Supernova ... Ka Boom!!!

A supernova (pl. supernovae) is a stellar explosion which produces an extremely bright object made of plasma that declines to invisibility over weeks or months. A supernova briefly outshines its entire host galaxy. It would take 10 billion years for the Sun to produce the energy output of an ordinary Type II supernova. Stars beneath the Chandrasekhar limit[1] , such as the Sun, are too light to ever become supernovae and will evolve into white dwarfs[2]

Types of Supernovae

Supernovae are divided into two basic physical types:

Type Ia.

These result in some Binary-Star[3] systems in which a carbon-oxygen white dwark is accreting matter from a companion. (What kind of companion star is best suited to produce Type Ia supernovae is hotly debated.) In a popular scenario, so much mass piles up on the white dwarf that its core reaches a critical density of 2 x 109 g/cm3. This is enough to result in an uncontrolled fusion of carbon and oxygen, thus detonating the star.

Type II.

These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star's iron core is massive enough then it will collapse and become a supernova.

However, these types of supernovae were originally classified based on the existence of hyderogen, Spectral Lines[4]. Type Ia do not show hydrogen lines, while Type II do.

In general this observational classification agrees with the physical classification outlined above, because massive stars have atmospheres (made of mostly hydrogen) while white dwarf stars are bare. However, if the original star was so massive that its strong Stellar Wind[5] had already blown off the hydrogen from its atmosphere by the time of the explosion, then it too will not show hydrogen Spectral Lines. These supernovae are often called Type Ib supernovae, despite really being part of the Type II class of supernovae. Looking at this discrepancy between our modern classification (based on a true difference in how supernovae explode), and the historical classification (based on early observations) shows how classifications in science can change over time as we better understand the natural world.

What Causes a Star to Blow Up?

Gravity gives the supernova its energy. For Type II supernovae, mass flows into the core by the continued making of iron from Nuclear-Fusion[6]. Once the core has gained so much mass that it cannot withstand its own weight, the core implodes (A violent inward collapse-An inward explosion). This implosion can usually be brought to a halt by neutrons, the only things in nature that can stop such a gravitational collapse (when a body falls under its own mass). Even neutrons sometimes fail depending on the mass of the star's core. When the collapse is abruptly stopped by the neutrons, matter bounces off the hard iron core, thus turning the implosion into an explosion: ka-BOOM!!!

For Type Ia supernova, the energy comes from the run-away fusion of carbon and oxygen in the core of the white dwarf.

Where Does the Core Go?

When the core is lighter than about 5 solar masses (A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 M_sun = 2 x 10³³ grams), it is believed that the neutrons are successful in halting the collapse of the star creating a neutron star. Neutron stars can sometimes be observed as Pulsars[7] or X-Ray binaries.

When the core is heavier (M_core > ~ 5 solar masses), nothing in the known universe is able to stop the core collapse, so the core completely falls into itself, creating a black hole(An object whose gravity is so strong that even light can not escape from it)

To understand the phenomenon of core collapse better, consider an analogy to a rocket escaping the Earth's gravity. According to Newton's law of Gravity, the energy it takes to completely separate two things is given by:

E = G M m / r

where G is the Gravitational constant, M is the mass of the Earth, m is the mass of the rocket and r is the distance between them (the radius of the Earth). When the rocket is shot off at a given velocity v, its energy is:

E = 1/2 m v²

For the rocket to escape the Earth's gravitational field, this energy must be as least as great as the gravitational energy described in the first equation. Thus, to determine if the rocket will completely break free from the Earth's grasp, we set the two equations equal to one another and solve for v:

v = ( 2 G M / r )^1/2

This result is called the escape velocity. For the Earth, the escape velocity is 11 km/sec.

Next imagine a star's central core in the role of the Earth in the above analogy. Consider what would happen if during the core collapse, the central core became so dense (i.e., the radius became very small while its mass stays the same) that something would have to travel faster than light to escape. Whenever this phenomenon occurs (i.e., M_core > ~ 5 solar masses), the supernova creates a black hole from the core of the original star. Now the escape velocity greater than the speed of light -- 300,000 km/sec.

Where Does Most of the Star Go?

The core is only the very small center of an extremely large star that for many millions of years had been making many (but not all) of the elements that we find here on Earth. When a star's core collapses, an enormous blast wave is created with the energy of about 10²⁸ mega-tons (The same amount energy as 1 million tons of TNT - 1 mega-ton = 4 x 10²² ergs = 4 x 10¹⁵ joules).This blast wave plows the star's atmosphere into interstellar space, propelling the elements created in the explosion outward as the star becomes a supernova remnant.

Are We Made of Stardust?

Many of the more common elements were made through nuclear-fusion in the cores of stars, but many were not as well. Because nuclear fusion reactions that make elements heavier than iron require more energy than they give off, such reactions do not occur under stable conditions that occur in stars. Supernovae, on the other hand, are not stable, so they can make these heavy elements beyond iron.

In addition to making elements, supernovae scatter the elements (made by both the star and supernova) out in to the Interstellar-Medium[8]. These are the elements that make up stars, planets and everything on Earth -- including ourselves.

How Often Do Supernovae Occur?

Although many supernovae have been seen in nearby galaxies, supernova explosions are relatively rare events in our own Galaxy, happening once a century or so on average. The last nearby supernova explosion occurred in 1680, It was thought to be just a normal star at the time, but it caused a discrepancy in the observer's star catalogue which historians finally resolved 300 years later, after the supernova remnant (Cassiopeia A) was discovered and its age estimated. Before 1680, the two most recent supernova explosions were observed by the great astronormers Tycho and Kepler in 1572 and 1604 respectively.

In 1987 there was a supernova explosion in the Large Magellanic Cloud, a companion galaxy to the Milky Way. Supernova 1987A, which is shown at the top of the page, is close enough to continuously observe as it changes over time thus greatly expanding astronomers' understanding of this fascinating phenomenon.

(this observation is updated as per 2004 - there may be some more supernovas explosion spotted between 2004-2006)

A picture of Supernova (Supernova 1987A) After & Before explosion is shown below.
The above two photographs are of the same part of the sky. The photo on the left was taken in 1987 during the supernova explosion of SN 1987A, while the right hand photo was taken beforehand. Supernovae are one of the most energetic explosions in nature, making them like a 10²⁸ mega-ton bomb (i.e., a few octillion nuclear warheads).

Definitions:

[1]Chandrasekhar limit: A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star

[2]White dwarf: A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a desnsity about 1 million times that of water!]

[3]Binary star: Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process)

[4]Spectral Line: Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm))

[5]Stellar wind: The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel)

[6]Nuclear Fusion: A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.

[7]Pulsar: A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.

[8]Interstellar medium: The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.

Resources:
http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernovae.html
http://en.wikipedia.org/wiki/Supernova