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Monday, February 17, 2020

Hypergiant

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Hypergiant
 
Comparison of (left to right) the Pistol Star, Rho Cassiopeiae, Betelgeuse, and VY Canis Majoris superimposed on an outline of the Solar System. The blue half-ring centered near the left edge represents the orbit of Neptune, the outermost planet of the Solar System.
 
A hypergiant (luminosity class 0 or Ia+) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.

 

Origin and definition

In 1956, the astronomers Feast and Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude brighter than MV = −7 (MBol will be larger for very cool and very hot stars, for example at least −9.7 for a B0 hypergiant). In 1971, Keenan suggested that the term would be used only for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today.

To be classified as a hypergiant, a star must be highly luminous and have spectral signatures showing atmospheric instability and high mass loss. Hence it is possible for a non-hypergiant, supergiant star to have the same or higher luminosity as a hypergiant of the same spectral class. Hypergiants are expected to have a characteristic broadening and red-shifting of their spectral lines, producing a distinctive spectral shape known as a P Cygni profile. The use of hydrogen emission lines is not helpful for defining the coolest hypergiants, and these are largely classified by luminosity since mass loss is almost inevitable for the class.

Formation

Stars with an initial mass above about 25 M☉ quickly move away from the main sequence and increase somewhat in luminosity to become blue supergiants. They cool and enlarge at approximately constant luminosity to become a red supergiant, then contract and increase in temperature as the outer layers are blown away. They may "bounce" backwards and forwards executing one or more "blue loops", still at a fairly steady luminosity, until they explode as a supernova or completely shed their outer layers to become a Wolf–Rayet star. Stars with an initial mass above about 40 M☉ are simply too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become red supergiants. The most massive stars, especially rapidly rotating stars with enhanced convection and mixing, may skip these steps and move directly to the Wolf–Rayet stage.

This means that stars at the top of the Hertzsprung–Russell diagram where hypergiants are found may be newly evolved from the main sequence and still with high mass, or much more evolved post-red supergiant stars that have lost a significant fraction of their initial mass, and these objects cannot be distinguished simply on the basis of their luminosity and temperature. High-mass stars with a high proportion of remaining hydrogen are more stable, while older stars with lower masses and a higher proportion of heavy elements have less stable atmospheres due to increased radiation pressure and decreased gravitational attraction. These are thought to be the hypergiants, near the Eddington limit and rapidly losing mass. 

The yellow hypergiants are thought to be generally post-red supergiant stars that have already lost most of their atmospheres and hydrogen. A few more stable high mass yellow supergiants with approximately the same luminosity are known and thought to be evolving towards the red supergiant phase, but these are rare as this is expected to be a rapid transition. Because yellow hypergiants are post-red supergiant stars, there is a fairly hard upper limit to their luminosity at around 500,000–750,000 L☉, but blue hypergiants can be much more luminous, sometimes several million L☉. 

Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors, but these are small except for two distinct instability regions where luminous blue variables (LBVs) and yellow hypergiants are found. Because of their high masses, the lifetime of a hypergiant is very short in astronomical timescales: only a few million years compared to around 10 billion years for stars like the Sun. Hypergiants are only created in the largest and densest areas of star formation and because of their short lives, only a small number are known despite their extreme luminosity that allows them to be identified even in neighbouring galaxies. The time spent in some phases such as LBVs can be as short as a few thousand years.

Stability

Great nebula in Carina, surrounding Eta Carinae

As the luminosity of stars increases greatly with mass, the luminosity of hypergiants often lies very close to the Eddington limit, which is the luminosity at which the radiation pressure expanding the star outward equals the force of the star's gravity collapsing the star inward. This means that the radiative flux passing through the photosphere of a hypergiant may be nearly strong enough to lift off the photosphere. Above the Eddington limit, the star would generate so much radiation that parts of its outer layers would be thrown off in massive outbursts; this would effectively restrict the star from shining at higher luminosities for longer periods.

