Gravitational binding energy
The gravitational binding energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. A gravitationally bound system has a lower gravitational potential energy than the sum of the energies of its parts when these are completely separated—this is what keeps the system aggregated in accordance with the minimum total potential energy principle.
Galaxy clusters are the largest known gravitationally bound structures in the universe.
A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.
SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526
The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova.
A 1414 text cites a 1055 report: since "the baleful star appeared, a full year has passed and until now its brilliance has not faded".
2015 supernova in galaxy cluster RXC J0949.8+1707. In 2011, two supernovae were observed in the same face-on spiral galaxy.