Type II supernova

A Type II supernova (plural: supernovae or supernovas) results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun (M) to undergo this type of explosion.[1] Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.

Stars generate energy by the nuclear fusion of elements. Unlike the Sun, massive stars possess the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium, albeit at increasingly higher temperatures and pressures, causing increasingly shorter stellar life spans. The degeneracy pressure of electrons and the energy generated by these fusion reactions are sufficient to counter the force of gravity and prevent the star from collapsing, maintaining stellar equilibrium. The star fuses increasingly higher mass elements, starting with hydrogen and then helium, progressing up through the periodic table until a core of iron and nickel is produced. Fusion of iron or nickel produces no net energy output, so no further fusion can take place, leaving the nickel–iron core inert. Due to the lack of energy output creating outward thermal pressure, the core contracts due to gravity until the overlying weight of the star can be supported largely by electron degeneracy pressure.

When the compacted mass of the inert core exceeds the Chandrasekhar limit of about 1.4 M, electron degeneracy is no longer sufficient to counter the gravitational compression. A cataclysmic implosion of the core takes place within seconds. Without the support of the now-imploded inner core, the outer core collapses inwards under gravity and reaches a velocity of up to 23% of the speed of light and the sudden compression increases the temperature of the inner core to up to 100 billion kelvins. Neutrons and neutrinos are formed via reversed beta-decay, releasing about 1046 joules (100 foe) in a ten-second burst. Also, the collapse of the inner core is halted by neutron degeneracy, causing the implosion to rebound and bounce outward. The energy of this expanding shock wave is sufficient to disrupt the overlying stellar material and accelerate it to escape velocity, forming a supernova explosion. The shock wave and extremely high temperature and pressure rapidly dissipate but are present for long enough to allow for a brief period during which the production of elements heavier than iron occurs.[2] Depending on initial size of the star, the remnants of the core form a neutron star or a black hole. Because of the underlying mechanism, the resulting supernova is also described as a core-collapse supernova.

There exist several categories of Type II supernova explosions, which are categorized based on the resulting light curve—a graph of luminosity versus time—following the explosion. Type II-L supernovae show a steady (linear) decline of the light curve following the explosion, whereas Type II-P display a period of slower decline (a plateau) in their light curve followed by a normal decay. Type Ib and Ic supernovae 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.

HST SN 1987A 20th anniversary
The expanding remnant of SN 1987A, a Type II-P supernova in the Large Magellanic Cloud. NASA image.


Evolved star fusion shells
The onion-like layers of a massive, evolved star just before core collapse. (Not to scale.)

Stars far more massive than the sun evolve in more complex ways. In the core of the star, hydrogen is fused into helium, releasing thermal energy that heats the sun's core and provides outward pressure that supports the sun's layers against collapse in a process known as stellar or hydrostatic equilibrium. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, fusion starts to slow down, and gravity 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 fewer than eight solar masses, the carbon produced by helium fusion does not fuse, and the star gradually cools to become a white dwarf.[3][4] 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 when the star contracts at the end of the helium-burning stage. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, surrounding a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon via the triple-alpha process, surrounding layers that fuse to progressively heavier elements. As a star this massive evolves, it undergoes repeated stages where fusion in the core stops, and the core collapses until the pressure and temperature are sufficient to begin the next stage of fusion, reigniting to halt collapse.[3][4]

Core-burning nuclear fusion stages for a 25-solar mass star
Process Main fuel Main products 25 M star[5]
hydrogen burning hydrogen helium 7×107 10 107 years
triple-alpha process helium carbon, oxygen 2×108 2000 106 years
carbon burning process carbon Ne, Na, Mg, Al 8×108 106 1000 years
neon burning process neon O, Mg 1.6×109 107 3 years
oxygen burning process oxygen Si, S, Ar, Ca 1.8×109 107 0.3 years
silicon burning process silicon nickel (decays into iron) 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 energy 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 neutrino production becomes significant, leading to a higher rate of reaction than would otherwise take place.[6] This continues until nickel-56 is produced, which decays radioactively into cobalt-56 and then iron-56 over the course of a few months. As iron and nickel have the highest binding energy per nucleon of all the elements,[7] energy cannot be produced at the core by fusion, and a nickel-iron core grows.[4][8] 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 states. However, this is forbidden for identical fermion particles, such as the electron – a phenomenon called the Pauli exclusion principle.

