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Type II supernova
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<h2 id="formation">Formation</h2> <p>Stars far more massive than the sun evolve in complex ways. In the core of the star, <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> is <a href="/facts/Thermonuclear_fusion/fzQUW49h">fused</a> into <a href="/facts/Helium/yRul93TG">helium</a>, releasing <a href="/facts/Thermal_energy/eVaKAC7K">thermal energy</a> that heats the star's core and provides outward <a href="/facts/Pressure/eyc37GRG">pressure</a> that supports the star's layers against collapse – a situation known as stellar or <a href="/facts/Hydrostatic_equilibrium/e75gVkh2">hydrostatic equilibrium</a>. The helium produced in the core accumulates there. 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 <a href="/facts/Gravitation/bqAN7DdQ">gravity</a> causes the core to contract. This contraction raises the temperature high enough to allow a shorter phase of helium fusion, which produces <a href="/facts/Carbon/ukrgQexo">carbon</a> and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, and accounts for less than 10% of the star's total lifetime. </p><p>In stars of less than eight solar masses, the <a href="/facts/Carbon/ukrgQexo">carbon</a> produced by helium fusion does not fuse, and the star gradually cools to become a <a href="/facts/White_dwarf/gIYlTtrC">white dwarf</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> If they accumulate more mass from another star, or some other source, they may become <a href="/facts/Type_Ia_supernova/W4rEFuDK">Type Ia supernovae</a>. But a much larger star is massive enough to continue fusion beyond this point. </p><p>The cores of these massive stars directly 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 core gradually becomes layered like an onion, 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 <a href="/facts/Triple-alpha_process/wV6lacSF">triple-alpha process</a>, 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.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> Core-burning nuclear fusion stages for a 25-<a href="/facts/Solar_mass/j6Auhku8">solar mass</a> star<table><tbody><tr><th rowspan="2">Process</th><th rowspan="2">Main fuel</th><th rowspan="2">Main products</th><th colspan="3">25 <a href="/facts/Solar_mass/j6Auhku8">M☉</a> star<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></th></tr><tr><th>Temperature(<a href="/facts/Kelvin/E3trzC4C">K</a>)</th><th>Density(g/cm3)</th><th>Duration</th></tr><tr><td><a href="/facts/Stellar_nucleosynthesis/qEQJdJe6">hydrogen burning</a></td><td><a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a></td><td><a href="/facts/Helium/yRul93TG">helium</a></td><td>7×107</td><td>10</td><td>107 years</td></tr><tr><td><a href="/facts/Triple-alpha_process/wV6lacSF">triple-alpha process</a></td><td><a href="/facts/Helium/yRul93TG">helium</a></td><td><a href="/facts/Carbon/ukrgQexo">carbon</a>, <a href="/facts/Oxygen/acyn6o8r">oxygen</a></td><td>2×108</td><td>2000</td><td>106 years</td></tr><tr><td><a href="/facts/Carbon-burning_process/MbHbTB03">carbon-burning process</a></td><td><a href="/facts/Carbon/ukrgQexo">carbon</a></td><td><a href="/facts/Neon/5bF1FwGL">Ne</a>, <a href="/facts/Sodium/eYuSPu2V">Na</a>, <a href="/facts/Magnesium/XyIhd2FG">Mg</a>, <a href="/facts/Aluminium/7wmR8jYE">Al</a></td><td>8×108</td><td>106</td><td>1000 years</td></tr><tr><td><a href="/facts/Neon-burning_process/gFaHhLDK">neon-burning process</a></td><td><a href="/facts/Neon/5bF1FwGL">neon</a></td><td><a href="/facts/Oxygen/acyn6o8r">O</a>, <a href="/facts/Magnesium/XyIhd2FG">Mg</a></td><td>1.6×109</td><td>107</td><td>3 years</td></tr><tr><td><a href="/facts/Oxygen-burning_process/o1uGg3kL">oxygen-burning process</a></td><td><a href="/facts/Oxygen/acyn6o8r">oxygen</a></td><td><a href="/facts/Silicon/cgWx0hdr">Si</a>, <a href="/facts/Sulfur/IK0amTfM">S</a>, <a href="/facts/Argon/QX6NRH6d">Ar</a>, <a href="/facts/Calcium/hf252CC0">Ca</a></td><td>1.8×109</td><td>107</td><td>0.3 years</td></tr><tr><td><a href="/facts/Silicon-burning_process/qpYtHo8c">silicon-burning process</a></td><td><a href="/facts/Silicon/cgWx0hdr">silicon</a></td><td><a href="/facts/Nickel/KwMCHYRP">nickel</a> (decays into <a href="/facts/Iron/DkICCKIC">iron</a>)</td><td>2.5×109</td><td>108</td><td>5 days</td></tr></tbody></table> <h2 id="core-collapse">Core collapse</h2> <p>The factor limiting this process is the amount of energy that is released through fusion, which is dependent on the <a href="/facts/Binding_energy/2Q3poXgg">binding energy</a> that holds together these atomic nuclei. Each additional step produces progressively heavier nuclei, which release progressively less energy when fusing. In addition, from <a href="/facts/Carbon-burning_process/MbHbTB03">carbon-burning</a> onwards, energy loss via <a href="/facts/Neutrino/yGWVfSg2">neutrino</a> production becomes significant, leading to a higher rate of reaction than would otherwise take place.