A good candidate for hosting a continuum-driven wind is Eta Carinae, one of the most massive stars ever observed. With an estimated mass of around 130 solar masses and a luminosity four million times that of the Sun, astrophysicists speculate that Eta Carinae may occasionally exceed the Eddington limit. The last time might have been a series of outbursts observed in 1840–1860, reaching mass loss rates much higher than our current understanding of what stellar winds would allow.

As opposed to line-driven stellar winds (that is, ones driven by absorbing light from the star in huge numbers of narrow spectral lines), continuum driving does not require the presence of "metallic" atoms — atoms other than hydrogen and helium, which have few such lines — in the photosphere. This is important, since most massive stars also are very metal-poor, which means that the effect must work independently of the metallicity. In the same line of reasoning, the continuum driving may also contribute to an upper mass limit even for the first generation of stars right after the Big Bang, which did not contain any metals at all.

Another theory to explain the massive outbursts of, for example, Eta Carinae is the idea of a deeply situated hydrodynamic explosion, blasting off parts of the star's outer layers. The idea is that the star, even at luminosities below the Eddington limit, would have insufficient heat convection in the inner layers, resulting in a density inversion potentially leading to a massive explosion. The theory has, however, not been explored very much, and it is uncertain whether this really can happen.

Another theory associated with hypergiant stars is the potential to form a pseudo-photosphere, that is a spherical optically dense surface that is actually formed by the stellar wind rather than being the true surface of the star. Such a pseudo-photosphere would be significantly cooler than the deeper surface below the outward-moving dense wind. This has been hypothesized to account for the "missing" intermediate-luminosity LBVs and the presence of yellow hypergiants at approximately the same luminosity and cooler temperatures. The yellow hypergiants are actually the LBVs having formed a pseudo-photosphere and so apparently having a lower temperature.

Relationships with Ofpe, WNL, LBV, and other supergiant stars

Very Large Telescope image of the surroundings of VY Canis Majoris
 
Hypergiants are evolved, high luminosity, high-mass stars that occur in the same or similar regions of the HR diagram to stars with different classifications. It is not always clear whether the different classifications represent stars with different initial conditions, stars at different stages of an evolutionary track, or is just an artifact of our observations. Astrophysical models explaining the phenomena show many areas of agreement. Yet there are some distinctions that are not necessarily helpful in establishing relationships between different types of stars. 

Although most supergiant stars are less luminous than hypergiants of similar temperature, a few fall within the same luminosity range. Ordinary supergiants compared to hypergiants often lack the strong hydrogen emissions whose broadened spectral lines indicate significant mass loss. Evolved lower mass supergiants do not return from the red supergiant phase, either exploding as supernovae or leaving behind a white dwarf.

Luminous blue variables are a class of highly luminous hot stars that display characteristic spectral variation. They often lie in a "quiescent" zone with hotter stars generally being more luminous, but periodically undergo large surface eruptions and move to a narrow zone where stars of all luminosities have approximately the same temperature, around 8,000K. This "active" zone is near the hot edge of the unstable "void" where yellow hypergiants are found, with some overlap. It is not clear whether yellow hypergiants ever manage to get past the instability void to become LBVs or explode as a supernova.

Blue hypergiants are found in the same parts of the HR diagram as LBVs but do not necessarily show the LBV variations. Some but not all LBVs show the characteristics of hypergiant spectra at least some of the time, but many authors would exclude all LBVs from the hypergiant class and treat them separately. Blue hypergiants that do not show LBV characteristics may be progenitors of LBVs, or vice versa, or both. Lower mass LBVs may be a transitional stage to or from cool hypergiants or are different type of object.

