Encyclopedia
A
supernova is a stellar
explosion which produces an extremely bright object made of
plasma that declines to invisibility over weeks or months. A supernova releases more than about 10
17 times the Sun's energy output, briefly outshining its entire host
galaxy.
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 collapse under the force of its own
gravity to form a
neutron star or
black hole. Alternatively, a white dwarf star may accumulate material from a companion star until it nears its Chandrasekhar limit and undergoes runaway nuclear fusion in its interior, completely disrupting it. Note that this second type of supernova should not be confused with a surface
thermonuclear explosion on a white dwarf, which is called a
nova. In either type of supernova, the resulting 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 example of this process is the remnant of
SN 1604, shown to the right.
"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, the name is inappropriate as it signals the end of a star .
Observation history
In 185 AD,
Chinese astronomers recorded the appearance of a bright star in the sky, and observed that it took about eight months to fade from the sky. It was observed to sparkle like a star and did not move across the heavens like a
comet. These observations are consistent with the appearance of a supernova, and this is believed to be the oldest recorded by humankind. SN 185 may also have been recorded in
Roman literature. The gaseous shell RCW 86 is suspected as being the remnant of this event, and recent X-ray studies show a good match for the expected age.
In 393 A.D., the Chinese recorded the appearance of another "guest star", SN 393, in the modern constellation
Scorpius. Additional unconfirmed supernovae events may have been observed in 369 A.D., 386 A.D., 437 A.D, 827 A.D. and 902 A.D. However these have not yet been associated with a supernova remnant, and so they remain only candidates. Over a span of about 2,000 years, Chinese astronomers would record a total of twenty such events, including later explosions noted by Islamic, European and possibly
Indian and other observers.
The supernova
SN 1006 appeared in the southern constellation of Lupus during the year 1006 A.D. This was the brightest recorded star ever to appear in the night sky, and its presence was noted in China, Egypt, Iraq, Italy, Japan and Switzerland. It may also have been noted in France, Syria and North America. Egyptian physician and astrologer Ali bin Ridwan gave the brightness of this star as one-quarter the brightess of the Moon. Modern astronomers have discovered the faint remnant of this explosion and determined that it was only 7,100 light years from the Earth.
Supernova SN 1054 was another widely-observed event, with Arab, Chinese and Japanese astronomers recording the star's appearance in 1054 A.D. It may also have been recorded by the
Anasazi indians as a
petroglyph. This explosion appeared in the constellation Taurus, where it produced the
Crab Nebula remnant. At its peak, the luminosity of SN 1054 may have been four times as bright as
Venus, and it remained visible for 23 days.
Curiously there are fewer records of supernova
SN 1181,
which occured in the constellation Cassiopeia just over a century after SN 1054. It was noted by Chinese and Japanese astronomers, however. The
pulsar 3C58 may be the left-over remnant from this event.
The
Danish astronomer
Tycho Brahe was noted for his careful observations of the night sky from his observatory on the island of
Hven. In 1572 he noted the appearance of a new star , also in the constellation Cassiopeia. Later called
SN 1572, this
supernova was associated with a remnant during the 1960s.
A common belief in Europe during this period was the
Aristotelian idea that the world beyond the Moon and planets was immutible. So observers argued that the phenomenon was something in the Earth's atmosphere. However Tycho noted that the object remained stationary from night to night—never changing its
parallax—so it must lay far away. He published his observations in the small book
De Stella Nova in 1573. It is from the title of this book that we derive the modern word
nova for cataclysmic variable stars.
The last supernova to be seen in the
Milky Way galaxy was
SN 1604, which was observed October 9, 1604. Several people noted the sudden appearance of this star, but it was
Johannes Kepler who became noted for his systematic study of the object. He published his observations in the work
De Stella nova in pede Serpentarii.
Galileo, like Tycho before him, tried in vain to measure the parallax of this new star, and then argued against the Aristotellian view of an immutable heavens. The remnant of this supernova was identified in 1941 at the
Mt. Wilson observatory.
| image = | epoch = J2000.0 [i]
...
, 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.
- 2006 – SNLS-03D3bb in a forming galaxy; observed in real time, it poses several major physical questions as it seems more massive than the Chandrasekhar limit would allow.
- – Cassiopeia A – Supernova in Cassiopeia, not observed on Earth, but estimated to be ~300 years old. Is the brightest remnant in the radio band.
Supernovae often leave behind
supernova remnants; the study of these objects has helped to increase knowledge of supernovae.
Supernova hunting
The explosion of supernovae in other galaxies 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 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, immediately followed by a one or two-letter designation. The first 26 supernovae of the year get an upper case letter from
A to
Z. Afterward, pairs of lower-case letters are used, starting with
aa,
ab, and so on. Four historical supernovae are known simply by the year they occurred ; starting with 1885, the letters are used, even if there was only one supernova that year —this last happened with SN 1947A. The standard abbreviation "SN" is an optional prefix. 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 341
st supernova found in 2005 .
Classification
As part of the attempt to understand supernovae, 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 the light curve of the supernova.
Spectral classification
| Type I | No hydrogen Balmer lines |
|---|
| Type Ia | Singly-ionized silicon line at 615.0 nm |
| Type Ib | Non-ionized helium line at 587.6 nm |
| Type Ic | Weak or no helium lines |
| Type II | Has hydrogen Balmer lines |
|---|
| Type II-P | Luminosity plateau |
| Type II-L | Linear decline in luminosity |
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 this 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 nears the Chandrasekhar limit. The current view is that this limit is never actually attained, so that the process of collapse is never initiated. Instead, 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 carbon
fusion is born, although the details of the ignition—the location and number of points where the flame begins—is still unknown. Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon. The 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. Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees.
