Supernova

A supernova represents the sudden brightening, within a matter of hours, of a star by as much as 21 magnitudes. We know that for a few days or weeks a supernova is likely to outshine all the other stars in its parent galaxy combined. And we know that for every bit of knowledge we gain by observation and modeling, we encounter another set of perplexing questions which require more observations and better models to answer.

The two major types of supernovae are, quite simply, Type I and Type II. The difference between the two is simply the presence or absence of the Balmer Hydrogen lines in the spectrum of the supernova. If, <> the spectrum of a supernova shows any Balmer lines in its spectrum, whether in absorption or emission or both, that supernova is classified as Type II. If there is no detectable Hydrogen signature in the spectrum then the supernova is classed as Type I. It is very important to keep in mind that supernova types are spectral types - no more and no less. All too often the amateur literature shows a misunderstanding of this fundamental truth as authors confuse the observed reality with various theoretical models of how these supernovae came about.

Within Type I, supernovae lacking any hydrogen lines in their maximum light spectra, there is a further subdivision into Type Ia, Ib, and Ic. Those designated Type Ia show a strong feature at 6150 Angstroms which is caused by singly-ionized Silicon (Si II). Type I SNe lacking this Si II feature are classified as Type Ib if they show substantial evidence of neutral Helium, especially the neutral Helium (He I) line at 5876 Angstroms. If the neutral Helium features are weak or missing entirely then the supernova is classified as Type Ic.

Supernovae of Type II are subdivided not by variations in spectra but by the way their luminosity changes over time, a property often called the light curve. In many cases SN II will reach maximum brightness, dim slightly, and then stay at almost the same brightness for many days. These SN II are known as Type IIP SNe, with the P meaning plateau. They eventually begin to dim again, with the decrease being monotonic and linear. Other SN II quickly reach maximum brightness and then dim regularly in a linear fashion. These are classified as Type IIL supernovae. Some SN II exhibit stranger behavior, dimming and brightening again, as in the case of the recent SN1993J.

Current models of supernovae suggest that all types of SNe <> Type Ia are caused by the catastrophic death of a star far more massive than our Sun. These progenitors are thought to be stars which started their main sequence lives with at least 12 times the Sun's mass - and possibly as much as 50 solar masses. After their brief (in stellar terms) main sequence lifetime these stars rapidly evolve into giant stars while successive stages of nuclear burning within their interiors build an onion-like structure of shells - each consisting of successively heavier groups of elements. When at last the star's core has been transmuted to iron and nickel it reaches a dead end, for fusion of these elements into heavier species takes energy out of the surrounding star rather than pumping energy into it as did earlier reactions. Still, the crush of gravity initiates iron fusion in the core, and the result is that in a matter of minutes the interior of the star collapses upon itself as a complex chain reaction is unleashed.

In the nuclear furnace of the stellar core where iron fusion is being driven by the star's inexorable gravity, the iron fusion draws heat out of the surrounding super-dense hot plasma. With the rapid decrease in free energy the core collapses while at the same time a vast number of high energy photons, or gamma rays, are released by the iron fusion. These gammas have sufficiently high energy that they are able to destroy most of the more complex atoms. When the gamma penetrates the nucleus it's energy is absorbed by the nucleus, and this energy is greater than the nuclear binding energy. This process is known as photodissociation.

In short order the collapsing core is convered from nickel and iron nuclei to mostly alpha particles, or helium nuclei. Still, the process doesn't stop here. As the core collapsed the mass of the star remained above it, held in place briefly by inertia. But deprived of support from the core, the overlying mass of the star falls freely. As this mass impacts onto the now largely Helium core it is further compressed and heated. The Helium is then dissociated into the fundamental subatomic particles - protons, neutrons, and electrons; and for a brief time the electrostatic force of the electrons resists the pressure of the star's overlying weight. But this resistance, known as electron degeneracy pressure, is not enough to resist the force of gravity given the tremendous mass of the star. As electron degeneracy pressure is overcome by gravity, a strange reaction known as <> occurs in which the electrons are absorbed into the protons, transmuting each electron-proton pair into a single neutron and releasing a neutrino in the process. Within fractions of a second the core is converted to a mass of neutrons far more dense than any earthly substance.

Now, the neutron core, compressed still by the force of the infalling stellar matter, rebounds from its maximum compression. The shock wave of the rebound travels outward through the star - tearing it apart and triggering a number of complex nuclear reactions in the process which produce elements spanning the periodic table.

