A nova is an exploding star; its brightness increases suddenly and temporarily by 10 to 15 magnitudes. Novae are split into dwarf novae, normal novae and supernovae. Dwarf and normal novae are usually composed of two very close stars: these are double stars of the same type as those forming X-ray stars, where one of the two stars is condensed (1 cm3 <=> 10 T). In a pair formed in this way, the following phenomena occur: a jet of gas is emitted by the atmosphere of the larger star, an accretion disk forms around the condensed star and a hot spot forms in the zone where the gas jet impacts the accretion disk. The material of the disk can sometimes fall onto the condensed star. Given that the gas in the disk reaches a very high temperature at the time of the impact, reactions occur that trigger a strong emission of light energy. As this phenomenon only involves the surface layer of the star, it only lasts a few days: in one week, or sometimes even less, the star returns to its normal brightness. Stars that undergo this phenomenon are called dwarf novae or U Geminorum-type variable stars, from the name of the star U in the constellation of Gemini, where the phenomenon is particularly clear.