Wolf–Rayet stars are extremely hot stars that have lost much or all of their outer layers. WNL is a term used for late stage (i.e. cooler) Wolf–Rayet stars with spectra dominated by nitrogen. Although these are generally thought to be the stage reached by hypergiant stars after sufficient mass loss, it is possible that a small group of hydrogen-rich WNL stars are actually progenitors of blue hypergiants or LBVs. These are the closely related Ofpe (O-type spectra plus H, He, and N emission lines, and other peculiarities) and WN9 (the coolest nitrogen Wolf–Rayet stars) which may be a brief intermediate stage between high mass main-sequence stars and hypergiants or LBVs. Quiescent LBVs have been observed with WNL spectra and apparent Ofpe/WNL stars have changed to show blue hypergiant spectra. High rotation rates cause massive stars to shed their atmospheres quickly and prevent the passage from main sequence to supergiant, so these directly become Wolf–Rayet stars. Wolf Rayet stars, slash stars, cool slash stars (aka WN10/11), Ofpe, Of+, and Of* stars are not considered hypergiants. Although they are luminous and often have strong emission lines, they have characteristic spectra of their own.

Known hypergiants

Hypergiants are difficult to study due to their rarity. Many hypergiants have highly variable spectra, but they are grouped here into broad spectral classes.

Luminous blue variables

Some luminous blue variables are classified as hypergiants, during at least part of their cycle of variation:
  • Eta Carinae, inside the Carina Nebula (NGC 3372) in the southern constellation of Carina. Eta Carinae is extremely massive, possibly as much as 120 to 150 times the mass of the Sun, and is four to five million times as luminous. Possibly a different type of object from the LBVs, or extreme for a LBV.
  • P Cygni, in the northern constellation of Cygnus. Prototype for the general characteristics of LBV spectral lines.
  • S Doradus, in the Large Magellanic Cloud, in the southern constellation of Dorado. Prototype variable, LBVs are still sometimes called S Doradus variables.
  • The Pistol Star (V4627 Sgr), near the center of the Milky Way, in the constellation of Sagittarius. The Pistol Star is possibly as much as 150 times more massive than the Sun, and is about 1.7 million times more luminous. Considered a candidate LBV, but variability has not been confirmed.
  • V4029 Sagittarii
  • V905 Scorpii
  • HD 6884, (R40 in SMC)
  • HD 269700, (R116 in the LMC)
  • LBV 1806-20 in the 1806-20 cluster on the other side of the Milky Way.

Blue hypergiants

A hypergiant star and its proplyd proto-planetary disk compared to the size of the Solar System

Usually B-class, occasionally late O or early A:         candidate with a supergiant companion:
In Galactic Center Region:
  • Star 13, type O, LBV candidate
  • Star 18, type O, LBV candidate
In Westerlund 1:
  • W5 (possible Wolf–Rayet)
  • W7
  • W13 (binary?)
  • W33
  • W42a

Yellow hypergiants

Field surrounding the yellow hypergiant star HR 5171

Yellow hypergiants with late A -K spectra.
In Westerlund 1:
  • W4
  • W8a
  • W12a
  • W16a
  • W32
  • W265

Plus at least two probable cool hypergiants in the recently discovered Scutum Red Supergiant Clusters: F15 and possibly F13 in RSGC1 and Star 49 in RSGC2.

Red hypergiants

Size comparison between the diameter of the Sun and VY Canis Majoris, a hypergiant which is among the largest known stars
 
M type spectra, the largest known stars.

A survey expected to capture virtually all Magellanic Cloud red hypergiants detected around a dozen M class stars Mv−7 and brighter, around a quarter of a million times more luminous than the Sun, and from about 1,000 times the radius of the Sun upwards.
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Type II supernova (Betelgeuse?)

From Wikipedia, the free encyclopedia
 
The expanding remnant of SN 1987A, a Type II-P supernova in the Large Magellanic Cloud. NASA image.

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. 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; those are generally composed of older low-mass stars, with few of the young highly massive stars necessary to cause a supernova. 

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. 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. 

Formation

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. 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.
Core-burning nuclear fusion stages for a 25-solar mass star
Process Main fuel Main products 25 M☉ star
Temperature
(K) 		Density
(g/cm3) 		Duration
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. 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, 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 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. 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. 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.

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.

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. These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos. 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).

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. 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, while the gallium-71-based Baksan instrument detected neutrinos (lepton number = 1) of either thermal or electron-capture origin. 