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> This continues until <a href="/facts/Silicon_burning_process/qpYtHo8c">nickel-56</a> is produced, which decays radioactively into <a href="/facts/Cobalt/gqhScEw4">cobalt-56</a> and then <a href="/facts/Iron/DkICCKIC">iron-56</a> over the course of a few months. As iron and nickel have the highest <a href="/facts/Binding_energy/2Q3poXgg">binding energy</a> per <a href="/facts/Nucleon/6fguU9Xk">nucleon</a> of all the elements,<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> energy cannot be produced at the core by fusion, and a nickel-iron core grows.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> 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 <a href="/facts/Degeneracy_pressure/S5bc5MwK">degeneracy pressure</a> of <a href="/facts/Electrons/L0KBo5cW">electrons</a>. In this state, matter is so dense that further compaction would require electrons to occupy the same <a href="/facts/Energy_level/66x51Khc">energy states</a>. However, this is forbidden for identical <a href="/facts/Fermion/LEckdDsR">fermion</a> particles, such as the electron – a phenomenon called the <a href="/facts/Pauli_exclusion_principle/lsBKGeHD">Pauli exclusion principle</a>. </p><p>When the core's mass exceeds the <a href="/facts/Chandrasekhar_limit/KojyANXK">Chandrasekhar limit</a> of about 1.4 <a href="/facts/Solar_mass/j6Auhku8">M☉</a>, degeneracy pressure can no longer support it, and catastrophic collapse ensues.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The outer part of the core reaches velocities of up to 70000 km/s (23% of the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a>) as it collapses toward the center of the star.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> The rapidly shrinking core heats up, producing high-energy <a href="/facts/Gamma_rays/5Drf913J">gamma rays</a> that decompose iron <a href="/facts/Atomic_nucleus/orpIm5Xl">nuclei</a> into helium nuclei and free <a href="/facts/Neutron/oRCM1geb">neutrons</a> via <a href="/facts/Photodisintegration/7TKyEJtc">photodisintegration</a>. As the core's <a href="/facts/Density/92p6hh9I">density</a> increases, it becomes energetically favorable for <a href="/facts/Electron/L0KBo5cW">electrons</a> and <a href="/facts/Proton/clCjepXP">protons</a> to merge via inverse <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>, producing neutrons and <a href="/facts/Elementary_particle/CTFDM97U">elementary particles</a> called <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a>. 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.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p><p>For Type II supernovae, the collapse is eventually halted by short-range repulsive neutron-neutron interactions, mediated by the <a href="/facts/Strong_force/JjRsKcLg">strong force</a>, as well as by <a href="/facts/Degeneracy_pressure/S5bc5MwK">degeneracy pressure</a> of neutrons, at a density comparable to that of an atomic nucleus. When the collapse stops, the infalling matter rebounds, producing a <a href="/facts/Shock_wave/CqrG1Xm5">shock wave</a> 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.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p><p>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 <a href="/facts/Electron_capture/EIccoV1u">electron capture</a>, an electron neutrino is produced. In a typical Type II supernova, the newly formed neutron core has an initial temperature of about 100 billion <a href="/facts/Kelvin/E3trzC4C">kelvins</a>, 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.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> These 'thermal' neutrinos form as neutrino-antineutrino pairs of all <a href="/facts/Neutrino_oscillation/c1cc5noc">flavors</a>, and total several times the number of electron-capture neutrinos.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> The two neutrino production mechanisms convert the <a href="/facts/Gravitational_energy/SJpNw6G5">gravitational potential energy</a> of the collapse into a ten-second neutrino burst, releasing about 1046 joules (100 <a href="/facts/Foe_(unit)/QhAzE4IG">foe</a>).<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p><p>Through a process that is not clearly understood, about 1%, or 1044 joules (1 foe), of the energy released (in the form of <a href="/facts/Supernova_neutrinos/p1w54oDl">neutrinos</a>) is reabsorbed by the stalled shock, producing the supernova explosion.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> <a href="/facts/Supernova_neutrinos/p1w54oDl">Neutrinos generated by a supernova</a> were observed in the case of <a href="/facts/Supernova_1987A/zHyoUpBC">Supernova 1987A</a>, leading astrophysicists to conclude that the core collapse picture is basically correct. The water-based <a href="/facts/Kamioka_Observatory/1qPLDlEz">Kamiokande II</a> and <a href="/facts/Irvine-Michigan-Brookhaven_(detector)/Yq6gkUYj">IMB</a> instruments detected antineutrinos of thermal origin,<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> while the <a href="/facts/Gallium/EucevDs9">gallium</a>-71-based <a href="/facts/Baksan_Neutrino_Observatory/4XnmbHAW">Baksan</a> instrument detected neutrinos (<a href="/facts/Lepton_number/vvocnLqR">lepton number</a> = 1) of either thermal or electron-capture origin. </p> <p>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 <a href="/facts/Neutron_star/4enfoxBA">neutron star</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> Above this mass, the remnant collapses to form a <a href="/facts/Black_hole/1r2lmkcF">black hole</a>.