an envelope of hydrogen-rich circumstellar material. These supernovae have been dubbed type Ia/IIn, type Ian, type IIa and type IIan. The quadruple star HD 74438, belonging to the open cluster IC 2391 the Vela constellation, has been predicted to become a non-standard type Ia supernova. Core collapse Very massive stars can undergo core collapse when nuclear fusion becomes unable to sustain the core against its own gravity; passing this threshold is the cause of all types of supernova except type Ia. The collapse may cause violent expulsion of the outer layers of the star resulting in a supernova. However, if the release of gravitational potential energy is insufficient, the star may instead collapse into a black hole or neutron star with little radiated energy. Core collapse can be caused by several different mechanisms: exceeding the Chandrasekhar limit; electron capture; pair-instability; or photodisintegration. When a massive star develops an iron core larger than the Chandrasekhar mass it will no longer be able to support itself by electron degeneracy pressure and will collapse further to a neutron star or black hole. Electron capture by magnesium in a degenerate O/Ne/Mg core (8–10 solar mass progenitor star) removes support and causes gravitational collapse followed by explosive oxygen fusion, with very similar results. Electron-positron pair production in a large post-helium burning core removes thermodynamic support and causes initial collapse followed by runaway fusion, resulting in a pair-instability supernova. A sufficiently large and hot stellar core may generate gamma-rays energetic enough to initiate photodisintegration directly, which will cause a complete collapse of the core. The table below lists the known reasons for core collapse in massive stars, the types of stars in which they occur, their associated supernova type, and the remnant produced. The metallicity is the proportion of elements other than hydrogen or helium, as compared to the Sun. The initial mass is the mass of the star prior to the supernova event, given in multiples of the Sun's mass, although the mass at the time of the supernova may be much lower. Type IIn supernovae are not listed in the table. They can be produced by various types of core collapse in different progenitor stars, possibly even by type Ia white dwarf ignitions, although it seems that most will be from iron core collapse in luminous supergiants or hypergiants (including LBVs). The narrow spectral lines for which they are named occur because the supernova