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. Above this mass, the remnant collapses to form a black hole. 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, 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][failed verification] 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 accelerators can produce particle interactions which are of much higher energy than are found in supernovae, but these experiments involve individual particles interacting with individual particles, and it is likely that the high densities within the supernova will produce novel effects. The interactions between neutrinos and the other particles in the supernova take place with the weak nuclear force, which is believed to be well understood. However, the interactions between the protons and neutrons involve the strong nuclear force, which is much less well understood.

The major unsolved problem with Type II supernovae is that it is not understood how the burst of neutrinos transfers its energy to the rest of the star producing the shock wave which causes the star to explode. From the above discussion, only one percent of the energy needs to be transferred to produce an explosion, but explaining how that one percent of transfer occurs has proven extremely 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.

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 shockwave forms and when and how it stalls and is reenergized.

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.

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.

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.

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 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.

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. The estimated circumstellar density required to explain the observational properties is much higher than that expected from the standard stellar evolution theory. 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. SN 1998S and SN 2005gl are examples of Type IIn supernovae; SN 2006gy, 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. 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. 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 SN 1993J, while another example is Cassiopeia A. The IIb class was first introduced (as a theoretical concept) by Woosley et al. in 1987, and the class was soon applied to SN 1987K and SN 1993J.
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Criticism of marriage

From Wikipedia, the free encyclopedia

Criticisms of marriage are arguments against the practical or moral value of the institution of matrimony or particular forms of matrimony. These have included the effects that marriage has on individual liberty, equality between the sexes, the relation between marriage and violence, philosophical questions about how much control can a government have over its population, the amount of control a person has over another, the financial risk when measured against the divorce rate, and questioning of the necessity to have a relationship sanctioned by government or religious authorities.

Feminist activists often point to historical, legal and social inequalities of wedding, family life and divorce in their criticism of marriage. Sheila Cronan claimed that the freedom for women "cannot be won without the abolition of marriage." "The institution of marriage – wrote Marlene Dixon of the Democratic Workers Party – is the chief vehicle for the perpetuation of the oppression of women; it is through the role of wife that the subjugation of women is maintained". Andrea Dworkin said that marriage as an institution, developed from rape, as a practice.

Early Second Wave feminist literature in the West, specifically opposed to marriage include personalities such as Kate Millett (Sexual Politics, 1969), Germaine Greer (The Female Eunuch, 1970), Marilyn French (The Women's Room, 1977), Jessie Bernard (The Future of Marriage, 1972), and Shulamith Firestone (The Dialectic of Sex: The Case for Feminist Revolution, 1970).

History

Sylvia Pankhurst (1882 – 1960), British feminist, refused to marry her son's father, creating public scandal.
 
In 380 BC, Plato criticised marriage in the Republic. He stated that the idea of marriage was a "natural enemy" of the "commonwealth," aiming for its own higher unity.

In the industrial age a number of notable women writers including Sarah Fielding, Mary Hays, and Mary Wollstonecraft, raised complaints that marriage in their own societies could be characterized as little more than a state of "legal prostitution" with underprivileged women signing in to support themselves. Sociologists Naomi Gerstel and Natalia Sarkisian wrote that marriage is also found to be often at odds with community, diminishing ties to relatives, neighbors, and friends. The Lonely American: Drifting Apart in the Twenty-First Century] by Jacqueline Olds and Richard S. Schwartz. According to Dan Moller's "Bachelor's Argument", modern marriage can be compared to the act of "forging professional credentials." Over 40 percent of them fail and therefore should be avoided similar to any high-risk venture.

Commentators have often been critical of individual local practices and traditions, leading to historical changes. Examples include the early Catholic Church's efforts to eliminate concubinage and temporary marriage, the Protestant acceptance of divorce, and the abolition of laws against inter-faith and inter-race marriages in the western countries.

The decision not to marry is a presumed consequence of Søren Kierkegaard's philosophy. His well-documented relationship with Regine Olsen is a subject of study in existentialism, as he called off their engagement despite mutual love. Kierkegaard seems to have loved Regine but was unable to reconcile the prospect of marriage with his vocation as a writer and his passionate and introspective Christianity.

