1. INTRODUCTION

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.

Dylan Thomas

Supernova 1998aq in the galaxy NGC 3982. The bright blue supernova in the two o'clock position is as bright as the 10 billion stars in this spiral galaxy. Source: Recent Supernovae, by Center of Astrophysics.

The subject of stellar explosions, is one of the most wonderful fields of astrophysics. As you will see, these explosions are truly tremendous. The quest to understand them takes us into the most extreme environments of the physical world -- temperatures of billions of degrees and densities of 1012 kg/cm3 (a cup full of such material would weigh more than the entire Long's Peak!). Moreover, these explosions are responsible for the presence of heavy elements in the universe -- the carbon, oxygen, nitrogen, iron, etc. The material of the Earth itself and everything on it was made in the explosion of star, billions of years ago. We are truly stardust!

We begin this lesson with a historical overview of the observed properties of stellar explosions. There are three major types: classical novae, and two entirely different types of supernovae.

Classical Novae: We see a bright classical nova about once per decade. A faint binary star system suddenly brightens to about 105 (100,000) Solar luminosities and then fades after a few months. The most recent classical nova visible to the naked eye (magnitude V = 3.6 at maximum brightness) was Nova Aquilae 1999 No. 2. Here is a recent Hubble Space Telescope image of another bright nova, Nova Cygni 1992, showing an expanding shell ejected by the explosion. When astronomers observe novae several years after outburst, they always find a close binary system containing a white dwarf. Evidently, nova explosions are not powerful enough to destroy the star systems that give rise to them.

Observing classical novae is a popular occupation of many amateur and professional astronomers. The link Variables shows the results of recent observations of novae, including light curves (graphs illustrating the fading of novae with time).

Supernovae: Supernovae are far more powerful events than novae. A star suddenly brightens to roughly 109 Solar luminosities. In the Milky Way galaxy, a supernova occurs about once every 30 years or so (on average), but most are hidden behind the dark dust clouds of the Milky Way and are invisible to the naked eye. However, some (about 10%) have been close enough and bright enough that they were visible to the naked eye. The table below lists several such events that have been recorded during the past 2000 years and provides links to a few of the original historical records. A few of these events were very bright. The supernova of 1006 AD, for example, was about 1/10 as bright as the full moon!

The supernova of 1572 AD, discovered by Tycho Brahe. With a maximum visual magnitude of -4, this exploding star was the brightest star in the sky for several months.

Observing supernovae is a very active field of astronomy. Supernovae in distant galaxies, bright enough to observe with modern telescopes, appear somewhere in the sky at a rate of about one per second -- if only we knew how to find them! Until recently, astronomers found only a few dozen per year. Today, with sophisticated search techniques, they are finding a few hundred each year. The International Supernovae Network is a group of amateur and professional astronomers dedicated to finding new supernovae. You should also check the Recent Supernovae Page of the Harvard-Smithsonian Center for Astrophysics. It has several images of recently discovered supernovae, plus a nice graphic showing a real supernova image blinking off (before explosion) and on (after) in a galaxy. See also Best of Supernova Images.

There are two entirely different kinds of supernovae, as summarized in the table below. But first, you should read this: An introduction to supernovae.

TWO VERY DIFFERENT TYPES OF SUPERNOVAE

Supernova Type

Thermonuclear (Type Ia*)

Core Collapse (Type II)

Maximum Luminosity

3 x 109 Suns (MB = -19.5)

3 x 108 Suns (MB = -18.5 +/- 1)

Spectrum

No hydrogen lines
Lines of many heavy elements

Hydrogen lines
Continuum

Where found

Among old star systems
(globular clusters, galactic bulge, elliptical galaxies)

Among young star systems
(young star clusters, star-forming regions in disk galaxies)

Parent Star

White dwarf in binary system

Massive star (usually a red supergiant)

Trigger mechanism

Mass transfer from companion

Collapse of iron core

Explosion mechanism

Thermonuclear explosion of carbon/oxygen core --> iron

Rebound shock from neutron star surface: neutrino pressure

Left behind

Nothing

Neutron star

Debris

Mostly iron

All kinds of elements

*Types Ib and Ic supernovae are unusual supernovae that have most of the properties of type II supernovae, except that their spectra show no hydrogen lines. They are believed to be core-collapse supernovae

The web is full of information about supernovae. Supernova Links is the best starting point I know.

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Last modified February 21, 2001
Copyright by Richard McCray