When the core's mass exceeds the Chandrasekhar limit of about 1.4 M, degeneracy pressure can no longer support it, and catastrophic collapse ensues.[9] The outer part of the core reaches velocities of up to 70000 km/s (23% of the speed of light) as it collapses toward the center of the star.[10] The rapidly shrinking core heats up, producing high-energy gamma rays that decompose iron nuclei into helium nuclei and free neutrons via photodisintegration. As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos. 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.[11]

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. When the collapse stops, the infalling matter rebounds, producing a shock wave 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.[12]

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 capture, an electron neutrino is produced. In a typical Type II supernova, the newly formed neutron core has an initial temperature of about 100 billion kelvins, 104 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.[13] These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos.[14] The two neutrino production mechanisms convert the gravitational potential energy of the collapse into a ten-second neutrino burst, releasing about 1046 joules (100 foe).[15]

Through a process that is not clearly understood, about 1%, or 1044 joules (1 foe), of the energy released (in the form of neutrinos) is reabsorbed by the stalled shock, producing the supernova explosion.[a][12] Neutrinos generated by a supernova were observed in the case of Supernova 1987A, leading astrophysicists to conclude that the core collapse picture is basically correct. The water-based Kamiokande II and IMB instruments detected antineutrinos of thermal origin,[13] while the gallium-71-based Baksan instrument detected neutrinos (lepton number = 1) of either thermal or electron-capture origin.

Core collapse scenario
Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming a nickel-iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.

When the progenitor star is below about 20 M – 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 star.[10] Above this mass, the remnant collapses to form a black hole.[4][16] The theoretical limiting mass for this type of core collapse scenario is about 40–50 M. Above that mass, a star is believed to collapse directly into a black hole without forming a supernova explosion,[17] although uncertainties in models of supernova collapse make calculation of these limits uncertain.

Theoretical models

The Standard Model of particle physics is a theory which describes three of the four known fundamental interactions between the elementary particles that make up all matter. This theory allows predictions to be made about how particles will interact under many conditions. The energy per particle in a supernova is typically 1–150 picojoules (tens to hundreds of MeV).[18] 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.[19] In particular, Earth-based particle accelerators can produce particle interactions which are of much higher energy than are found in supernovae,[20] 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.[21]

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 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 material from which the star originally formed.[22]

Neutrino physics, which is modeled by the Standard Model, is crucial to the understanding of this process.[19] 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 shockwave forms and when and how it stalls and is reenergized.[23]

In fact, some theoretical models incorporate a hydrodynamical instability in the stalled shock known as the "Standing Accretion Shock Instability" (SASI). This instability comes about as a consequence of non-spherical perturbations oscillating the stalled shock thereby deforming it. The SASI is often used in tandem with neutrino theories in computer simulations for re-energizing the stalled shock.[24]

Computer models have been very successful at calculating the behavior of Type II supernovae when 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.[25][26][27]

Light curves for Type II-L and Type II-P supernovae

This graph of the luminosity as a function of time shows the characteristic shapes of the light curves for a Type II-L and II-P supernova.

When the spectrum of a Type II supernova is examined, it normally displays Balmer absorption lines – reduced flux at the characteristic frequencies where hydrogen atoms absorb energy. The presence of these lines is used to distinguish this category of supernova from a Type I supernova.

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 magnitudes per day; much lower than the decay rate for Type Ia supernovae. Type II is subdivided into two classes, depending on the shape of the light curve. The light curve for a Type II-L supernova shows a steady (linear) decline following the peak brightness. By contrast, the light curve of a Type II-P supernova has a distinctive flat stretch (called a 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.[28]

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.[28] The plateau phase in Type II-P supernovae is due to a change in the opacity 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 opacity. This prevents photons from the inner parts of the explosion from escaping. When the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.[29]

Type IIn supernovae

The "n" denotes narrow, which indicates the presence of narrow or intermediate width hydrogen 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.[30][31] The estimated circumstellar density required to explain the observational properties is much higher than that expected from the standard stellar evolution theory.[32] It is generally assumed that the high circumstellar density is due to the high mass-loss rates of the Type IIn progenitors. The estimated mass-loss rates are typically higher than 10−3 M per year. There are indications that they originate as stars similar to Luminous blue variables with large mass losses before exploding.[33] SN 1998S and SN 2005gl are examples of Type IIn supernovae; SN 2006gy, an extremely energetic supernova, may be another example.[34]

