase in luminosity.
The theory of these type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not reach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a fusion reaction of material near its surface but does not cause the star to collapse.
Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.
Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticals. They show no preference for regions of current star formation.
The similarity in the shapes of the luminosity profiles of all known Type Ia supernovae has led to their use as a standard candle in extragalactic astronomy. The cause of this similarity in the luminosity curve is still an open question. In 1998, observations of Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion.
Type Ib and Ic
The early spectra of Types Ib and Ic do not show lines of hydrogen, nor the strong silicon absorption feature near 615 nanometers. These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their envelopes due to strong stellar winds or interaction with a companion. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing. There is some evidence that Type Ic supernovae may be the progenitors of gamma ray bursts, though it is also thought that any supernova may be a GRB dependent upon the geometry of the explosion.
Type II
Stars far more massive than the sun evolve in far more complex fashions. In the core of the sun, 589 million tonnes of hydrogen fuse into 584 million tonnes of helium every second, the extra 4.3 millon tonnes of mass is converted into pure energy which then radiates outwards. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, having been either fused to helium or progressively diluted by the ongoing build-up of helium "ash", fusion begins to slow down and gravity begins to cause the core to contract. This contraction spikes the temperature high enough to initiate a shorter phase of helium fusion, which accounts for less than 10% of the star's total lifetime. In stars with a mass less than ten times than that of the sun, the carbon produced by helium fusion does not fuse, and the star gradually cools to being a white dwarf. White dwarf stars can then become Type Ia supernovae.
A much larger star, however, has the kind of gravity needed to create temperatures and pressures sufficient to cause the carbon in the core to begin to fuse once the star contracts. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, sinking down on a layer of hydrogen fusing into helium, with the helium sinking down into a layer of helium fusing into carbon, with the carbon sinking down to fuse into even heavier elements. These stars go through progressive stages where the core will shrink, built-up atomic nuclei which were previously unfusable begin to fuse, and the core springs back into equilibrium with gravity. This causes them to be irregular variables, as each new burst of fusion pushes elements out of the fusing core into what is called the stellar envelope, and dims the star, causing gravity to pull mass back into the fusing core and begin the cycle over again.
The limiting factor in this process is the amount of energy that is released through fusion, which is dependent on the binding energy of these atomic nuclei. Each additional step produces progressively heavier nuclei, which is also more and more tightly bound by the strong force, this means it releases less energy per fusion reaction than lighter elements fusing.
Among most tightly bound of all nuclei is iron, with a chemical symbol of Fe. It represents the the landmark point where there is no energy relased when heavier elements fuse and light elements undergo nuclear fission. As iron "ash" begins to accumulate in the core of the star, gravity pulls more and more mass into the area of fusion, which, in turn, goes through all of the steps of fusion: Hydrogen to helium by the proton chain, helium to carbon by the triple-alpha process, carbon and helium combine into oxygen, oxygen fuses into neon, neon into magnesium, magnesium into silicon and silicon into iron.
The iron core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by degeneracy pressure of electrons, components of atoms, where electronds are pushing against other electrons. If it builds up to the Chandrasekhar limit at which electron degeneracy pressure cannot sustain it, the iron core begins to collapse. The collapsing core produces high energy gamma rays, which decompose some iron nuclei into thirteen helium atoms plus 4 neutrons, another component of atoms, a process known as photodissociation. However, no nuclear reaction of an iron nucleus can create energy; it can only absorb it. Thus, where reactions in the core have for its entire lifetime have been balancing the star against gravity, they suddenly begin sucking energy inwards, an effect which is combined with gravity to cause the core, a structure with more mass than the Sun and a diameter comparable to Earth, to collapse within a fraction of a second.
