What is Supernova?
{{Distinguish|Nova}} [[Image:SN2005cs.jpg|right|300px|Photograph of supernova in another galaxy. The supernova is pointed by the arrow. The other bright spots are stars of our own galaxy that happen to be in front of the other galaxy]] A '''supernova''' is the explosion of a [[giant star]]. It usually happens when the [[nuclear fusion]] in the star cannot hold the core against its own gravity.<ref>This is the cause of all types of supernova except type Ia.</ref> The core collapses, and then explodes. The biggest stars that make supernovae are [[hypergiant]]s and smaller ones are [[supergiant]]s. They are [[mass]]ive: because of [[gravity]], they use up their energy very quickly. Normally they only live for a few million years. During the explosion, the total energy radiated by supernovae may briefly outshine the entire output of a [[galaxy]]. They [[emission|emit]] energy equal to that of the whole lifetime of a solar-like star.<ref> Giacobbe, Frederick W. 2005. How a Type II supernova explodes. ''Electronic Journal of Theoretical Physics'' '''2''' (6): 30–38. [http://adsabs.harvard.edu/abs/2005EJTP....2f..30G]</ref> The explosion blows off its stellar material away from the star, at velocities up to 30,000 km/s or 10% of the speed of light.<ref>Heger, Alexander ''et al'' 2003. How massive single stars end their life. ''Astrophysical Journal''. '''591''': 288. [https://arxiv.org/abs/astro-ph/0212469#]</ref> This drives a [[shock wave]] into the surrounding [[interstellar medium]]. This sweeps up an expanding shell of gas and dust, which we see as a supernova remnant. After exploding, what is left becomes a [[black hole]] or a [[neutron star]]. Most stars are small and do not explode. After their [[red giant]] phase, they become colder and smaller and become [[white dwarf]] stars.<ref name=nasa>{{cite web |url=http://heasarc.gsfc.nasa.gov/docs/snr.html |title=Supernova |last=Tyler |first=Pat |date=26 June 2003 |publisher=NASA HEASARC: Education and Public Information |accessdate=29 January 2013}}</ref> Su