A similar argument is found in Franz Kafka's journal entry titled "Summary of all the arguments for and against my marriage":
I must be alone a great deal. What I accomplished was only the result of being alone.
As a high-profile couple, Jean-Paul Sartre and Simone de Beauvoir always expressed opposition to marriage. Brian Sawyer says "Marriage, understood existentially, proposes to join two free selves into one heading, thus denying the freedom, the complete foundation, of each self."

Presently, the high divorce rates are leading to questioning of the purpose of marriage. Some contemporary critics of marriage question why governments (in Western countries) continue to support marriage, when it has such a high failure rate. Anthropologist Lionel Tiger wrote:
It is astonishing that, under the circumstances, marriage is still legally allowed. If nearly half of anything else ended so disastrously, the government would surely ban it immediately. If half the tacos served in restaurants caused dysentery, if half the people learning karate broke their palms, if only 6 percent of people who went on roller coaster rides damaged their middle ears, the public would be clamoring for action. Yet the most intimate of disasters...happens over and over again.
In response to the passage of California Proposition 22 and the current controversy regarding same-sex unions in the United States, a group of people have banded together to boycott marriage until all people can legally marry. The argument is that since marriage is not an inclusive institution of society, the members of the boycott refuse to support the institution as it exists.

In the West, cohabitation and births outside marriage are becoming more common. In the United States, conservative and religious commentators are highly critical of this trend. They are also often critical of present-day marriage law and the ease of divorce. John Witte, Jr., Professor of Law and director of the Law and Religion Program at Emory University, argues that contemporary liberal attitudes toward marriage produce a family that is "haphazardly bound together in the common pursuit of selfish ends" exactly as prophesied by Nietzsche. In his From Sacrament to Contract, Witte has argued that John Stuart Mill's secular and contractarian model of marriage, developed during the Enlightenment, provided the theoretical justification for the present-day transformation of Anglo-American marriage law, promoting unqualified "right to divorce" on plaintiff's demand, one-time division of property, and child custody without regard for marital misconduct. A Catholic professor Romano Cessario, in a review of Witte's book published in an ecumenical journal the First Things, suggested that a solution to the current crisis of marriage in the West, could come from the possible revival of the sacramental marriage among Christians, thus counterbalancing Nietzsche's pessimism (as echoed by Witte).

Cultural criticisms

"Esposas de Matrimonio" ("Wedding Cuffs"), a wedding ring sculpture expressing the criticism of marriages' effects on individual liberty. Esposas is a Spanish pun, in which the singular form of the word esposa refers to a spouse, and the plural refers to handcuffs.
 

Male dominance

Critics of marriage argue that it is an institution which contributes to the maintaining of traditional gender roles, thus preventing women from achieving social equality, and reinforcing the idea that women exist to serve men, which in turn increases the abuse of women. They argue that marriage reinforces the traditional paradigm of male-female interaction: subordination of the woman to the man in exchange of subsistence. According to Sheila Jeffreys "the traditional elements of marriage have not completely disappeared in western societies, even in the case of employed, highly educated and well paid professional women". She argues that even such women remain in abusive marriages out of fear of leaving and out of duty. Even in Western countries, married women "feel they have no choice but to stay and endure and may be 'loving to survive".

Normalization and discrimination

Some commentators criticize government authorities for promotion of marriage. They also criticize the romanticized image that marriage is given in films and romance novels. Over 40% of books sold in America were romance novels.

Some critics argue that people cannot form an objective image of what marriage is if they are from early childhood indoctrinated into believing marriage is desirable and necessary.

Critics of marriage argue that this institution represents a form of state sponsored discrimination, in a generalized way against people who do not marry, and in a particular way against certain racial or ethnic groups who are less likely to marry and more likely to have children outside marriage, such as African Americans in the US - by stigmatizing such individuals, presenting their lifestyle as abnormal and denying them rights. Dean Spade and Craig Willse write that
The idea that married families and their children are superior was and remains a key tool of anti-Black racism. Black families have consistently been portrayed as pathological and criminal in academic research and social policy based on marriage rates, most famously in the Moynihan Report.