As the density in the collapsing core skyrockets, electrons and protons are pushed together until their electrical attraction overcomes their inherent nuclear repulsion from each other. This process of electron capture, creates a neutron and releases a neutrino. The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star and reaches the density of neutronium, where the neutrons press against each other and the entire core is the density of an atomic nucleus. This is the core collapse. At this point neutrino degeneracy pressure is sufficient to balance gravity; however the core has actually overshot the equilibrium point and undergoes a slight bounce, creating a shock wave which slams into the collapsing outer layers of the star. A "proto-neutron star" begins to form at the core, though if it is massive enough, it will continue collapsing to form a black hole.
The core collapse phase is known to be so dense and energetic that only neutrinos are able to escape the collapsing star. Most of gravitational potential energy of the collapse gets converted to a 10 second neutrino burst, releasing about 1046 joules (100 foes). Of this energy, about 1044 J (1 foe) is reabsorbed by the star producing an explosion. The energy per particle in a supernova is typically 1 to 150 picojoules (tens to hundreds of MeV). The neutrinos produced by a supernova have been actually observed in the case of Supernova 1987A leading astronomers to conclude that the core collapse picture is basically correct. Several currently operational neutrino detectors have established a Supernova Early Warning System, which will attempt to notify the astronomical community in the event of a supernova in the Milky Way Galaxy.
This energy is small enough that the predictions gained from the standard model of particle physics is likely to be basically correct, but the high densities may include corrections to the standard model. In particular, earth based particle accelerators can produce particle interactions which are of much higher energy than are found in supernova, but these experiments involve individual particles interacting with individual particles, and it is likely that the high densities within the supernova will produce novel effects. The interactions between neutrinos and the other particles in the supernova take place with the weak nuclear force which is believed to be well understood. However, the interactions between the protons and neutrons involve the strong nuclear force which is much less well understood.
The major unsolved problem with type II supernova is that it is not understood how the burst of neutrinos transfers its energy to the rest of the star producing the shock wave which causes the star to explode. From the above discussion, only one percent of the energy needs to be transferred to produce an explosion, but getting that one percent of transfer has proven very difficult. In the 1990's, one model for doing this involved convective overturn, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the one the star originally formed from.
Neutrino physics, which is modeled by the standard model, is crucial to the understanding of this process. The other crucial area of investigation is the hydrodynamics of the plasma that makes up the dying star, how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is re-energized. Computer models have been very successful at calculating the behavior of type II supernova once the shock has been created. By ignoring the first second of the explosion, and assuming that an explosion is started, astrophysicists have been able to make detailed predictions about the elements produced by the supernova and of the expected light curve from the supernova.
The remaining core of the star may become a neutron star or a black hole, depending on its mass, although because the processes of supernova collapse are poorly understood, it is unknown what the cutoff mass is.
Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L. Type II-P reach a "plateau" in their light curve while II-Ls have a "linear" decrease in their light curve, where it is "linear" in magnitude versus time, or exponential in luminosity versus time. This is believed to result from differences in the envelope of the stars. II-Ps have a large hydrogen envelope that traps energy released in the form of gamma rays and releases it slowly, while II-Ls are believed to have much smaller envelopes converting less of the gamma ray energy into visible light.
One can also sub-divide supernovae of Type II based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of km/s, some have relatively narrow features which may be produced by the interaction of the ejecta with circumstellar material; these are called Type IIn, where the "n" stands for "narrow".
A few supernovae, such as SN 1987K and 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib. These are likely massive stars which have lost most, but not all, of their hydrogen envelopes. As the ejecta expand, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.
There has been some speculation that some exceptionally large stars may instead produce a "hypernova" when they die. In the proposed hypernova mechanism, the core of a very massive star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts.
Naming of supernovae
Supernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, and a one or two-letter designation. The first 26 supernovae of the year get a letter from A to Z. After Z, they start with aa and ab likewise.
Notable supernovae
There have been several supernovae that have been observed throughout history. The dates for these supernovae listed were the time when they were first observed on Earth, rather than their actual occurrence dates. The supernovae themselves are at distances hundreds or thousands of light years from Earth, varying how long it took for the light of each supernova to reach it.