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. However, later on the H emission becomes undetectable, and there is also a second peak in the light curve that has a spectrum which more closely resembles a Type Ib supernova. The progenitor could have been a massive star that expelled most of its outer layers, or one 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.[35] As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes more transparent and reveals the deeper layers.[35] The classic example of a Type IIb supernova is SN 1993J,[36][37] while another example is Cassiopeia A.[38] The IIb class was first introduced (as a theoretical concept) by Woosley et al. in 1987,[39] and the class was soon applied to SN 1987K[40] and SN 1993J.[41]


Hypernovae are a rare type of supernova substantially more luminous and energetic than standard supernovae. Examples are SN 1997ef (type Ic) and SN 1997cy (type IIn). Hypernovae are produced by more than one type of event: relativistic jets during formation of a black hole from fallback of material onto the neutron star core, the collapsar model; interaction with a dense envelope of circumstellar material, the CSM model; the highest mass pair instability supernovae; possibly others such as binary and quark star model.

Stars with initial masses between about 25 and 90 times the sun develop cores large enough that after a supernova explosion, some material will fall back onto the neutron star core and create a black hole. In many cases this reduces the luminosity of the supernova, and above 90 M the star collapses directly into a black hole without a supernova explosion. However, if the progenitor is spinning quickly enough, the infalling material generates relativistic jets that emit more energy than the original explosion.[42] They may also be seen directly if beamed towards us, giving the impression of an even more luminous object. In some cases these can produce gamma-ray bursts, although not all gamma-ray bursts are from supernovae.[43]

In some cases a type II supernova occurs when the star is surrounded by a very dense cloud of material, most likely expelled during luminous blue variable eruptions. This material is shocked by the explosion and becomes more luminous than a standard supernova. It is likely that there is a range of luminosities for these type IIn supernovae with only the brightest qualifying as a hypernova.

Pair instability supernovae occur when an oxygen core in an extremely massive star becomes hot enough that gamma rays spontaneously produce electron-positron pairs.[44] This causes the core to collapse, but where the collapse of an iron core causes endothermic fusion to heavier elements, the collapse of an oxygen core creates runaway exothermic fusion which completely unbinds the star. The total energy emitted depends on the initial mass, with much of the core being converted to nickel-56 and ejected which then powers the supernova for many months. At the lower end stars of about 140 M produce supernovae that are long-lived but otherwise typical, while the highest mass stars of around 250 M produce supernovae that are extremely luminous and also very long lived; hypernovae. More massive stars die by photodisintegration. Only population III stars, with very low metallicity, can reach this stage. Stars with more heavy elements are more opaque and blow away their outer layers until they are small enough to explode as a normal type Ibc supernova. It is thought that even in our own galaxy, mergers of old low metallicity stars may form massive stars capable of creating a pair instability supernova.

See also


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Messier 106

Messier 106 (also known as NGC 4258) is an intermediate spiral galaxy in the constellation Canes Venatici. It was discovered by Pierre Méchain in 1781. M106 is at a distance of about 22 to 25 million light-years away from Earth. M106 contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole has been demonstrated from radio-wavelength observations of the rotation of a disk of molecular gas orbiting within the inner light-year around the black hole. NGC 4217 is a possible companion galaxy of Messier 106. A Type II supernova was observed in M106 in May 2014.

Messier 81

Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away, with a diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major. Due to its proximity to Earth, large size, and active galactic nucleus (which harbors a 70 million M☉supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers.Messier 81 was first discovered by Johann Elert Bode on December 31, 1774. Consequently, the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, which was subsequently listed in the Messier Catalogue.

Messier 81 is located approximately 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group.Messier 81 and Messier 82 can both be viewed easily using binoculars and small telescopes. The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a very dark sky. Telescopes with apertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy. Its far northern declination makes it generally visible for observers in the northern hemisphere. It is not visible to most observers in the southern hemisphere, except those in a narrow latitude range immediately south of the equator.

Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and thus enhancing the infrared dust emission from these regions.

Only one supernova has been detected in Messier 81. The supernova, named SN 1993J, was discovered on 28 March 1993 by F. García in Spain. At the time, it was the second brightest supernova observed in the 20th century. The spectral characteristics of the supernova changed over time. Initially, it looked more like a type II supernova (a supernova formed by the explosion of a giant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib.Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernova, but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a type IIb, a transitory class between type II and type Ib. The scientific results from this supernova suggested that type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in type II supernovae. The supernova was also used to estimate a distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81. As a local galaxy, the Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M81 along with M31 and M33.