Social isolation

What is it about modern coupledom that makes policing another person's behaviour a synonym for intimacy? (Or is it something about the conditions of modern life itself: is domesticity a venue for control because most of us have so little of it elsewhere?) Then there's the fundamental premise of monogamous marriage: that mutual desire can and will last throughout a lifetime. And if it doesn't? Well apparently you're just supposed to give up on sex, since waning desire for your mate is never an adequate defence for 'looking elsewhere'. At the same time, let's not forget how many booming businesses and new technologies have arisen to prop up sagging marital desire. Consider all the investment opportunities afforded: Viagra, couples pornography, therapy. If upholding monogamy in the absence of desire weren't a social dictate, how many enterprises would immediately fail?
— American cultural critic and essayist Laura Kipnis, 2003
A criticism of marriage is that it may lead to the social isolation of a person, who is often expected to diminish other relations with friends, relatives or colleagues, in order to be fully dedicated to their spouse. Julie Bindel writes that: "Maybe those at the most risk of ending up alone are not the folk who never marry, but rather the people who chuck all their eggs in one basket. [...] During their marriage, believing as they did that they only needed each other, both parties would have neglected friendships, or indeed, failed to cultivate new ones".

Symbolism

Some critics assert that marriage will always remain a symbolic institution signifying the subordination of women to men. Clare Chambers points to the sexist traditions surrounding marriage and weddings; she writes:
Symbolically, the white wedding asserts that women's ultimate dream and purpose is to marry, and remains replete with sexist imagery: the white dress denoting the bride's virginity (and emphasising the importance of her appearance); the minister telling the husband "you may now kiss the bride" (rather than the bride herself giving permission, or indeed initiating or at least equally participating in the act of kissing); the reception at which, traditionally, all the speeches are given by men; the wife surrendering her own name and taking her husband's.
The history of marriage in relation to women makes it an institution that some critics argue cannot and should not be accepted in the 21st century; to do so would mean to trivialize the abuses it was responsible for. Some critics argue that it is impossible to dissociate marriage from its past. Clare Chambers argues that:
(...) it is impossible to escape the history of the institution. Its status as a tradition ties its current meaning to its past". Past abuses of marriage are sometimes depicted in documentaries. A documentary in Ireland presented the story of elderly women who described their experiences with repeated acts of rape in marriage and the children born from these rapes, during the time when marital rape was not criminalized, contraception, abortion and divorce were all illegal, and the marriage bar restricting married women's employment outside home was in force. Marital rape in Ireland was made illegal in 1990, and divorce was legalized in 1996.

Violence against women

Violence related to female virginity is considered a problem. In many parts of the world it is socially expected for the bride to be a virgin; if the husband has sex with his wife after marriage and she does not bleed (it is possible for a woman to not bleed when she has sex for the first time ), this can end in extreme violence, including an honor killing.

The common view of marital life as "private" and outside the sphere of public intervention allows violence to flourish. Elizabeth Brake writes that ""privacy" protects unequal divisions of domestic labor, domestic violence, and exclusion of health coverage for abortion and contraception." Mary Lyndon Shanley writes that police often "ignore complaints of domestic violence because they do not want to "intrude" on the private realm of the married couple".

Legal criticisms


Economic dependence

Marriage has been criticized in its complicity of wives' economic dependence on husbands due to the gendered division of labour and that women's' work typically pays less than men's work. Women are more likely to downgrade or drop out of their careers to assist in child rearing or when their career conflicts with their husband's. Absent a career, women become dependent on legally granted marriage benefits such as a husband's health insurance, and are thus increasingly dependent on their husband. This dependence can facilitate abuse because the marriage becomes economically difficult to leave.

Immobility

In some conservative cultures, married women are not allowed to leave home without the consent of the husband - a prohibition that is supported by the law itself in many of these countries. For instance, in Yemen marriage regulations state that a wife must obey her husband and must not leave home without his permission.

