[Note - this article was originally written as suggested revision of Supernova
A supernova is the mindbogglingly powerful explosion of certain kinds of stars at the end of their lives, typically giant stars that have exhausted their available fuel supply so that the core collapses while the outer layers are blasted away, the eruption briefly outshining an entire galaxy. Another kind of supernova occur when a small, compact star called a white dwarf pulls in so much material from a companion star it initiates an uncontrolled nuclear reaction, resulting in a thermonuclear explosion that leaves nothing behind.
In this way, there are two types of supernovae (the plural of "supernova") depending on the physical mechanism involved, either a core collapse or a white dwarf thermonuclear explosion. However, the current classification of supernovae is based on observations of the type of light emitted, where the information in this light tells astronomers which elements were produced by the star and blasted into space. This "spectroscopic" analysis amounts to a fingerprint of the elements produced by the star either during its lifetime or by the supernova -- elements that seed the galaxies and make other stars, planets, and ultimately life possible.
Type I versus Type II Supernovae
A "Type I" supernova has no hydrogen present in its spectroscopic signature while a "Type II" supernova does. That is, hydrogen lines are seen only in Type II supernovae.
Type I supernovae were subsequently found to have three subcategories, referred to as Type Ia, Ib, and Ic. The Type Ia have the element silicon in their spectroscopic signature. Both Type Ib and Ic have sodium and calcium but the Type Ib has helium whereas the Type Ic does not. (There may be some overlap between Type Ib and Ic).
Fortunately astronomers soon realized that the Type Ia supernovae were a fundamentally different kind of explosion than the Type Ib, Ic, and Type II. In this way, we can again simplify all these kinds of supernovae into two types based on physical mechanism involved -- the white dwarf detonation versus the core collapse. Overall, there are about twice as many core collapse supernovae as there are white dwarf nuclear explosions. And for astronomers studying exotic objects such as neutron stars, pulsars, and black holes, as well as the phenomenon of gamma-ray bursts, all of these are thought to be formed by or related to the core collapse supernova.
A White Dwarf Goes Thermonuclear:
The Type Ia supernova involves the sudden explosion of a white dwarf star when that star has drawn too much matter onto its surface from a companion star. The white dwarf began its life as a star with a mass less than 8 times that of the Sun and eventually ran out of fuel with its core contracting into an Earth sized object with a mass less than 1.4 times that of the Sun. The life cycle of such a star (including our Sun) is available at "Life Cycles of Stars".
Because stars often form in pairs (binary systems) -- our Sun is not part of a binary system -- a white dwarf star in a binary system will draw material off its companion star if that star gets too close to it. This is due to the strong gravitational pull of an object as dense as a white dwarf.
Should the in-falling matter from the companion star cause the white dwarf to exceed a mass of 1.4 times that of the Sun, a mass called the Chandrasekhar mass after the scientist who discovered it, the white dwarf will suddenly have enough mass for the fusion process to restart. The oxygen and carbon elements making up the star begin to fuse uncontrollably, resulting in a thermonuclear detonation of the entire star. Nothing is left behind, except whatever elements were left over from the white dwarf or forged in the supernova blast. Among the new elements is nickel, which then undergoes fission, liberating huge amounts of energy, including visible light. Eventually, the gas ejected into space becomes a what astronomers long ago and somewhat misleadingly call a "planetary nebula."
The Core Collapse Supernova
A core-collapse supernova is the result of the collapse of a massive stars -- one with an initial mass at least 8 times the mass of the Sun, that has exhausted its available fuel supply to drive the fusion process in the core. Because these stars are more massive (some up to 50 times or more the mass of the Sun), they can fuse oxygen and carbon into heavier elements still, including neon, magnesium, silicon, and iron, each time liberating energy to keep the star shining. Such a star is often depicted as resembling "onion layers" of progressively heavier elements.
But once iron has been created, the star encounters a dire problem: iron requires more energy to fuse it into heavier elements than is released in the process.
As a result, once the core begins producing iron, the fusion process can't continue. At a certain moment, the fusion process shuts off and -- lacking the enormous outward pressure generated by the heat energy -- the star's own massive gravitational pull causes the core to collapse. In about 1 second, the core collapses from an object the size of the Earth into a neutron star, an object about 10 miles across, or into a black hole, an object theoretically of zero size.