Messier 81 is the largest galaxy in the M81 Group, a group of 34 galaxies located in the constellation Ursa Major. At approximately 11.7 Mly (3.6 Mpc) from the Earth, it makes this group and the Local Group, containing the Milky Way, relative neighbors in the Virgo Supercluster.

Gravitational interactions of M81 with M82 and NGC 3077 have stripped hydrogen gas away from all three galaxies, forming gaseous filamentary structures in the group. Moreover, these interactions have allowed interstellar gas to fall into the centers of M82 and NGC 3077, leading to vigorous star formation or starburst activity there.

NGC 1087

NGC 1087 is an intermediate spiral galaxy in Cetus. The central bar/core is very small with many irregular features in the surrounding disk of material. With the many strange features of NGC 1087, its true nature is still uncertain. It has an extremely small nucleus and a very short stellar bar. Unlike most barred galaxies, the bar apparently has some new star-formation taking place. There is a multiple spiral structure defined more by the dust lanes than by luminous matter. Overall, the disc has a very low surface brightness. Even though it appears close to another galaxy (NGC 1090), these two galaxies are not interacting and should be considered isolated from one another.

NCG 1087 lies near the small M77 (NGC 1068) galaxy group that also includes NGC 936, NGC 1055, and NGC 1090. However, because of its distance, it probably is not an actual group member.

Based on the published red shift, (Hubble Constant of 62 km/s per Mpc) a rough distance estimate for NGC 1087 is 80 million light-years, with a diameter of about 86,800 light-years. The Type II Supernova 1995V is the only recorded supernova in NGC 1087.

NGC 150

NGC 150 (also known as PGC 2052) is a barred spiral galaxy in the constellation Sculptor. It is about 70 million light years away from the solar system, and it has a diameter of about 55,000 light years. It was discovered on 20 November 1886, by Lewis A. Swift. The Type II supernova SN 1990K was detected in NGC 150, and was reported to be similar to SN 1987A.

NGC 2280

NGC 2280 is a spiral galaxy located in the constellation Canis Major. It is located at a distance of circa 75 million light years from Earth, which, given its apparent dimensions, means that NGC 2280 is about 135,000 light years across. It was discovered by John Herschel on February 1, 1835. One supernova has been observed in NGC 2280, SN 2001fz, a type II supernova discovered by the Beijing Astronomical Observatory Supernova Survey on November 15, 2001. It had a peak magnitude of 17.4.

NGC 2770

NGC 2770 is a type SASc spiral galaxy located about 88 million light years away, in the constellation Lynx.

Three Type Ib supernovae have occurred there recently: SN 1999eh, SN 2007uy, and SN 2008D. The last of these is famous for being the first supernova detected by the X-rays released very early on in its formation, rather than by the optical light emitted during the later stages, which allowed the first moments of the outburst to be observed. It is possible that NGC 2770's interactions with a suspected companion galaxy may have created the massive stars causing this activity.SN 2015bh, a Type II supernova, was discovered in NGC 2770 in February 2015.NGC 2770 was also the target for the first binocular image produced by the Large Binocular Telescope.

NGC 4242

NGC 4242 is a spiral galaxy in the northern constellation of Canes Venatici. The galaxy is about 18 million light years (5.5 megaparsecs) away. It was discovered on 10 April 1788 by William Herschel, and it was described as "very faint, considerably large, irregular, round, very gradually brighter in the middle, resolvable" by John Louis Emil Dreyer, the compiler of the New General Catalogue.NGC 4242's galaxy morphological type is SABdm. This means that it is an intermediate spiral galaxy, with loosely wound spiral arms and is generally irregular in appearance. It was photographed by the Hubble Space Telescope in 2017. The image shows an asymmetric center and a small galactic bar. NGC 4242 has a relatively low surface brightness and rate of star formation. NGC 4242 may be a satellite galaxy of Messier 106 and is a member of the Canes II Group.SN 2002bu was detected in NGC 4242, brightening to its peak magnitude of 15.5 in 2002. It was originally classified as a type II supernova, but it may be a supernova impostor, like SN 2008S.

NGC 5668

NGC 5668 is a nearly face-on spiral galaxy located about 81 million light years away in the constellation Virgo. As seen from the Earth, it is inclined by an angle of 18° to the line of sight along a position angle of 145°. The morphological classification in the De Vaucouleurs system is SA(s)d, indicating a pure spiral structure with loosely wound arms. However, optical images of the galaxy indicate the presence of a weak bar structure spanning an angle of 12″ across the nucleus. There is a dwarf galaxy located around 650×10^3 ly (200 kpc) to the southeast of NGC 5668, and the two may be gravitationally interacting.Three supernovae have been observed in this galaxy: SN 1952G, SN 1954B, and SN 2004G. The last, a type II supernova, was initially imaged on January 19, 2004, at 43" to the west and 12".5 south of the galaxy core. High velocity clouds of neutral hydrogen have been observed in NGC 5668, which may have their origin in supernova explosions and strong stellar winds.