This energy release from thermonuclear burning is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able to fly apart from each other. The star explodes violently and releases a
shock wave in which matter is typically ejected at speeds on the order of 10,000 km/s, or roughly 3% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical absolute magnitude of Type Ia supernovae is -19.5 , with little variation.
The theory of this type of supernovae is similar to that of
novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star.
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 generally occur in all types of
galaxies, including
ellipticals. They show no preference for regions of current stellar 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 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 . 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, 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 eight 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 , 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, reigniting 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 ensues.
As the core collapses, it heats up, producing high energy
gamma rays which decompose iron nuclei into helium nuclei and free neutrons . 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 collapse is eventually halted by short-range repulsive neutron-neutron interactions mediated by the strong force, as well as by degeneracy pressure of
neutrons, at a density comparable to that of an atomic nucleus . Once collapse stops, the infalling matter rebounds, producing a shock wave 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. Most of gravitational potential energy of the collapse gets converted to a ten second neutrino burst, releasing about 10
46 joules . Of this energy, about 10
44 J is reabsorbed by the star producing an explosion. The energy per particle in a supernova is typically one to one hundred and fifty picojoules . 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 reenergized. Computer models have been very successful at calculating the behavior of Type II supernovae once the shock has been formed. 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 light visible to us.
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
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
hypernova explosion. In the proposed hypernova mechanism 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.
Type I versus Type II supernovae
A fundamental difference between Type I and Type II supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Type II supernovae are stars with extended envelopes that can attain a degree of transparency with a relatively small amount of expansion. Most of the energy powering emission at peak light is derived from the shock wave that heats and ejects the envelope. The progenitors of Type I supernovae, on the other hand, are compact objects much smaller than Sol that must expand enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type I supernovae is thus entirely attributable to the decay of
radionuclides produced in the explosion, principally
Nickel-56 and its daughter
Cobalt-56 . Gamma rays emitted during the decays are absorbed by the ejected material, heating it to incandescence. As the material ejected by a Type II supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission in this case also. A bright Type Ia supernova may expel 0.5-1.0 solar mass of Nickel-56, while a Type Ib, Ic or Type II supernova probably ejects closer to 0.1 solar mass of Nickel-56.
Interstellar impact
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 , 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.
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.
Role of supernovae in stellar evolution
In standard astronomy, the Big Bang produced
hydrogen,
helium, and traces of
lithium, while all heavier elements are synthesized in stars and supernovae.
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 used in
chemistry.
These injected elements ultimately enriching the molecular clouds that are the sites of star formation. 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 formed in a star during its period of nuclear fusion, throughout space. The different abundances of elements in the material that forms a star have important influences on the star's life, and may decisively influence the possibility of having
planets orbiting it.
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 astronomic periods of time ultimately made the
chemistry of life on
Earth possible.
Impact of supernovae on Earth
A
near-Earth supernova is an explosion resulting from the death of a
star that occurs close enough to the
Earth to have noticeable effects on its
biosphere.
Gamma rays are responsible for most of the adverse effects a supernova can have on a living
terrestrial planet. In Earth's case, gamma rays induce a
chemical reaction in the upper
atmosphere, converting
ozone into
nitrous oxide, depleting the
ozone layer enough to expose the surface to harmful
solar and cosmic radiation. The gamma ray burst from a nearby supernova explosion has been proposed as the cause of the Ordovician extinction, which resulted in the death of nearly 60% of the oceanic life on Earth.
Speculation as to the effects of a nearby supernova on Earth often focuses on large stars, such as
Betelgeuse, a red supergiant four hundred and twenty-seven light years from Earth which is a type II supernova candidate. Several prominent stars within a few light centuries from Sol are candidates for becoming supernovae in as little as a millenium. 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 a thousand
parsecs to affect the Earth.
Recent estimates predict that a Type II supernova would have to be closer than eight parsecs to destroy half of the Earth's
ozone layer. Such estimates are mostly concerned with atmospheric modeling 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 at Urbana-Champaign 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 the
Technical University of Munich.
See also
- Accelerating universe
- Dwarf nova
- Nova
- Red giant
- Timeline of white dwarfs, neutron stars, and supernovae
Further reading
Filippenko, . "Optical Spectra of Supernovae".
Annual Review of Astronomy and Astrophysics Volume 35, 1997, pp. 309-355 - an article describing spectral classes of supernovae.
A popular-science account is included in Ken Croswell's The Alchemy of the Heavens.
References
External links
-
- - a searchable catalog at Sternberg Astronomical Institute, Moscow University
-
- , a list of supernovae reported since 1885
-
-
- The SNEWS project uses neutrino detectors to build a network that will provide advance notice of a supernova explosion
- A on SNEWS
- A technical article on Type Ia supernovae
- A article on a mechanism of explosion of Type Ia supernovae
- Another good of supernova events
- An on the connection between Supernovae and neutrinos
- A mpeg of a supernova explosion
- A explaining the supernova process using Lite Brite diagrams
- - a project that attempts to find and catalog Type Ia supernovae in nearby galaxies to better understand the phenomenon
-
-
- Jul 25, 2006