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> 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,<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> although uncertainties in models of supernova collapse make calculation of these limits uncertain. </p> <h2 id="theoretical-models">Theoretical models</h2> <p>The <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a> of <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a> is a theory which describes three of the four known <a href="/facts/Fundamental_interaction/WrFEJaFl">fundamental interactions</a> between the <a href="/facts/Elementary_particles/CTFDM97U">elementary particles</a> that make up all <a href="/facts/Matter/osyc04UN">matter</a>. 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 <a href="/facts/Picojoule/9ndq9jD2">picojoules</a> (tens to hundreds of <a href="/facts/MeV/YMT1fqX9">MeV</a>).<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> 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.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> In particular, Earth-based <a href="/facts/Particle_accelerator/Pkuc08xC">particle accelerators</a> can produce particle interactions which are of much higher energy than are found in supernovae,<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> 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 <a href="/facts/Weak_nuclear_force/f6upmKMs">weak nuclear force</a>, which is believed to be well understood. However, the interactions between the protons and neutrons involve the <a href="/facts/Strong_nuclear_force/JjRsKcLg">strong nuclear force</a>, which is much less well understood.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p><p>The major unsolved problem with Type II supernovae is that it is not understood how the burst of <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a> 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 <a href="/facts/Convective_overturn/RWFv24LI">convective overturn</a>, which suggests that convection, either from <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a> 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 <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a>, 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.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> </p><p><a href="/facts/Weak_interaction/f6upmKMs">Neutrino physics</a>, which is modeled by the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>, is crucial to the understanding of this process.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> The other crucial area of investigation is the <a href="/facts/Hydrodynamics/y7uzSCfg">hydrodynamics</a> of the plasma that makes up the dying star; how it behaves during the core collapse determines when and how the <a href="/facts/Shockwave/CqrG1Xm5">shockwave</a> forms and when and how it stalls and is reenergized.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p><p><a href="/facts/Computer_model/Re6o8vzN">Computer models</a> 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, <a href="/facts/Astrophysicist/krFPRWLA">astrophysicists</a> have been able to make detailed predictions about the elements produced by the supernova and of the expected <a href="/facts/Light_curve/zN0wdtaR">light curve</a> from the supernova.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a><a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> </p> <h2 id="light-curves-for-typeii-l-and-typeii-p-supernovae">Light curves for Type II-L and Type II-P supernovae</h2> <p>When the <a href="/facts/Stellar_spectrum/NDGQ7mmM">spectrum</a> of a Type II supernova is examined, it normally displays <a href="/facts/Balmer_series/Vc4s4SIt">Balmer absorption lines</a> – reduced flux at the characteristic <a href="/facts/Frequency/QUSbPaW8">frequencies</a> where hydrogen atoms absorb energy. The presence of these lines is used to distinguish this category of supernova from a <a href="/facts/Type_Ia_supernova/W4rEFuDK">Type I supernova</a>. </p><p>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 <a href="/facts/Absolute_magnitude/R8RVxkLx">magnitudes</a> 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 (<a href="/facts/Linear/fChtg3KC">linear</a>) decline following the peak brightness. By contrast, the light curve of a Type II-P supernova has a distinctive flat stretch (called a <a href="/facts/Plateau/RtbmsmfB">plateau</a>) 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.