新星是激变变星的一类，是由吸积在白矮星表面的氢造成剧烈的核子爆炸的现象。这类星通常原本都很暗，难以发现，爆发时突然增亮，被认为是新产生的恒星，因此而得名。新星按光度下降速度分为快新星（NA）、中速新星（NAB）、慢新星（NB）和甚慢新星（NC），爆发时亮度会增加几万、几十万甚至几百万倍，持续几星期或几年。但不能和Ia超新星或其他恒星的爆炸混淆，包括加州理工学院在2007年5月首度发现的发光红新星。

目前在银河系中已发现超过200颗新星。

A nova (plural novae or novas) is a cataclysmic nuclear explosion on a white dwarf, which causes a sudden brightening of the star. Novae are not to be confused with other brightening phenomena such as supernovae or luminous red novae. Novae are thought to occur on the surface of a white dwarf in a binary system when they are sufficiently near to one another, allowing material (mostly hydrogen) to be pulled from the companion star's surface onto the white dwarf. The nova is the result of the rapid fusion of the accreted hydrogen on the surface of the star, commencing a runaway fusion reaction.