Novae
Nova means "new star", but they are actually "newly visible" stars caused by brightening of an existing star by 5 to 15 magnitudes. This can be brightening by hundreds of millions of times. The brightening may occur within a few days or weeks, but usually fades again within months. There are some recurring novae, but most are one-time events. One model of novae suggests that they occur in binary systems where one is a white dwarf and the other is on its way to becoming a red giant . The red giant can lose mass which would trigger hydrogen fusion as it falls on the white dwarf. This would blow the gas off and the process could repeat itself. A notable nova example is Nova Cygni 1975.
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