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> The plateau phase in Type II-P supernovae is due to a change in the <a href="/facts/Opacity_(optics)/l90q0sAV">opacity</a> of the exterior layer. The shock wave <a href="/facts/Ionize/W13Ceavd">ionizes</a> the hydrogen in the outer envelope – stripping the electron from the hydrogen atom – resulting in a significant increase in the <a href="/facts/Opacity_(optics)/l90q0sAV">opacity</a>. This prevents photons from the inner parts of the explosion from escaping. When the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p> <h2 id="typeiin-supernovae">Type IIn supernovae</h2> <p>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.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a><a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> The estimated circumstellar density required to explain the observational properties is much higher than that expected from the standard <a href="/facts/Stellar_evolution/Qa4vAGmn">stellar evolution</a> theory.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> 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 <a href="/facts/Luminous_blue_variable/thWkdoWc">luminous blue variables</a> with large mass losses before exploding.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> <a href="/facts/SN_1998S/tFdfWraL">SN 1998S</a> and <a href="/facts/SN_2005gl/TfjXLCsP">SN 2005gl</a> are examples of Type IIn supernovae; <a href="/facts/SN_2006gy/r6UergHc">SN 2006gy</a>, an extremely energetic supernova, may be another example.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> </p><p>Some supernovae of Type IIn show interactions with the circumstellar medium, which leads to an increased temperature of the <a href="/facts/Circumstellar_dust/rCpC6KL0">cirumstellar dust</a>. This warm dust can be observed as a brightening in the <a href="/facts/Mid-infrared/44XdJNcc">mid-infrared</a> light. If the circumstellar medium extends further from the supernova, the mid-infrared brightening can cause an <a href="/facts/Cassiopeia_A/wqKS2fqb">infrared echo</a>, causing the brightening to last more than 1000 days. These kind of supernovae belong to the rare 2010jl-like supernovae, named after the archetypal <a href="/facts/SN_2010jl/hqecbvzp">SN 2010jl</a>. Most 2010jl-like supernovae were discovered with the decommissioned <a href="/facts/Spitzer_Space_Telescope/Ic1xXIYb">Spitzer Space Telescope</a> and the <a href="/facts/Wide-field_Infrared_Survey_Explorer/7st7krr6">Wide-Field Infrared Survey Explorer</a> (e.g. SN 2014ab, SN 2017hcc).<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a><a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a><a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a><a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a> </p> <h2 id="typeiib-supernovae">Type IIb supernovae</h2> <p>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 <a href="/facts/Type_Ib_and_Ic_supernovae/UgGGLu6C">Type Ib supernova</a>. 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.<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes <a href="/facts/Optical_thickness/XQwtIdFH">more transparent</a> and reveals the deeper layers.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> The classic example of a Type IIb supernova is <a href="/facts/SN_1993J/EJAfREaR">SN 1993J</a>,<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a><a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> while another example is <a href="/facts/Cassiopeia_A/wqKS2fqb">Cassiopeia A</a>.<a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a> The IIb class was first introduced (as a theoretical concept) by Woosley et al. in 1987,<a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> and the class was soon applied to SN 1987K<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a> and <a href="/facts/SN_1993J/EJAfREaR">SN 1993J</a>.<a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/History_of_supernova_observation/x9w8uJV2">History of supernova observation</a></li> <li><a href="/facts/Supernova_remnant/tYboe2n4">Supernova remnant</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20170110014854/https://sne.space/?claimedtype=ii">"List of all known Type II supernovae"</a>. <i>The Open Supernova Catalog</i>. Archived from <a href="https://sne.space/?claimedtype=ii">the original</a> on 2017-01-10. Retrieved 2017-01-09.</li> <li>Merrifield, Michael. <a href="/facts/Brady_Haran/7oNb0Tez">Haran, B.</a> (ed.). <a href="http://www.sixtysymbols.com/videos/supernova.htm"><i>Type II supernovae</i></a>. <i>Sixty Symbols</i> (video lecture). Nottingham, UK: <a href="/facts/University_of_Nottingham/PR8osn19">University of Nottingham</a>.</li> <li>Gibney, Elizabeth (2018-04-18). <a href="https://doi.org/10.1038%2Fd41586-018-04601-7">"How to blow up a star"</a>. <i><a href="/facts/Nature_(journal)/XmlJXkFx">Nature</a></i>. 556 (7701): 287–289. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2018Natur.556..287G">2018Natur.556..287G</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1038%2Fd41586-018-04601-7">10.1038/d41586-018-04601-7</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/29670276">29670276</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:4956943">4956943</a>.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Prantzos, N (1996). "Stellar nucleosynthesis and γ-ray line astronomy". 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