Supernovae

Classification of novae and supernovae

The ancient astronomers had already noted that sometimes new stars became visible in the sky and after some time disappeared again. In the Middle Ages the astronomers called these stars novae, which is the Latin word for new stars. Some of these new stars were exceedingly bright, and were later called supernovae. Three of these supernovae were observed in historic times: Tycho de Brahe's supernova, which occurred in the year 1572, Kepler's supernova which became very bright in the year 1604, and a supernova which was observed by Chinese astronomers in the year 1054. At the location of the Chinese supernova we now see the Crab nebula in the constellation of Taurus. The nebula got its name from its appearance which reminds us of a crab. The Crab nebula still expands with velocities of about 1400 km s−1, showing that a truly gigantic explosion must have occurred 900 years ago.

What are these novae and supernovae? How often do they occur? What kinds of objects are their progenitors? What leads to such gigantic explosions? What distinguishes novae and supernovae? Are all supernovae similar events or do we have to distinguish different kinds of novae or supernovae? These are questions for which we would like to find the answers.

Both novae and supernovae are objects which suddenly increase their light output by many orders of magnitude.

General discussion

In the previous sections we have discussed stars which are generally considered to be normal stars, which means their spectra fit into the two-dimensional classification scheme according to spectral type and luminosity. True, the weak-lined stars, or population II stars, do not fit into that scheme, but generally their peculiarity can be understood by the change of just one parameter, the ratio of the metal abundance to the hydrogen abundance, though recently it has been found that this may not always be the case. More than one parameter may actually be necessary to describe the abundances of the heavy elements. The population II stars are still generally considered to be ‘normal’ stars because we believe that all their peculiarities can apparently be traced back to different chemical abundances. For the stars we are going to discuss in this chapter, this does not seem to be the case. There are, of course, a large number of different kinds of peculiar stars, but we are not able to discuss all of them in the framework of this introduction to stellar astrophysics. We shall only discuss the most frequent kinds of peculiar stars and those which are of special interest in the framework of understanding stellar structure and evolution.

Peculiar A stars, or magnetic stars

The observations

In the previous section we saw that some stars with very strong magnetic fields are found among the early A stars.

This document was originally distributed in 1975 by the Mathematics Department of King's College, University of London, as a technical report. (The research was supported by the Science Research Council.) A brief account of its most novel conclusions was published as Fulling 1976.

It is reproduced here verbatim, except for certain improvements connected with the revolution in scientific typography, and the updating of references to some journal articles that were not yet in print at that time.

Analogous studies of the Klein effect for fermions have since been conducted by Bilodeau 1977 for the neutrino field and by Manogue 1988 for the massive Dirac field. Ambjorn & Wolfram 1983 investigate the Schiff–Snyder–Weinberg scenario further; they present evidence that the reaction of the quantized field on the electric field suffices to suppress the instabilities.

Recent years have seen considerable attention to the implications of strong-field effects (on fermions, primarily) for realistic nuclear physics. I understand that the experimental evidence is still inconclusive. From this literature I will cite only these reviews: Rafelski et al. 1978; Soffel et al. 1982; Greiner et al. 1985.

Abstract

Part One A relativistic scalar field is quantized in a one-dimensional “box” comprising two broad electrostatic potential wells. As the potential difference increases, the phenomena found by Schiff, Snyder, and Weinberg in such a model occur: merging of mode frequencies and disappearance of the vacuum as a discrete state, followed by appearance of complex frequencies and unboundedness below of the total energy.