A
Type II supernovaA supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
(plural:
supernovae) results from the rapid collapse and violent explosion of a massive
starA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
. A star must have at least 9 times, and no more than 40–50 times the
mass of the SunThe solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its
spectrumA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
. Type II supernovae are mainly observed in the spiral arms of galaxies and in
H II regionAn H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
s, but not in elliptical galaxies.
Massive stars generate energy by the
nuclear fusionNuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
of elements. Unlike the Sun, these stars possess the mass needed to fuse elements that have an
atomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
greater than hydrogen and helium, albeit at increasingly high
temperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s and
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, and for increasingly shorter periods of time. The degeneracy pressure of electrons and the energy generated by these
fusion reactionsNuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
is sufficient to counter the force of gravity and prevent the star from collapsing. The star fuses increasingly higher mass elements, starting with
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and then
heliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, until finally a core of iron and nickel is produced. Fusion of iron or nickel produces no net energy, so further fusion is unable to take place, leaving the nickel-iron core inert.
When the mass of the inert core exceeds the
Chandrasekhar limitWhen a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
of about 1.44 solar masses, electron degeneracy alone is no longer sufficient to counter gravity. A cataclysmic implosion takes place within seconds, in which the outer core reaches an inward
velocityIn physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of up to 23% of the
speed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
and the inner core reaches temperatures of up to 100 billion
kelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
.
NeutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s and
neutrinoA neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s are formed via reversed beta-decay, releasing about 10
46 joules (100 foes) in a ten-second burst. The collapse is halted by neutron degeneracy, causing the implosion to bounce outward. The energy of this expanding
shock waveA shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
is sufficient to detach the surrounding stellar material, forming a supernova explosion, while the shock wave and extreme conditions briefly allows the
production of elementsSupernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
heavier than iron. Because of the underlying mechanism, the resulting
variable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
is also described as a
core-collapseCore collapse can refer to:* The collapse of the stellar core of a massive star, such as the core collapse that produces a Type II supernova.* The dynamic process that leads to a concentration of stars at the core of a Globular cluster....
supernova.
There exist several categories of Type II supernova explosions, which are categorized based on the resulting
light curveIn astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
—a graph of luminosity versus time—following the explosion. Type II-L supernovae show a steady (
linearIn mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
) decline of the light curve following the explosion, whereas Type II-P display a period of slower decline (a
plateauIn geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
) in their light curve followed by a normal decay.
Type Ib and Ic supernovaeTypes Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...
are a type of core-collapse supernova for a massive star that has shed its outer envelope of hydrogen and (for Type Ic) helium. As a result they appear to be lacking in these elements.
Formation
Stars far more massive than the sun evolve in more complex ways. In the core of the sun, hydrogen is fused into helium, releasing
thermal energyThermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
which heats the sun's core and provides
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
that supports the sun's layers against collapse in a process known as
hydrostatic equilibriumHydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
. 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 starts to slow down and
gravityGravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
causes 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
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
produced by helium fusion does not fuse, and the star gradually cools to become a
white dwarfA white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
. White dwarf stars, if they have a near companion, may then become
Type Ia supernovaA Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
e.
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 processThe triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...
, 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-burning nuclear fusion stages for a 25-solar massThe solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
star
Process |
Main fuel |
Main products |
25 M☉ star |
Temperature
(KelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
) |
Density
(g/cm3) |
Duration |
| hydrogen burning In the context of stellar nucleosynthesis, a hydrogen-burning process can refer to either the proton-proton chain reactions dominant in main sequence stars lighter than at most 5 solar masses, or to the CNO cycle dominant in heavier stars. Both processes produces stellar energy by burning hydrogen...
|
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
|
heliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
|
7×107 |
10 |
107 years |
| triple-alpha process The triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...
|
heliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
|
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
|
2×108 |
2000 |
106 years |
| carbon burning process The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in massive stars that have used up the lighter elements in their cores...
|
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
|
NeNeon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
, NaSodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, MgMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, AlAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
|
8×108 |
106 |
103 years |
| neon burning process The neon-burning process is a set of nuclear fusion reactions that take place in massive stars . Neon burning requires high temperatures and densities ....
|
neonNeon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
|
OOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, MgMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
|
1.6×109 |
107 |
3 years |
| oxygen burning process The oxygen-burning process is a set of nuclear fusion reactions that take place in massive stars that have used up the lighter elements in their cores. It occurs at temperatures around 1.5×109 K / 130 keV and densities of 1010 kg/m3....
|
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
|
SiSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, SSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, ArArgon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, CaCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
|
1.8×109 |
107 |
0.3 years |
silicon burning processIn astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main...
