Observational and Physical Classification of Supernovae

This chapter describes the current classification scheme of supernovae (SNe). This scheme has evolved over many decades and now includes numerous SN types and subtypes, many universally recognized, others controversial. The types are defined according to observational properties, mostly visible-light spectra near maximum light, as well as photometric properties. However, a long-term goal of SN classification is to associate observationally defined classes with specific physical explosive phenomena. We show here that this aspiration is now finally coming to fruition, and we establish the SN classification scheme upon direct observational evidence connecting SN groups with specific progenitor stars. Observationally, the broad class of Type II SNe contains objects showing strong spectroscopic signatures of hydrogen, while objects lacking such signatures are of Type I, which is further divided to numerous subclasses. Recently a class of superluminous SNe (SLSNe, typically ten times more luminous than standard events) has been identified. Finally we describe an alternative classification scheme that presents our emerging physical understanding of SN explosions, while clearly separating robust observational properties from physical inferences that can be debated. This new system is quantitative, with events distributed along a continuum, rather than divided into discrete classes. Thus, it may be more suitable to the coming era where SN numbers will reach millions.