In astronomy the term guest star refers to a star which has suddenly appeared visible in the place where no star had previously been observed and becomes invisible again after some time. The term is a literal translation from ancient Chinese astronomical records. Modern astronomy recognizes that guest stars are manifestations of cataclysmic variable stars: novae and supernovae. Still, the term "guest star" is still used in in the context of ancient records, since the exact classification of an astronomical event in question is based on interpretations of old records, rather than on direct observations. In ancient Chinese astronomy guest stars (ke xing 客星) were one of the three types of "new stars", the other two being comets in modern understanding. The earliest Chinese record of guest stars is contained in Han Shu (漢書), the history of Han Dynasty (206 BCE — 220 CE), and all subsequent dynastic histories had such records.

Of ancient European chronicles the possible early indications at supernovae are vague references to astronomical events which may be interpreted as the supernova of 185 recorded by Chinese. At the same time astronomers are in dispute why a notable supernova of 1054 is missing from European records.