Marital rape

Historically, in many cultures marriage has been used to regulate sexuality, rather than consent regulating it. That is, non-marital sex was banned regardless of consent, while marital sex was an enforceable obligation. From the mid-20th century onward, changing social norms have led to, among other things, the decriminalization of consensual non-marital sex and the criminalization of marital rape. These changes are not universal around the world, and in many countries they have not occurred. One of the concerns about marriage is that it may contradict the notion of sexual self-determination, due to cultural, religious, and in many countries also legal norms. For instance, sex outside marriage is still punishable by death in some jurisdictions. In 2014, Amnesty International's Secretary General stated that "It is unbelievable that in the twenty-first century some countries are condoning child marriage and marital rape while others are outlawing abortion, sex outside marriage and same-sex sexual activity – even punishable by death."

In various places, men have sexual authority over their wives, in law and in practice. The men decide when and where to have sex, and wives have no power to stop unwanted sex. In certain countries marital rape is legal, and even where it is illegal it is infrequently reported or prosecuted. Often, married women also cannot stop unwanted pregnancy, because in various countries modern contraception is not available, and in some countries married women need legal permission from the husband to use contraception (and even in countries where the husband's consent is not legally required in practice it is asked for), and abortion is illegal or restricted, and in some countries married women need the consent of husband for abortion. Therefore, marriage leads to a situation which allows not only forced sex, but also forced pregnancy, and in some of these countries pregnancy and childbirth remain dangerous because of lack of adequate medical care. The effects of sexual violence inside marriage are exacerbated by the practice of child marriage; in 2013 an 8-years-old Yemeni girl died from internal bleeding after she was raped by her 40-year-old new husband. Sheila Jeffreys argues that the very institution of marriage is based on the idea that heterosexual sex is the absolute right of the man and the absolute duty of the woman; that men are entitled to demand sex on their terms and to coerce sex, and women are not allowed to ever refuse it. Lack of economic opportunity means that wives have no choice but to "allow sexual access to their bodies in return for subsistence".

Relationship favoritism

Another issue is the question on why relations that are (or are believed to be) sexual are favored in law with regard to legal protections and promotion, and those that are not (or are believed not to be) are not. This is especially the case as marriage rates are quite low in many Western countries, and the state has been criticized for ingoing other living arrangements that are not sexual relations; and there have been increased objections to legal concepts such as consummation or adultery that critics argue do not belong in modern law. It is argued that with regard to family life, the state should regulate the parental rights and responsibilities of parents, not focus on whether there is an ongoing sexual/romantic relation between the parents.

State control

A criticism of marriage is that it gives the state an undue power and control over the private lives of the citizens. The statutes governing marriage are drafted by the state, and not by the couples who marry under those laws. The laws may, at any time, be changed by the state without the consent (or even knowledge) of the married people. The terms derived from the principles of institutionalized marriage represent the interests of the governments.

Critics of marriage argue that it is an institution based on control, domination and possession, and that attempting to exercise control over another person's life is immoral and dangerous, and should not be encouraged by the state. Claudia Card, professor of Philosophy at the University of Wisconsin-Madison, writes that:
The legal rights of access that married partners have to each other's persons, property, and lives makes it all but impossible for a spouse to defend herself (or himself), or to be protected against torture, rape, battery, stalking, mayhem, or murder by the other spouse... Legal marriage thus enlists state support for conditions conducive to murder and mayhem.

Violence against women

Anti-dowry poster in Bangalore, India.
 
The United Nations General Assembly defines "violence against women" as "any act of gender-based violence that results in, or is likely to result in, physical, sexual or mental harm or suffering to women, including threats of such acts, coercion or arbitrary deprivation of liberty, whether occurring in public or in private life." The 1993 Declaration on the Elimination of Violence Against Women noted that this violence could be perpetrated by assailants of either gender, family members and even the "State" itself.

