ago, hot supergiants were not considered likely to explode, but observations have shown otherwise. Blue supergiants form an unexpectedly high proportion of confirmed supernova progenitors, partly due to their high luminosity and easy detection, while not a single Wolf–Rayet progenitor has yet been clearly identified. Models have had difficulty showing how blue supergiants lose enough mass to reach supernova without progressing to a different evolutionary stage. One study has shown a possible route for low-luminosity post-red supergiant luminous blue variables to collapse, most likely as a type IIn supernova. Several examples of hot luminous progenitors of type IIn supernovae have been detected: SN 2005gy and SN 2010jl were both apparently massive luminous stars, but are very distant; and SN 2009ip had a highly luminous progenitor likely to have been an LBV, but is a peculiar supernova whose exact nature is disputed. The progenitors of type Ib/c supernovae are not observed at all, and constraints on their possible luminosity are often lower than those of known WC stars. WO stars are extremely rare and visually relatively faint, so it is difficult to say whether such progenitors are missing or just yet to be observed. Very luminous progenitors have not been securely identified, despite numerous supernovae being observed near enough that such progenitors would have been clearly imaged. Population modelling shows that the observed type Ib/c supernovae could be reproduced by a mixture of single massive stars and stripped-envelope stars from interacting binary systems. The continued lack of unambiguous detection of progenitors for normal type Ib and Ic supernovae may be due to most massive stars collapsing directly to a black hole without a supernova outburst. Most of these supernovae are then produced from lower-mass low-luminosity helium stars in binary systems. A small number would be from rapidly-rotating massive stars, likely corresponding to the highly-energetic type Ic-BL events that are associated with long-duration gamma-ray bursts. External impact Supernovae events generate heavier elements that are scattered throughout the surrounding interstellar medium. The expanding shock wave from a supernovae can trigger star formation. Galactic cosmic rays are generated by supernova explosions. Source of heavy elements Supernovae are a major source of elements in the interstellar medium from oxygen through to rubidium, though the theoretical abundances of the elements produced or seen in the spectra varies significantly depending on the various supernova types. Type Ia supernovae produce mainly silicon and iron-peak elements, metals such as