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Supernova :: Supernovae_and_Remnants
 

A supernova is a stellar explosion that produces an extremely bright object made of plasma that declines to invisibility over weeks or months. There are several different types of supernovae and two possible routes to their formation. 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 (see Nova). 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 to the right.

"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).

Supernovae as a source of heavy elements

Supernovae are the main source of all the elements heavier than oxygen. These elements are produced by fusion (for iron and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. The only competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead. In standard cosmology, the Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. Supernovae inject these heavy elements into the interstellar medium, ultimately enriching the molecular clouds that are the sites of star formation. Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago. Supernova production of heavy elements over cosmic time 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, such as plutonium and californium.

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 a 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 Ia
Si II line at 615.0 nm
Type Ib
He I line at 587.6 nm
Type Ic
Weak or no Helium lines
Type II
Has hydrogen Balmer lines
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 typical absolute magnitude of Type Ia supernovae is -19.5 (~5 billion times brighter than our Sun), with little variation. 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 supernovae, 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 core-collapse supernova (Type Ib, Ic, or II) could be a GRB dependent upon the geometry of the explosion.

Type II

Stars far more massive than the sun evolve in much more complex fashions. In the core of the sun, hydrogen is fused into helium, releasing energy which heats the sun's core, and providing pressure which supports the sun's layers against collapse (see hydrostatic equilibrium). 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 largely 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 raises 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 less than ten solar masses, the carbon produced by helium fusion does not fuse, and the star gradually cools to become a white dwarf. White dwarf stars, if they have a near companion, may then become Type Ia supernovae.

A much larger star, however, is massive enough to create temperatures and pressures needed to cause the carbon in the core to begin to fuse once the star contracts at the end of the helium-burning stage. 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, surrounding a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon (via the triple-alpha process), surrounding layers that fuse to progressively heavier elements. As a star this massive evolves, it undergoes repeated stages where fusion in the core stops, and the core collapses until the pressure and temperature is sufficient to begin the next stage of fusion, re-igniting to halt collapse.

Core collapse
The factor limiting 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 release progressively less energy when fusing, until iron is produced. As iron has the highest binding energy per nucleon of all the stable elements, it cannot produce energy when fused, and an iron core grows. This iron core is under huge gravitational pressure. As there is no fusion to further raise the star's temperature to support it against collapse, it is supported only by degeneracy pressure of electrons. When the core's size exceeds the Chandrasekhar limit, degeneracy pressure can no longer support it, and catastrophic collapse occurs.

As the core collapses, it heats up, producing high energy gamma rays which decompose iron nuclei into helium nuclei and free neutrons (via photodissociation). As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and neutrinos. 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. Some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion. For Type II supernovae, the core collapse is eventually halted by degeneracy pressure of neutrons, at a density comparable to that of an atomic nucleus (forming a neutron star). As neutron degeneracy pressure exerts itself, the infalling matter rebounds, producing a shockwave which blows off the rest of the star's material.

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.

Type II supernovae and theoretical models
The per-particle energy involved in a supernova is small enough that the predictions gained from the Standard Model of particle physics are 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 supernovae, 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 supernovae 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 supernovae 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.

Sub-types of Type II supernovae
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 SN 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.

Hypernovae (Collapsars)
The core collapse of sufficiently massive stars may not be halted by neutron degeneracy pressure. In these cases, the core collapses to directly form a black hole, perhaps producing a (still theoretical) hypernova explosion. In the proposed hypernova mechanism (known as a collapsar) two extremely energetic jets of plasma are emitted from the star's rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts. The cutoff point for neutron star vs. black hole formation is not precisely known, but is expected to be in the range of 25 to 50 times the mass of the Sun.

Supernova hunting

The explosion of a supernovae in another galaxy cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress. Most uses for supernovae — as standard candles, for instance — require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomers, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs. Towards the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and CCDs for hunting supernovae. While such systems are popular with amateurs, there are also larger installations like the Katzman Automatic Imaging Telescope.

Supernova searches fall into two regimes: high redshift and low redshift, with the boundary falling somewhere around a redshift of z = 0.2. High redshift searches for supernovae involve the observation of (usually) Type Ia supernova light curves for use as standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. At low redshift, supernova spectroscopy is more practical than at high redshift, and these data can be used to study the physics and environments of supernovae. Low redshift observations also anchor the low redshift end of the Hubble curve.

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, AB, and likewise. Professional and amateur astronomers currently find between 300 and 400 supernovae a year. For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 341st supernova found in 2005.

If only one supernova is observed in a given year (as with SN 1006), no letter suffix is added to the name.

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.

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 means all of the elements other than hydrogen and helium and is a different definition than that use in chemistry. 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. Supernovae are the dominant mechanism for distributing these heavier elements, which are created in a star during its lifetime of nuclear fusion, throughout space. The different abundances of elements in the material that creates a star have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.

Impact of supernovae on Earth

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars, such as Betelgeuse, a red supergiant 427 light years from Earth which is a type II supernova candidate. Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovae in as little as 1000 years. Though spectacular, these "predictable" supernovae are thought to have little potential to affect Earth. Type Ia supernovae, though, are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from 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. One theory suggests that a Type Ia supernova would have to be closer than 1000 parsecs (3300 light years) to affect the Earth.

Recent estimates predict that a Type II supernova would have to be closer than 8 parsecs (26 light years) to destroy half of the Earth's protective ozone layer. 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. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from once every 100 million years to once every one to ten billion years.

In 1996, astronomers at the University of Illinois theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, iron-60 enrichment has been reported in deep-sea rock of the Pacific Ocean by researchers from Technical University of Munich.

See also

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.

A popular-science account is included in Ken Croswell's The Alchemy of the Heavens.

References

External links

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