User:Xrayburst1/Stellar Explosion

Depending on their initial mass, proximity to other stars, composition, and other factors, some stars will undergo energetic explosions as a part of, or as an end to, their life cycle. There are numerous explosion types that differ in their energies, mechanisms, progenitors, emission spectra, and remnants. Our own Sun will eventually evolve into a faint white dwarf star and will not undergo an explosion.

Core Collapse Supernovae
[[Image:N49 (Chandra).jpg|thumb|

A composite image of N49, a remnant of a core collapse supernova in the Large Magellanic Cloud. X-ray observations (in blue) are from the Chandra X-ray Observatory, while the visible light observations (yellow and purple) are from the Hubble Space Telescope.]]

Stars larger than roughly 8 solar masses will end their life in a core collapse supernova. They occur roughly every 50 years per galaxy, but the last supernova in our Milky Way Galaxy was in 1604. The total explosion energies are typically over 1053 ergs, with 99 % of it carried away by neutrinos and 1 % (1051 ergs, one foe) converted to kinetic energy of the ejected envelope. The optical light is approximately 0.01 foe over many months, roughly equivalent to 1037 watts.

The mechanism for these explosions involves the collapse or implosion of the inner Iron core of a massive star to densities beyond nuclear matter. The inner core then rebounds out and generates a shock wave as it collides with infalling material from the outer core. The shock wave propagates outwards through the core, losing energy as it dissociates nuclei, and proceeds to obliterate the outer layers of the star. Various mechanisms  have been proposed to explain how the shock wave can maintain sufficient energy to successfully leave the core and cause an explosion and ejection of the outer layers.

These dramatic events leave behind a neutron star or black hole remnant, two of the most exotic objects in space. The rest of the projenitor star is completely disintegrated and dispersed into an enormous debris cloud. This remnant contains elements up to Iron created during the star's long life in a variety of nucleosynthesis processes, as well as heavier elements that were created during the explosion itself. This may include elements made by rapid neutron capture process (r-process) nucleosynthesis in the region above the newly-formed neutron star remnant, as well as those elements formed as the shock wave passes through and heats the outer regions of the star. Supernovae are a major contributor to the production and distribution of heavy elements in the cosmos.

In addition to this role in element formation, supernovae have also been linked to Gamma-ray bursts, Galactic cosmic ray acceleration, the formation of new stars including our Sun and Solar system, and possibly to prehistoric mass extinctions on Earth.

Type II Supernovae
Type II Supernovae are Core Collapse supernovae that have Hydrogen and Helium in their emission spectrum. This indicates that they have not experienced the loss of their outer Hydrogen and Helium layers, in contrast to Type Ib and Type Ic Supernovae. Type II explosions cannot be the precursors to Gamma-ray bursts because of the presence of these light element emission lines.

Type Ib and Ic Supernovae
Type Ib supernovae are the subset of core collapse supernovae wherein the progenitor star has lost its outer layer of Hydrogen, so no Hydrogen lines appear in their emission spectrum. Type Ic are a similar subset where both the outer layers of Hydrogen and Helium have been lost. One model for this mass loss is a strong stellar wind such as hypothesized in an isolated, massive Wolf-Rayet star, while another model involves the transfer of material from the star to a binary companion star. In both cases, the underlying explosion mechanism is thought to be collapse of the stellar core like a Type II supernova. Some Type Ib and Type Ic supernovae may be the progenitors of Gamma-ray bursts.

Hypernovae and Collapsars


A Hypernova is a supernova resulting from the collapse of a very massive (100 - 300 solar mass) progenitor star. Energy releases from hypernovae are often 100 times that released by typical supernovae of lower mass progenitor stars. Hypernovae may be responsible for long-duration gamma ray bursts. Hypernovae may be distinguishable from other explosions by the heavy elements produced in the explosion and ejected, such as the large amount of 56Ni -- roughly 10 times the value from a typical supernova. The decay of this 56Ni powers the long-term light curve of the explosion.

A Collapsar is an explosion model wherein the core of a massive, fast-rotating Wolf-Rayet star collapses into a black hole that relativistically pulls in and consumes the outer layers of the star. These explosion models generate relativistic jets that may be responsible for Gamma-ray Bursts. The term collapsar is sometimes used to refer to the black hole formation in such explosions, as an abbreviation for "collapsed star".

Gamma-ray Bursts


Gamma-ray bursts are thought to predominantly occur with Collapsar formation during a Hypernova explosion. Relativistic jets of material produced with black hole formation these systems interact with material in the outer layer of the star to form intense, beamed bursts of high energy photons (gamma-rays).

But by no means all core collapse supernovae produce a GRB. Theoretical considerations dictate that several other factors must also be present. For one thing, the supernova must result in the formation of a black hole that is massive enough to support a large accretion disk of matter orbiting around it. A neutron star, which is the alternative remnant of a supernova, just isn't massive enough. To get a sufficiently massive black hole, the progenitor star must be at least 40 Solar masses.

High mass alone, however, is not enough. The progenitor star must also be rotating rapidly enough that the angular momentum of the system is large enough to cause most of the matter and energy from the supernova explosion to be focused into a jet of angular width at most about 20 degrees. This concentrates most of the energy of the explosion into a narrow beam, so that the energy emitted in our direction matches what we actually observe. If the beam were not so narrow, the energy would not appear to be the magnitude that we observe.

There are additional factors that affect the varying characteristics of GRBs that we observe. In particular, the distribution of matter in the interstellar medium surrounding the supernova is important. It is the collision between the jets and this matter that determines the intensity and duration of the afterglow we observe for some time after the original burst.

-- those massive enough to form a black hole in a Hypernova explosion --

as well as from the merger of two compact objects, such as a binary star system containing neutron stars or black holes.