|
siliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
|
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
(decays into ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
) |
2.5×109 |
108 |
5 days |
Core collapse
The factor limiting this process is the amount of energy that is released through fusion, which is dependent on the
binding energyBinding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
that holds together these atomic nuclei. Each additional step produces progressively heavier nuclei, which release progressively less energy when fusing. In addition from carbon-burning onwards energy loss via
neutrinoA neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
production becomes significant, leading to a higher rate of reaction than would otherwise take place. This continues until
nickel-56In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main...
is produced, which decays radioactively into
cobalt-56Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
and then
iron-56Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
over the course of a few months. As iron and nickel have the highest
binding energyBinding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
per nucleon of all the elements, energy cannot be produced at the core by fusion, and a nickel-iron core grows. This 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. In this state, matter is so dense that further compaction would require electrons to occupy the same
energy statesA quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
. However, this is forbidden for identical
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
particles, such as the electron—a phenomenon called the
Pauli exclusion principleThe Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles...
.
When the core's mass exceeds the
Chandrasekhar limitWhen a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
, degeneracy pressure can no longer support it, and catastrophic collapse ensues. The outer part of the core reaches velocities of up to 70,000 km/s (23% of the
speed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
) as it collapses toward the center of the star. The rapidly shrinking core heats up, producing high-energy gamma rays that decompose iron
nucleiThe nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
into helium nuclei and free
neutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s via
photodisintegrationPhotodisintegration is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the...
. As the core's
densityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
increases, it becomes energetically favorable for
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s to merge via inverse
beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
, producing neutrons and
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s called
neutrinoA neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s. Because neutrinos rarely interact with normal matter they can escape from the core, carrying away energy and further accelerating the collapse, which proceeds over a timescale of 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 waveA shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
that propagates outward. The energy from this shock dissociates heavy elements within the core. This reduces the energy of the shock, which can stall the explosion within the outer core.
The core collapse phase is so dense and energetic that only neutrinos are able to escape. As the protons and electrons combine to form neutrons by means of
electron captureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
, an electron neutrino is produced. In a typical Type II supernova, the newly formed neutron core has an initial temperature of about 100 billion
kelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
; 10
4 times the temperature of the sun's core. Much of this thermal energy must be shed for a stable neutron star to form; otherwise the neutrons would "boil away". This is accomplished by a further release of neutrinos. These 'thermal' neutrinos form as neutrino-antineutrino pairs of all
flavorsNeutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
, and total several times the number of electron-capture neutrinos. The two neutrino production mechanisms convert the gravitational
potential energyIn physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
of the collapse into a ten second neutrino burst, releasing about 10
46 joules (100 foes).
Through a process that is not clearly understood, about 10
44 joules (1 foe) is reabsorbed by the stalled shock, producing an explosion. The neutrinos generated by a supernova were actually observed in the case of Supernova 1987A, leading astronomers to conclude that the core collapse picture is basically correct. The water-based
Kamiokande IIThe is a neutrino physics laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades...
and
IMBIMB, the Irvine-Michigan-Brookhaven detector, was a nucleon decay experiment and neutrino observatory located in a Morton Salt company's Fairport mine on the shore of Lake Erie in the United States. It was a joint venture of the University of California, Irvine, the University of Michigan, and...
instruments detected antineutrinos of thermal origin, while the
galliumGallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
-71-based
BaksanThe Baksan Neutrino Observatory is a scientific laboratory of INR RAS located in the Baksan gorge in the Caucasus mountains in Russia. It started operations in 1977, becoming the first such observatory in the USSR...
instrument detected neutrinos (
lepton numberIn particle physics, the lepton number is the number of leptons minus the number of antileptons.In equation form,so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0...
= 1) of either thermal or electron-capture origin.
When the progenitor star is below about 20
solar massThe solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es—depending on the strength of the explosion and the amount of material that falls back—the degenerate remnant of a core collapse is a
neutron starA neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
. Above this mass the remnant collapses to form a
black holeA black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
. The theoretical limiting mass for this type of core collapse scenario is about 40–50 solar masses. Above that mass, a star is believed to collapse directly into a black hole without forming a supernova explosion, although uncertainties in models of supernova collapse make calculation of these limits uncertain.
Theoretical models
The
Standard ModelThe Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of
particle physicsParticle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
is a theory which describes three of the four known
fundamental interactionIn particle physics, fundamental interactions are the ways that elementary particles interact with one another...
s between the elementary particles that make up all
matterMatter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
. This theory allows predictions to be made about how particles will interact under many conditions. The energy per particle in a supernova is typically one to one hundred and fifty picojoules (tens to hundreds of
MeVMeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
). 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 require corrections to the Standard Model. In particular, Earth-based
particle acceleratorA particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s 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 explaining how that one percent of transfer occurs has proven very difficult, even though the particle interactions involved are believed to be well understood. In the 1990s, one model for doing this involved
convective overturnThe convective overturn model of supernovae was proposed by Bethe and Wilson in 1985, and received a dramatic test with SN 1987A, and the detection of neutrinos from the explosion...