The current understanding of this process is far from complete. Nobody has any more than the vaguest idea of how to formulate the equations of state given the incredible temperatures and pressures involved. Even careful study of the most powerful nuclear bombs tested on Earth do not approach the conditions involved in these supernovae. The most fruitful studies involve powerful computer models attempting to replicate the observed properties of real SNe. While some of these models have finally achieved behaviors representative of real SNe in the last several years, the theoreticians admit they lack accuracy.

The explanation for how this basic model can produce the various spectral types observed, such as SN Ib, Ic, IIP and IIL, is that by the time a large star reaches the iron fusion stage its outer layers are mostly out of touch with the core. In the cases where no Hydrogen is observed (SNe Ib and Ic) the accepted explanation is that a strong stellar wind or a nearby binary companion has swept away the star's outer layer of Hydrogen prior to the core collapse. In the case of SN Ic the Helium layer beneath the Hydrogen layer has been largely depleted as well. Conversely, SN II have retained a large portion of their Hydrogen envelopes, and we see this Hydrogen in the spectrum of the supernova.

The Type IIL supernovae show a linear decline in their light curves, similar to the light curves of SNe I. SNe IIP, the plateau supernovae, are thought to have this property because the exploding star is expanding just fast enough for the increase in radius to compensate for decreasing surface temperature. (Stars emit light proportional to the square of their radii and the fourth power of temperature.) The few SN that have been observed to dim and then re-brighten before entering the plateau phase are postulated to have experienced an accelerated expansion after the initial brightening began to decrease.

This leaves supernovae of Type Ia. These are special in several ways, perhaps most importantly because they are the brightest in absolute terms. There is strong evidence to suggest that all Type Ia SNe have the same absolute maximum brightness, and if this is true they provide an excellent "standard candle" for measuring the distances to distant galaxies.

But Type Ia supernovae can not be caused by the deaths of massive stars. First, no compact neutron stars are found in the remnants of type Ia SNe. Second, no physical process, no matter how violent, can produce the luminosity observed from SNe Ia by core collapse of even a supergiant star 100 times the mass of the Sun. The outer layers of a giant star would absorb too much of the energy. Rather, it is necessary that a mass of something more than the Sun's mass be completely destroyed while unshielded by overlaying material for anything as bright as a Type Ia SN to be possible.

So what is left, what can they be?

White Dwarf stars, those dense hot corpses of stars slowly cooling to cinders, have the curious property that the more massive White Dwarves are smaller in radius. While this is completely against all common sense it is nonetheless true. The WD stars are only supported against gravity by electron degeneracy pressure, and as more mass produces more gravity, the electrons are squeezed together more tightly - thus decreasing radius with increasing mass.

But obviously there must be some limit to this. At what point will the mass of a White Dwarf become so much that it is compressed into a point mass of zero radius? And what happens then? S. Chandrasekhar (University of Chicago) has shown that the limit at which a star supported only by electron degeneracy compresses to a point is 1.44 solar masses. Other considerations related to the internal physics of real stars can be shown to reduce the upper limit for the mass of a White Dwarf star to ~1.2 solar masses. So imagine a White Dwarf star of greater than the Sun's mass, bound to a binary companion. If that companion swells enough for its outer layers to be pulled across the space to the White Dwarf, then the WD star could gain sufficient mass to go beyond Chandrasekhar's limit.

Stan Woosley of the University of California, Santa Cruz; and Kenichi Nomoto and co-workers in Japan, have shown conclusively that a carbon-oxygen White Dwarf with a mass of 1.2 solar masses can be torn apart by a <> wave, producing an event similar to observed Type Ia SNe. The mechanics of this process are complicated, but briefly deflagration means that run-away nuclear burning is touched off near the core of the WD star. A shock wave propagates outward at <>, eventually becoming supersonic as it approaches the surface. A supersonic shock wave would produce a detonation, not a deflagration, and detonation models do not adequately produce the observed elemental abundances seen in the ejecta of SNe Ia.

White Dwarves have been suspected as the progenitors of SNe Ia for many years. Recent improvements in the capabilities of high speed computers have made possible the detailed modeling necessary to confirm them as the prime candidates. But many questions still remain, awaiting future observations of supernovae with more sensitive instrumentation to confirm or refute the work of the current generation of theorists.

But beyond this, supernovae inspire and awe us. They are perhaps the single most energetic events in the universe, representing the violent destruction of a star and the concurrent creation and release into the interstellar medium of all those elements heavier than Helium which otherwise would not exist. The carbon which is the building block of our flesh, and the iron in our blood, were forged in the fire of a supernova long ago, along with the silicon which makes the sand of our beaches and the rocks of our mountains; and the oxygen and nitrogen we breathe.