NGC 5921

NGC 5921 is a barred spiral galaxy located approximately 65 million light-years from the Solar System in the constellation Serpens Caput. It was discovered by William Herschel on 1 May 1786. In February 2001 a type II supernova (SN 2001X) was discovered in NGC 5921.

NGC 655

NGC 655 is a lenticular galaxy located 400 million light-years away in the constellation Cetus. It was discovered in a sky-survey by Ormond Stone on December 12, 1885.On July 23, 2010, supernova SN 2010gp in NGC 655 was announced, reaching magnitude 15.5 on July 22. It was offset by 22″ west and 47″ south of the nucleus. Earlier in 2000, the type II supernova SN 2000bg had also appeared in this galaxy.

NGC 6845

NGC 6845 (also known as Klemola 30) is an interacting system of four galaxies in the constellation Telescopium. The cluster has certain similarities with Stephan's Quintet. Its distance is estimated to be about 90 Mpc.

The components of the galaxy cluster are the two spiral galaxies NGC 6845A and NGC 6845B as well as the two lenticular galaxies NGC 6845C and NGC 6845D. The four galaxies occupy an area of about 4' x 2' in the sky. The largest galaxy in this compact galaxy cluster is NGC 6845A, a barred spiral galaxy. SN 2008DA was a Type II supernova observed in NGC 6845A in June 2008. The dwarf galaxy ATCA J2001-4659, which is found around 4.4' northeast of NGC 6845B, was identified as a companion of NGC 6845.NGC 6845 was discovered on July 7, 1834 by John Herschel.

NGC 6925

NGC 6925 is an unbarred spiral galaxy in the constellation Microscopium of apparent magnitude 11.3. It is lens-shaped, as it lies almost edge on to observers on Earth. It lies 3.7 degrees west-northwest of Alpha Microscopii.SN 2011ei, a Type II supernova in NGC 6925, was discovered by Stu Parker in New Zealand in July 2011.

NGC 877

NGC 877 is an intermediate spiral galaxy located in the constellation Aries. It is located at a distance of circa 160 million light years from Earth, which, given its apparent dimensions, means that NGC 877 is about 115,000 light years across. It was discovered by William Herschel on October 14, 1784. It interacts with NGC 876.

NGC 877 features two spiral arms with a grand design pattern and slightly disturbed morphology. When pictured in H-alpha, the arms have numerous knots and appear brighter than the nucleus. The northwest part of the galaxy has higher polarised emission than the rest of the galaxy. A bar appears in radio waves.

The nucleus has activity that resembles that of a HII region. The galaxy has been categorised as a luminous infrared galaxy, a category of galaxies associated with high star formation rate. The total infrared luminosity of the galaxy is estimated to be between 1011.04 L☉ and 1011.1 L☉, lying near the threshold to classify a galaxy as luminous infrared. The total star formation rate in NGC 877 is estimated to be between 20 and 53 M☉ per year.One possible supernova has been observed in NGC 877, SN 2019rn. It was discovered by the robotic sky survey ATLAS on January 12.30, 2019, using a twin 0.5m telescope system. It had apparent magnitude 18.9 on discovery. The supernova was initially classified as a type II supernova with spectroscopic observations by Keck-II, and further spectographic observations categorised it as type IIb, although it could also be a cataclysmic variable or another type of variable star.NGC 877 forms a pair with the edge-on spiral galaxy NGC 876, which lies 2.1 arcminutes to the southwest. At the distance of NGC 877, this corresponds to a projected distance of 30 kpc. A low surface brightness bridge connects the two galaxies. NGC 870 and NGC 871 are two other nearby galaxies. NGC 877 is the brightest and most massive member of a galaxy group known as the NGC 877 group or LGG 35. Other members of the group include NGC 876 and NGC 871, as well as UGC 1693, IC 1791, UGC 1773, and UGC 1817. The group contains large amounts of HI gas.