Critics of marriage argue that it is complicit in the mistreatment and subjugation of women across the world. Common concerns raised today focus on the health and general well-being of women, who, in parts of the world, have virtually no protection in law or in practice, against domestic violence within marriage. It is also nearly impossible for women there to get out of abusive relationships. Abuses are upheld by claims of possession and entitlement in some cultures and the well-being of women is undermined by a powerful act of subordination. According to Gerstel and Sarkisian, domestic violence, isolation, and housework tend to increase for women who sign marriage contracts. Those with lower income draw even fewer benefits from it. Bad marriages, according to Gerstel and Sarkisian, result in higher levels of stress, suicide, hypertension, cancer, and slower wound healing in women.

Opponents of legal marriage contend that it encourages violence against women, both through practices carried out within a marriage (such as beating and rape inside marriage - which are legal in some countries and tolerated in many more), and through acts related to marital customs (such as honor killings for refusing arranged marriages; forcing rape victims to marry their rapist, marriage by abduction; or executions for sex outside marriage). In some parts of the world, the extreme stigma cast on women who have reached a certain age and are still unmarried often leads these women to suicide. Suicide is also a common response of women caught in abusive marriages with no possibility of leaving those marriages. Women who are faced with the prospect of forced marriage may commit suicide. Violence and trafficking related to payment of dowry and bride price are also problems. Dowry deaths especially occur in South Asia, and acid throwing is also a result of disputes related to dowry conflicts.

In various countries married men have authority over their wives. For instance, Yemeni marriage regulations state that a wife must obey her husband and must not leave home without his permission. In Iraq husbands have a legal right to punish their wives. The criminal code states that there is no crime if an act is committed while exercising a legal right. Examples of legal rights include: "The punishment of a wife by her husband, the disciplining by parents and teachers of children under their authority within certain limits prescribed by law or by custom". In the Democratic Republic of Congo the Family Code states that the husband is the head of the household; the wife owes her obedience to her husband; a wife has to live with her husband wherever he chooses to live; and wives must have their husbands' authorization to bring a case in court or to initiate other legal proceedings.

Specific criticisms


Anarchist

Famous anarchist Emma Goldman wrote how marriage was not a love pact but an economic agreement that restricts men's and mainly women's freedoms. She criticized how wives were surrendered freedoms permanently for the sake of marriage, and how sexuality and child rearing outside of marriage is shamed. 
 

Feminist

Marriage is a focus of many feminist concerns. Of these many cultural concerns include the fact that within many marriages women are generally expected to do most of the work in the home, even if they had careers outside the home. A more economic concern is that marriage may also foster economic dependence since women's work is underpaid and women are expected to downgrade their careers when their careers conflict with their husband's or with work in the home. Without appropriate finances women can become dependent on their husband's marriage benefits like health insurance.

Some feminists have argued for the reform of marriage while others have argued for its abolition arguing it is entrenched in sexist cultural norms and a legal structure that promotes it.

Marxist

The separation of the family from the clan and the institution of monogamous marriage were the social expressions of developing private property; so-called monogamy afforded the means through which property could be individually inherited. And private property for some meant no property for others, or the emerging of differing relations to production on the part of different social groups. The core of Engels’ formulation lies in the intimate connection between the emergence of the family as an economic unit dominated by the male and this development of classes.
— Anthropologist and social theorist Eleanor Leacock
Within early Marxist texts there existed critiques of marriage. Friedrich Engels wrote how the origins of marriage were not for the purposes of love but instead for private property rights. Monogamous marriage became an institution to be the base of the family and solidify a system for the family to handle private property and its inheritance. Monogamy would later spur on adultery and the business of prostitution.

In the book The Second Sex author Simone de Beauvoir argues that marriage is an alienating institution. Men can become tied to supporting a wife and children and women can become dependent on their husbands, and for children who become the target of rage when the stresses of marriage overwhelm their parents. She argues about marriage that "Any institution which solders one person to another, obliging people to sleep together who no longer want to is a bad one".

Queer theory

Within Queer theory a critique exists that the legalization of same-sex marriage simply normalizes the cultured gender norms and economic inequalities of marriage into the LGBT community. Also that the normalization of marriage delegitimizes non-monogamous relationships which are considered common in the LGBT community.
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