A nova (plural novae) is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner. Novae are not to be confused with supernovae or luminous red novae.

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle.

A bus ( /ˈbʌs/; plural "buses", /ˈbʌsɨz/, archaically also big car, omnibus, multibus, or autobus) is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker rigid bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are used for longer distance services. Bus manufacturing is increasingly globalised, with the same design appearing around the world.

Buses may be used for scheduled bus transport, scheduled coach transport, school transport, private hire, tourism; promotional buses may be used for political campaigns and others are privately operated for a wide range of purposes.

Horse drawn buses were used from the 1820s, followed by steam buses in the 1830s, and electric trolleybuses in 1882. The first buses powered by internal combustion engines were used 1895^{[citation needed]} and this is still the most common power source. Recently there has been growing interest in hybrid electric buses, fuel cell buses, electric buses as well as ones powered by compressed natural gas or bio-diesel.