NGC 988

NGC 988 is a spiral galaxy located in the constellation Cetus. It lies at a distance of 50 million light years from Earth, which, given its apparent dimensions, means that NGC 988 is about 75,000 light years across. Magnitude 7.1 HD 16152 is superposed 52" northwest of the center of NGC 988. The galaxy was discovered by Édouard Jean-Marie Stephan in 1879. One ultraluminous X-ray source has been detected in NGC 988.NGC 988 is the brightest galaxy in NGC 1052 group (which is also known as NGC 988 group),which also includes the elliptical galaxy NGC 1052, NGC 991, NGC 1022, NGC 1035, NGC 1042, NGC 1047, NGC 1051, NGC 1084, NGC 1110. It belongs in the same galaxy cloud as Messier 77.One supernova has been discovered in NGC 988, SN 2017gmr, a Type II supernova discovered on 4 September 2017.

SN 2005ap

SN 2005ap was an extremely energetic type Ic supernova in the galaxy SDSS J130115.12+274327.5. With a peak absolute magnitude of around −22.7, it is the second-brightest hypernova yet recorded, twice as bright as the previous record holder, SN 2006gy, though SN 2005ap was eventually surpassed by ASASSN-15lh. It was initially classified as type II-L, but later revised to type Ic. It was discovered on 3 March 2005, on unfiltered optical images taken with the 0.45 m ROTSE-IIIb (Robotic Optical Transient Search Experiment) telescope, which is located at the McDonald Observatory in West Texas, by Robert Quimby, as part of the Texas Supernova Search that also discovered SN 2006gy. Although it was discovered before SN 2006gy, it was not recognized as being brighter until October 2007. As it occurred 4.7 billion light years from Earth, it was not visible to the naked eye.

Although SN 2005ap was twice as bright at its peak than SN 2006gy, it was not as energetic overall, as the former brightened and dimmed in a typical period of a few days whereas the latter remained very bright for many months. SN 2005ap was about 300 times brighter than normal for a type II supernova. It has been speculated that this hypernova involved the formation of a quark star. Quimby has suggested that the hypernova is of a new type distinct from the standard type II supernova, and his research group have identified five other supernovae similar to SN 2005ap and SCP 06F6, all of which were extremely bright and lacking in hydrogen.

SN 2005cs

SN 2005cs was a supernova in the Whirlpool Galaxy. It was a type II supernova, discovered in 2005 by Wolfgang Kloehr, a German amateur astronomer.

Silicon-burning process

In 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 sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion Kelvin (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

Texas Supernova Search

Texas Supernova Search is one of many ongoing projects to identify and record supernova events. The project is led by Robert Quimby and to date has found 35 supernovae, 29 of which they were the first to report on. In addition they have discovered 12 novae (including a probable LBV), in M31 and M33 and 6 dwarf novae.[1]

The project's most notable successes are SN 2005ap and SN 2006gy, the 2 most powerful supernovae yet recorded. SN 2005ap was an extremely energetic type II supernova. It is reported to be the brightest supernova yet recorded, twice as bright as the previous record holder, SN 2006gy. [2]

Although SN 2005ap was twice as bright at its peak than SN 2006gy it was not as energetic overall as the former brightened and dimmed in a typical period of a few days whereas the latter remained very bright for many months. SN2005ap was about 300 times brighter than normal for a type II supernova. It has been speculated that this supernova involved the formation of a quark star. [3]

Time magazine listed the discovery of SN 2006gy as third in its Top 10 Scientific Discoveries for 2007.[4]

Vela Supernova Remnant

The Vela supernova remnant is a supernova remnant in the southern constellation Vela. Its source Type II supernova exploded approximately 11,000–12,300 years ago (and was about 800 light-years away). The association of the Vela supernova remnant with the Vela pulsar, made by astronomers at the University of Sydney in 1968, was direct observational evidence that supernovae form neutron stars.

The Vela supernova remnant includes NGC 2736. It also overlaps the Puppis Supernova Remnant, which is four times more distant. Both the Puppis and Vela remnants are among the largest and brightest features in the X-ray sky.

The Vela supernova remnant (SNR) is one of the closest known to us. The Geminga pulsar is closer (and also resulted from a supernova), and in 1998 another near-Earth supernova remnant was discovered, RX J0852.0-4622, which from our point of view appears to be contained in the southeastern part of the Vela remnant. One estimate of its distance puts it only 200 parsecs away (about 650 ly), closer than the Vela remnant, and, surprisingly, it seems to have exploded much more recently, in the last thousand years, because it is still radiating gamma rays from the decay of titanium-44. This remnant was not seen earlier because in most wavelengths, it is lost because of the presence of the Vela remnant.

Physics of

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