Annu. Rev. Astron. Astrophys. 1997. 35: 309-355 Copyright © 1997 by Annual Reviews. All rights reserved

1. INTRODUCTION

The study of supernovae (SNe) has expanded tremendously during the past decade. A major motivation, of course, was provided by SN 1987A, by far the most thoroughly observed supernova (SN) in history. Advances in the field have also been driven by technology: The advent of sensitive detectors, especially charge-coupled devices (CCDs), and the proliferation of moderately large telescopes made it possible to obtain excellent photometry and spectroscopy of large numbers of SNe. In addition to their intrinsic interest, SNe are relevant to nucleosynthesis and galactic chemical evolution, the production of neutron stars and black holes, the origin of cosmic rays, the physical state of the interstellar medium, and induced star formation; thus, they have been investigated from a wide range of perspectives. Finally, the enormous potential of SNe as cosmological distance indicators is inspiring many new studies.

This review concentrates primarily on the observed optical spectra of SNe, illustrating the temporal evolution of the major classes and subclasses. When combined with other observations and properly interpreted, such data can reveal the chemical composition of the ejecta, the nature of the progenitors, the explosion mechanisms, and even the distances of SNe. Optical light curves are briefly summarized; it is difficult to entirely decouple discussions of the photometric and spectral evolution of SNe. Details concerning light and color curves can be found in Kirshner (1990), Ford et al (1993), Leibundgut (1994, 1996), Patat et al (1994), Suntzeff (1996), Richmond et al (1996b) and other articles.

Conference proceedings or collections of reviews devoted to many different aspects of SNe (in some cases largely to SN 1987A) include those edited by Wheeler (1980), Rees & Stoneham (1982), Bartel (1985), Danziger (1987), Kafatos & Michalitsianos (1988), Brown (1988), Proust & Couch (1988), Wheeler et al (1990), Petschek (1990), Woosley (1991), Danziger & Kjär (1991), Ray & Velusamy (1991), Audouze et al (1993), Clegg et al (1994), Bludman et al (1995), McCray & Wang (1996), Ruiz-Lapuente et al (1997). Additional reviews include those of Oke & Searle (1974), Trimble (1982, 1983), Woosley & Weaver 1986, Dopita (1988), Weiler & Sramek (1988), Arnett et al (1989), Imshennik & Nadëzhin (1989), Hillebrandt & Höflich (1989), Wheeler & Harkness (1990), Branch et al (1991), Branch & Tammann (1992), McCray (1993), Chevalier (1981, 1995), Arnett (1996, especially Chapter 13). Infrared (IR) spectra of SNe are discussed by Meikle et al (1993, 1997) and others. A thorough atlas of International Ultraviolet Explorer (IUE) spectra of SNe, together with some optical spectra, light curves, and many useful references, has been published by Cappellaro et al (1995c). Recently, Wheeler & Benetti (1997) have concisely summarized many of the basic observed properties of SNe.

The most extensive catalogs of SNe are those of the Asiago Observatory (Barbon et al 1989, with an update by van den Bergh 1994) and the Sternberg Astronomical Institute. These are now regularly maintained and available electronically on the World Wide Web (http://athena.pd.astro.it/~supern/snean.txt and http://www.sai.msu.su/sn/sncat/, respectively). The Palomar catalog of SNe (Kowal & Sargent 1971) is no longer updated.

Unless otherwise noted, the optical spectra illustrated here were obtained by the author or his collaborators, primarily with the 3-m Shane reflector at Lick Observatory. They have been shifted to their rest frame; in each case the adopted redshift is listed in the caption. Telluric lines were generally removed, but the spectra were not dereddened. Although the relative spectrophotometry (i.e. the shape of each spectrum) is accurate, the absolute scale is arbitrary. Universal time (UT) dates are used throughout this review. When referring to phase of evolution, the variables t and denote time since maximum brightness (usually in the B passband) and time since explosion, respectively.