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Supernovae

An Overview

Ancient astronomers knew that the sky is not static.  They observed the regular motions of the planets across the sky, and they noted the dramatic and unexpected appearances of comets in the sky.  The most striking events they saw were the stars that would suddenly appear in the sky and then fade away after a month.  In Europe these transient stars were given the Latin name “nova,” meaning new.  The ancient Chinese astronomers cataloged many novae as part of their duties to keep the emperor informed of occurrences in the sky—if an emperor were truly the intermediary between heaven and Earth, as he proclaimed, he had to show his subjects that we was aware of what the heavens were up to.

Today several different events in astronomy go by the name nova.  The classical novae are outbursts by cataclysmic variable stars.  These binary stars repeatedly produce outbursts.  More striking than the classical nova are the supernovae, which are given this name because of the tremendous amount of energy they produce.  The energy generated by a supernova is a significant fraction of the rest-mass energy of a solar mass star.  A supernova at its peak outshines its host galaxy.  It is so luminous that it can be seen across the universe by the most powerful telescopes.  The nova seen by the Danish astronomer Tycho Brahe in 1572 was a supernova.

The search for supernovae is intense, and dozens are found every year through automated searches with ground-based telescopes.  Supernovae are very rare events.  They are expected to take place in the Milky Way only at a rate of about once every 50 years, and this despite being in an unusually large galaxy with many young, massive stars.  To observe large numbers of supernovae in a short time, one must observe many galaxies, which means observing out to the high-redshift limits of the universe.  The supernovae that are found are labeled by their year and a letter that indicates their sequence of discover, so the first supernova of 2009 is called SN 2009a.

Over the years, observers have developed a classification scheme for supernovae based on their spectra.  The first broad distinction is between supernova that have the emission lines for helium—type I supernovae, where I is the Roman numeral 1—and those that have the emission lines for hydrogen—type II supernovae.  These two types are further subdivided based on the specific pattern of spectral lines they possess.  As with the spectral classification of stars, the supernova spectral subtypes are labeled as letters, starting with the letter a.  So there is a type Ia supernova, for instance (this particular supernova type is widely used in cosmological studies to establish distance).

Like observers, theorists also divide supernovae into two classes: the core-collapse supernovae, and the thermonuclear detonation supernovae.  These two classes are not aligned with the observers' type I and type II classes.  The thermonuclear detonation supernova is associated only with the type Ia supernova, while the core-collapse supernova is associated with the type II and several of the type I supernovae.

The core collapse supernova occurs when a massive star has consumed all of its thermonuclear fuel, so that the core is composed of iron.  If the core of the star exceeds the Chandrasekhar mass limit, it collapses under its own gravity.  The core shrinks from a radius of tens of thousands of kilometers to a radius of tens of kilometers, where the star is stabilized by the degeneracy exerted by protons and neutrons.  The collapse liberates gravitational potential energy that blows away the layers overlying the star's core in an enormous explosion; the energy travels from the core to the outer layers as neutrinos.

Under the most popular theory for the thermonuclear detonation supernova, an explosion occurs when a white dwarf is pushed above the Chandrasekhar limit.  This would happen in cataclysmic variable systems, where a white dwarf is pulling mass from a companion star onto itself.  As the white dwarf grows in mass, it becomes gravitationally unstable, much as the core of a massive star becomes gravitationally unstable.  The difference for the white dwarf is that it was formed from a fusion-powered star of several solar masses before all of the thermonuclear fuel was consumed.  Many white dwarfs are composed of carbon and oxygen.  When such a white dwarf collapses gravitationally, the pressure and temperature inside the star increase until the explosive thermonuclear fusion of carbon and oxygen commences.  This thermonuclear release of energy is sudden, and the amount of energy released far exceeds the star's gravitational potential energy, so the star is blown apart.

Regardless of the energy source, whether gravitational or thermonuclear, one ends up with high-temperature stellar debris flung into space at high velocity.  The brightening we see as a new star is the expansion of the photosphere of this debris.  Eventually the brightening caused by the expansion is countered by the cooling of the debris, and the supernova fades from view.

The supernova shock we are so familiar with in the pictures of ancient supernova remnants is caused by the stellar debris plowing into the surround interstellar gas, driving a shock wave into the gas.

One of the more interesting features of the core-collapse supernovae is that they create elements heavier than iron.  The interior of a fusion-powered star is cool compared to the energy required to convert iron into any other element, so the natural thermodynamic equilibrium is for hydrogen to combine into heavier elements until the matter is in its lowest energy state, which is pure iron.  In the stellar debris of a supernova, however, the temperature exceeds the temperature at the core of the hottest star, and this drives the material in this debris to a thermal equilibrium comprised of elements much heavier than iron.

We owe our existence to supernovae, because many of the basic elements that make up our bodies were created in a supernova explosion.  Extremely heavy elements like silver and gold are created only in supernovae, so if gold is the root of all evil, then supernovae provide the soil for that root.  Supernovae created the radioactive elements like uranium, so when we create energy with a solar panel, we are harnessing the thermonuclear fusion of hydrogen in the Sun, but when we create energy from the nuclear fission of uranium, we are harnessing the power of an ancient supernova.  We find mankind linked to the supernova, with our life, our well-being, the object of our greed, and the means to our destructiveness directly provided by an ancient supernova explosion.

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