Supernova refers to several types of stellar explosions that produce extremely bright objects that decline to invisibility over weeks to months. There are two possible routes to this end: either a massive star may cease to generate fusion energy in its core, and collapse inward under the force of its own gravity, or a white dwarf star may accumulate material from a companion star until it reaches a critical mass 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. Supernova explosions produce all the elements heavier than iron by nuclear fusion in their blast waves, as well as expelling vast quantities of lighter elements produced in the star prior to explosion. It is believed that this process is responsible for the existence of all the elements heavier than iron in our Solar System and the diffusion of heavy elements throughout the universe.
"Nova" 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).
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. See "Optical Spectra of Supernovae" by Filippenko (Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355) for a good description of the classes.
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.
- Type I
No hydrogen Balmer lines
- Si II line at 615.0 nm
- He I line at 587.6 nm
- Weak or no Helium lines
- Has hydrogen Balmer lines
- Type II-P
- Type II-L
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 ~100 years begins. At some point in this simmering phase, a fusion flame 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 into a 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 (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 mark. In the late 1990s, observations of Type Ia supernovae produced the unexpected result that the universe seems to undergo an accelerating expansion.
A Type Ia supernova releases the highest amounts of energy amongst all known classifications of supernovae. The farthest single object ever detected in the universe (galaxies or globular clusters do not count) was a Type Ia supernova located billions of light-years (tens of yottameters) away.
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.
Stars far more massive than our sun evolve in far more complex fashions. In the core of our sun, 589 teragrams of hydrogen fuse into 584 teragrams of helium every second, the extra 4.3 teragrams of mass 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 burning, this accounts for less than 10% of the star's total lifetime. In stars with less than about 10 solar masses, the carbon produced by helium fusion does not ignite, and the star gradually cools to being a "white dwarf". White dwarf stars can become Type I supernovae as described above.
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 ever 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 and 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.
The most tightly bound of all nuclei is iron, chemical symbol Fe. It represents the "bottom of the hill" for lighter elements to fuse, and for heavier elements to fission. Lighter elements release energy when they fuse and heavier elements release energy when they 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 (Fe) core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by electron degeneracy pressure, the electrons 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 13 He plus 4 neutrons, a process known as photodisassociation. However, no nuclear reaction of an iron nucleus can create energy; it can only absorb it. Thus, where reactions in the core have for millions of years been radiating energy outward, balancing the star against gravity, they suddenly begin sucking energy inwards, joining hands with gravity to cause the core, a massive structure the size of our sun, 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 combination, a process called "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 nuclear matter, 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 neutron 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 the case of Supernova 1987A leading astronomers to conclude that the core collapse picture is basically correct.
This energy is small enough that 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 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 ("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, ab, and so on.
The Crab Nebula
is an expanding cloud of gas created by the 1054 supernova.
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; last 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.
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 our knowledge of supernovae.
Role of supernovae in stellar evolution
Supernovae tend to enrich the surrounding interstellar medium with metals (for astronomers, metals are all the elements 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.
Last updated: 10-24-2005 05:59:00