, 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 material from which the star originally formed.
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 curveIn astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
from the supernova.
Light curves
When the
spectrumA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of a Type II supernovae is examined, it normally displays
Balmer absorption linesThe Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....
—the characteristic
frequenciesFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
where hydrogen atoms absorbs energy. The presence of these lines are used to distinguish this category of supernova from a
Type Ia supernovaA Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
.
When the luminosity of a Type II supernova is plotted over a period of time, it shows a characteristic rise to a peak brightness followed by a decline. These light curves have an average decay rate of 0.008
magnitudeAbsolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
s per day; much lower than the decay rate for Type Ia supernovae. Type II are sub-divided into two classes, depending on the shape of the light curve. The light curve for a Type II-L supernova shows a steady (
linearIn mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
) decline following the peak brightness. By contrast, the light curve of a Type II-P supernova has a distinctive flat stretch (called a
plateauIn geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
) during the decline; representing a period where the luminosity decays at a slower rate. The net luminosity decay rate is lower, at 0.0075 magnitudes per day for Type II-P, compared to 0.012 magnitudes per day for Type II-L.
The difference in the shape of the light curves is believed to be caused, in the case of Type II-L supernovae, by the expulsion of most of the hydrogen envelope of the progenitor star. The plateau phase in Type II-P supernovae is due to a change in the
opacityOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
of the exterior layer. The shock wave ionizes the hydrogen in the outer envelope—stripping the electron from the hydrogen atom—resulting in a significant increase in the
opacityOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.
Type IIn supernovae
The "n" denotes narrow, which indicates the presence of intermediate or very narrow width H emission lines in the spectra. In the intermediate width case, the ejecta from the explosion may be interacting strongly with gas around the star – the circumstellar medium.
There are indications that they originate as stars similar to
Luminous blue variableLuminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
s with large mass losses before exploding.
SN 2005glSN 2005gl was a supernova in the barred-spiral galaxy NGC 266. It was discovered using CCD frames taken October 5, 2005, from the 60 cm automated telescope at the Puckett Observatory in Georgia, and reported by Tim Puckett in collaboration with Peter Ceravolo...
is one example of Type IIn;
SN 2006gySN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and...
, an extremely energetic supernova, may be another example.
Type IIb supernovae
A
Type IIb supernova has a weak hydrogen line in its initial spectrum, which is why it is classified as a Type II. After the initial peak in its light curve there is a second peak that has a spectrum which more closely resembles a
Type Ib supernovaTypes Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...
. The progenitor could have been a giant star which lost most of its hydrogen envelope due to interactions with a companion in a binary system, leaving behind the core that consisted almost entirely of helium. As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes more transparent and reveals the deeper layers.
The classic example of a Type IIb supernova is
Supernova 1993JSN 1993J is a supernova observed in the galaxy M81. It was discovered on 28 March 1993 by F. Garcia in Spain. At the time, it was the second brightest supernova observed in the twentieth century behind SN 1987A....
, while another example is
Cassiopeia ACassiopeia A is a supernova remnant in the constellation Cassiopeia and the brightest astronomical radio source in the sky, with a flux density of 2720 Jy at 1 GHz. The supernova occurred approximately away in the Milky Way. The expanding cloud of material left over from the supernova is now...
.
Hypernovae (collapsars)
The core collapse of sufficiently massive stars may not be halted. Degeneracy pressure and repulsive neutron-neutron interactions can only support a neutron star whose mass does not exceed the
Tolman-Oppenheimer-Volkoff limitThe Tolman–Oppenheimer–Volkoff limit is an upper bound to the mass of stars composed of neutron-degenerate matter . The TOV limit is analogous to the Chandrasekhar limit for white dwarf stars.-History:...
of very roughly 4 solar masses. Above this limit, the core collapses to directly form a
black holeA black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
, perhaps producing a (still theoretical)
hypernovaHypernova , also known as a type 1c Supernova, refers to an incredibly large star that collapses at the end of its lifespan...
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 rayGamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s, and are one of many candidate explanations for
gamma ray burstGamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
s.
See also
- History of supernova observation
The known history of supernova observation goes back to 185 CE, when supernova SN 185 appeared, the oldest appearance of a supernova recorded by humankind...
- Supernova nucleosynthesis
Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
- Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...