- 1006 – SN 1006 – Extremely bright supernova; accounts found in Egypt, Iraq, Italy, Switzerland, China, Japan, and possibly France and Syria
- 1054 – SN 1054 – the formation of the Crab Nebula, recorded by Chinese astronomers and possibly by Native Americans
- 1181 – SN 1181 – Recorded by Chinese and Japanese astronomers, supernova in Cassiopeia most likely left as its remnant the strange star 3C 58.
- 1572 – SN 1572 – Supernova in Cassiopeia, observed by Tycho Brahe, whose book De Nova Stella on the subject gives us the word "nova"
- 1604 – SN 1604 – Supernova in Ophiuchus, observed by Johannes Kepler; latest supernova to be observed in the Milky Way
- 1885 – S Andromedae in the Andromeda Galaxy, discovered by Ernst Hartwig
- 1987 – Supernova 1987A in the Large Magellanic Cloud, observed within hours of its start, it was the first opportunity for modern theories of supernova formation to be tested against observations.
- – Cassiopeia A – Supernova in Cassiopeia, not observed on Earth, but estimated to be ~300 years old. Is the brightest remnant in the radio band.
The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed.
Supernovae often leave behind supernova remnants; the study of these objects has helped to increase knowledge of supernovae.
Role of supernovae in stellar evolution
Supernovae tend to enrich the surrounding interstellar medium with metals, which for astronomers, include chemical nonmetals after helium. Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different chemical abundances have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.
Possible threats to Earth
Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovas in a relatively short time, perhaps as little as 1000 years into the future. Speculations as to the effects of a nearby supernova on Earth often focus on these large stars, such as Betelgeuse, a red supergiant at a distance of about 400 light years from Earth. Of interest is the conclusion that Type Ia supernovas are the most potentially dangerous, if they occur close enough to the Earth. Since these supernovas are the result of accretion onto relatively dim, common, white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably, and take place in a star system that is not well studied. The predictable supernovas, such as Betelgeuse, while spectacular, will have little effect on Earth. There is an estimation that a Type Ia supernova would have to be closer than 1000 parsecs - roughly 3300 light years - to affect the Earth [1]. There are likely to be many Type Ia candidates within this distance. However the typical rate for Type Ia supernovas in a galaxy is about 1 per 1000 years[2], and therefore the probability of one occurring within 1000 parsecs of Earth, given that the Milky Way is about 30,000 parsecs in diameter and 1000 parsecs thick, is probably less than 1 per 1 million years. The probability of a Type Ia within 100 parsecs is about 1 per billion years or less. Thus it is likely that a nearby Type Ia about 100-1000 parsecs away has occurred several times within the history of life on Earth, about 500 million years ago, but is unlikely to occur anytime within the lifespan of the human species.
Recent estimates predict that a Type II supernova would have to be closer than 8 parsecs, which is about 26 light years, to destroy half of the Earth's protective ozone layer [3]. Such estimates are mostly concerned with atmospheric modelling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud.
See also
- Accelerating universe
- Hypernova
- Near-Earth Supernova
- Supernova nucleosynthesis
- Timeline of white dwarfs, neutron stars, and supernovae
Further reading
Filippenko, (1997). "Optical Spectra of Supernovae". Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355 Annual Review of Astronomy and Astrophysics Volume 35, 1997, pp. 309-355 - an article descriving spectrial classes of supernovae.
External links
- Supernova produces cosmic rays
- List of recent supernovae
- List of bright supernovae, including finding charts
- The SNEWS project (SuperNova Early Warning System) uses neutrino detectors to build a network that will (hopefully) provide advance notice of a supernova explosion
- A review article on SNEWS
- A technical review article on Type Ia supernovae
- A Science article on a mechanism of explosion of Type Ia supernovae
- Another good review of supernova events.
- An article on the connection between Supernovae and neutrinos.
- A mpeg animation of a supernova explosion.
- The Nearby Supernova Factory - a project that attempts to find and catalog Type Ia supernovas in nearby galaxies to better understand the phenomenon