Gamma-ray bursts are intense flashes of energetic photons (gamma-rays) that last from milliseconds to hundreds of seconds and originate from all directions in the sky. These bursts have a subsequent afterglow in visible light, X-rays, and radio waves that can last for weeks. The first burst was detected in 1967 by the Vela Satellite, and now there are dedicated observatories such as the Swift Gamma-Ray Burst MIssion which is recording roughly a hundred bursts each year.

Long-duration bursts, those lasting more than 2 seconds, have now been associated with Collapsar formation during a Hypernova explosion. Short-duration bursts may result from the merger of two compact objects, such as a binary star system containing neutron stars or black holes. Matching the gamma-ray emission of these models to observations is an ongoing challenge.

Type Ia supernova are not progenitors as their energy release is far too small.

But by no means all core collapse supernovae produce a GRB. Theoretical considerations dictate that several other factors must also be present. For one thing, the supernova must result in the formation of a black hole that is massive enough to support a large accretion disk of matter orbiting around it. A neutron star, which is the alternative remnant of a supernova, just isn't massive enough. To get a sufficiently massive black hole, the progenitor star must be at least 40 Solar masses.

High mass alone, however, is not enough. The progenitor star must also be rotating rapidly enough that the angular momentum of the system is large enough to cause most of the matter and energy from the supernova explosion to be focused into a jet of angular width at most about 20 degrees. This concentrates most of the energy of the explosion into a narrow beam, so that the energy emitted in our direction matches what we actually observe. If the beam were not so narrow, the energy would not appear to be the magnitude that we observe.

There are additional factors that affect the varying characteristics of GRBs that we observe. In particular, the distribution of matter in the interstellar medium surrounding the supernova is important. It is the collision between the jets and this matter that determines the intensity and duration of the afterglow we observe for some time after the original burst.

Energy released in GRB = ????

Most observed GRBs are believed to be a narrow beam of intense radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, possibly the merger of binary neutron stars.

Pair-instability supernovae


Pair-instability supernova explosions are thought to occur in very massive stars (130 to 250 solar masses). The core temperature of these stars rises so high that the gamma rays produced from thermonuclear fusion have enough energy to create electron-positron pairs when interacting with heavy nuclei. This reduces the mean free path of the fusion photons, impeding their ability to transfer energy out of the core that prevents the gravitational collapse of the star. The rising core temperature accelerates the thermonuclear reactions, leading to a runaway explosion that, in contrast to a typical supernova, does not leave a neutron star or black hole remnant.

Thermonuclear Supernovae
Composite X-ray (red and green)/optical (blue) image of Dorado from the Chandra X-ray Observatory showing a Type Ia supernova remnant on the upper left and a Type II supernova remnant on the lower right. The apparent proximity of the remnants is probably the result of a chance alignment.]]

Lower-mass stars may also undergo explosions if they are in a binary star system where matter is transferred from one star to the other by the process of accretion. Thermonuclear or Type Ia supernovae are one example, where fast mass transfer drives a white dwarf star to become too massive -- beyond the Chandrasekhar Limit for stability. This leads to an explosion by the propagation of a thermonuclear flame through the star triggered by carbon burning. The explosion could result from subsonic flame propagation (deflagrations), flames accelerated by turbulence, and supersonic detonations. Combinations of these scenarios, such as turbulent deflagration and delayed detonation, are also possible.

These explosions complete destroy the star, leaving only the dust cloud as a remnant. These explosions have a relatively uniform luminosity, making them useful as distance indicators or "standard candles" in cosmology research.

Nova Explosions


Nova explosions occur in a cataclysmic binary system, where lower mass transfer (accretion) rates results in a thermonuclear explosion on the surface of a white dwarf star. These explosions leave behind the compact, faint white dwarf star and eject a debris cloud of the accreted material and perhaps some of the underlying white dwarf material that was mixed in. Classical novae, where the donor is typically a main sequence star, recur roughly every 104 years. For recurrent novae, the donors are usually red giant stars, the accretion rates are higher, and the white dwarfs more massive, leading to frequent outbursts.

Dwarf Nova are xxxxxxxxxx.

A U Geminorum-type variable star, or dwarf nova (pl. novae) is a type of cataclysmic variable star[1] consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion. They are similar to classical novae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different: classical novae result from the fusion and detonation of accreted hydrogen, while current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a collapse onto the white dwarf that releases large amounts of gravitational potential energy.[2][3] Dwarf novae are distinct from classical novae in other ways; their luminosity is lower, and they are typically recurrent on a scale from days to decades.[1] The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the Hubble space telescope suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.[2][3]

Observations have suggested a new type of outburst, a Luminous Red Nova yyyyy. Some novae can occur inside the envelope of their binary star, such as the recurrent nova RS Ophiuchi.

rp-process

Nova Remnant



A nova remnant is made up of the material either left behind by the gigantic explosion of a star in a nova, or from the bubbles of gas blasted away in a recurrent nova. It has an expansion velocity of around 1000 km/s, and has a lifetime of a few centuries. Nova remnants are much less massive than supernova remnants or planetary nebulae. they are usually very old and when we see them in space, they are already gone

X-ray Bursts
X-ray bursts are a similar phenomenon, where the mass transfer is from a low-mass donor star onto a neutron star. These explosions generate 1045 ergs in X-rays but do not likely eject significant amounts of material due to the strong gravitational field of the neutron star.

rp-process

nova remnant is made up of the material either left behind by the gigantic explosion of a star in a nova, or from the bubbles of gas blasted away in a recurrent nova. It has an expansion velocity of around 1000 km/s, and has a lifetime of a few centuries. Nova remnants are much less massive than supernova remnants or planetary nebulae. they are usually very old and when we see them in space, they are already gone

Other resources
Supernova imposter