What is Nova?
{{distinguish|Supernova}} [[Image:Making a Nova.jpg|frame|right|Artist's conception of a white dwarf pulling hydrogen from a larger companion]] A '''nova''' (plural ''[[wiktionary:nova|novae]]'' or ''novas'') is a huge [[nuclear explosion]] on a [[white dwarf]] star. It causes a sudden brightening of the star. Novae are not to be confused with other brightening phenomena such as [[supernova]]e. Novae occur on the surface of a white dwarf in a [[binary star|binary system]]. If the two stars of the system are close enough, material is pulled from the companion star's surface onto the white dwarf. This collection of hydrogen onto the surface of the star starts a runaway [[nuclear fusion|fusion]] reaction.<ref>Darnley M.J. ''et al'' 2012. On the progenitors of galactic novae. ''Astrophysical Journal'' '''746''' (61). [http://iopscience.iop.org/0004-637X/746/1/61/article]</ref> In spite of their violence, the amount of material ejected in novae is usually only about 1⁄10,000 of a [[solar mass]], quite small relative to the mass of the white dwarf. Only five percent of the accreted mass is fused during the power outburst. This is enough energy to make nova ejecta to go outward as fast as several thousand kilometers per second—higher for some novae than others. The star also brightens, from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope were surprised to discover, for the first time, that a nova can also emit gamma-rays (>100 MeV).<ref>JPL/NASA 2010. Fermi detects 'shocking' surprise from supernova's little cousin. ''PhysOrg.'' [http://phys.org/news200849593.html]</ref> A white dwarf can potentially generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. An example is RS Ophiuchi, which is known to have flared six times (in 1898, 1933, 1958, 1967, 1985, and 2006). Eventually, the white dwarf could explode as a type Ia supernova if it approaches the [[Chandrasekhar limit]]. Occasionall