Annu. Rev. Astron. Astrophys. 1992. 30: 359-89 Copyright © 1992 by Annual Reviews. All rights reserved

1. INTRODUCTION

As extremely luminous point sources, supernovae are attractive indicators of extragalactic distances. On the basis of their optical spectra supernovae are classified as type Ia, Ib, Ic, or II (Harkness & Wheeler 1990, Branch et al 1991). Those of type Ia (SN Ia), the most luminous and homogeneous kind, are the subject of this review.

Observations of SNe Ia and the present understanding of their physics have been reviewed by Wheeler & Harkness (1990). There is a characteristic light-curve shape that can be understood in terms of the trapping and thermalization of the decay products of radioactive ⁵⁶Ni and ⁵⁶Co in a Chandrasekhar mass (1.4 M) of ejected matter. There also is a characteristic development of the SN Ia optical spectrum. Near the time of maximum light the spectrum contains lines of intermediate-mass elements from oxygen to calcium, ejected at high velocities 10,000 km s^-1. At later times the spectrum becomes dominated by lines of the first several ionization stages of iron, most of which is presumed to have formed by ⁵⁶Co decay. Unlike other supernova types, SNe Ia are found in all kinds of galaxies, including ellipticals, and they show no obvious preference for regions of current star formation. Thus the initial mass of the SN Ia stellar progenitors must be lower than that of the SNe II and SNe Ib/Ic progenitors. All of this evidence suggests that SNe Ia are the explosions of white dwarfs that accrete matter from binary companions. In numerical simulations, a white dwarf that accretes matter at a rate in the range 10^-6-10^-8 M yr^-1 is found to ignite degenerate carbon at its center. A suitably parameterized nuclear burning front can then propagate outwards through the white dwarf, incinerating an inner fraction of the star to nuclear statistical equilibrium (mainly ⁵⁶Ni) while ejecting it at low velocity, and burning the outer layers into elements of intermediate mass while ejecting them at high velocity. Such models can account for both the spectra and the light curves. A white dwarf progenitor that accretes matter until it reaches the Chandrasekhar mass also would be consistent with the very impressive observed homogeneity of SNe Ia.

Section 2 reviews the status of the observational homogeneity, and Section 3 is concerned with the calibration of the SN Ia absolute magnitude. A final section discusses the prospects for applications of SNe Ia as distance indicators for cosmology: to provide an independent (but low-resolution) probe of the deviations from a linear expansion law; to measure the Hubble constant, H₀: to test the fundamental premise that the universe is expanding; and to measure the deceleration parameter, q₀.