The core collapse crushes electrons and protons together forming electrically neutral neutrons and many ghostly elementary particles called neutrinos. Nearly mass-less and seldom interacting with ordinary matter -- countless solar neutrinos pass through the Earth and everything on it all the time without ever "touching" matter -- the pressure inside the star as the core collapses is so great that the out-rushing neutrinos crash into the in-falling layers of the star. The collision is with such force that the outer layers undergo a titanic "bounce" that ejects them into space even as the violence of the bounce triggers fusion of even heavier elements occurs. This bounce caused by the release of so much gravitational energy is the supernova event. As in the case of the Type Ia supernova, the production of nickel and its subsequent decay causes the explosion to appear dazzlingly bright -- brighter than an entire galaxy for a week or so. Elements such as zinc, gold, silver, platinum, cobalt, and even uranium are forged in the supernova event.
As the supernovae runs its course, the outward expanding material from the exploding star collides both with gas and dust existing in the interstellar medium and internally as blobs of gas moving at different speeds strike each other. The result of such collisions are emissions of a whole range of light photons, including visible and X-ray light. Colorful nebulae are sometimes created. And since different elements produce different spectroscopic signatures when their light is analyzed at different frequencies, it is possible to identify which element is present.
Traveling out through space, the light of a supernova is visible across tens of millions of light years. The neutrinos also travel outward -- their initial detection usually proceeds the supernova light by a brief period.
The elements fused by the star and in the subsequent supernova are the basis of a new generation of stars and planets, the latter including both the gas giants and the small, rocky worlds such as Earth. Thus, in a very direct way, supernovae make possible life on Earth.
But what of the core? How do we know if it will collapse into a neutron star or a black hole? And just what are these things?
After Core Collapse: A Neutron Star, Pulsar, or Black Hole?
A Neutron Star
For the collapsed core left behind, if the mass is less than two to three times that of the Sun, it will either collapse into a neutron star. About 10 miles in diameter, intensely and uniformly hot, and almost perfectly spherical, a neutron star consists of the dead star's stellar core collapsed so tightly together that a teaspoon of matter weighs more than a mountain on Earth. The name "neutron star" comes from the fact matter is compressed so tightly protons and electrons are squeezed together inside atomic nuclei to form neutrons.
Rapidly spinning neutron stars (possessing powerful magnetic fields are called pulsars if their electromagnetic radiation is detectable from Earth in a regular pulse. The pulsation results from a favorable alignment of the Earth with the star's magnetic field, which is channeled into a tight cone or beam by the star's rapid rotation. It is possible that most or even all neutron stars are pulsars if observed from the correct angle and/or with sensitive enough equipment to pick up their pulse. [LINK to the Jodrell Bank Observatory web site to "hear" the sounds of various pulsars from slow ones to the millisecond ones.]
The spin frequency of pulsars can reach fantastic speeds, spinning hundreds per second, which is to say faster than a kitchen blender. Such pulsars are called millisecond pulsars and they can reach such speeds because they pull matter off of a companion star. The in-falling matter strikes the pulsar and causes it to speed up. Recently, scientists using data from the NASA Rossi Timing X-ray Explorer satellite were able to determine an upper limit on how fast a pulsar (that is, a neutron star) can spin. LINK TO BEYOND EINSTEIN PRESS PAGE: "Einstein's Gravitational Waves May Set Speed Limit for Pulsar Spin".]
A Black Hole
But what if the core is more massive than about 2.5 times that of the Sun? In that case, the gravitational collapse is so great that a "runaway" collapse occurs. The resulting "object" is certainly the most awesome known: the black hole. This is an object whose gravitational pull inside a certain radius called the "event horizon" is so enormous that nothing can escape it not even light. The laws of physics as we know them are literally stretched to a breaking point as space and time are "infinitely" curved/warped/distorted.
To understand this, think of the three dimensions of space (front-back, side-to-side, and up-down) and the dimension of time as being a single fabric that curves or bends in response to the presence of concentrations of mass. Such curves are "wells" created by gravity; that is, they are gravity wells. By using the analogy of a trampoline net on which stars of different sizes and masses (and hence gravity well strengths) are denoted by various objects of different weights (here used interchangeably with masses), we can get an idea of what this means.
Our Sun's gravity well in the fabric of space-time would be like a golf ball on the trampoline net -- causing a very small warp. The gravity well of a massive star would be like a bowling ball, causing a big warp. (A planet would be akin to a grain of salt, if that, on the net.) A neutron star would be akin to a small but very heavy shot put. But for a black hole, we would have to put an object so massive that the trampoline would within a certain small area be pulled downward infinitely -- an infinite gravity well. In practice, even if we could find such a concentrated heavy object, eventually we'd either put a hole in the trampoline net or rip the entire thing from the frame.
What actually goes on inside the event horizon is effectively cut off from our Universe -- no information can come to us. Effectively and maybe even literally, it isn't in our Universe anymore!