supernovae provide events of known intrinsic luminosity ("standard candles") that can be used for distance measurement to, for instance, nearby galaxies they can also be used to determine when other stars will have a supernova. They are also the death of a star and the birth of a black hole.

That depends on how far away the star is. If the exploding star is 1,000 light years away we would see the supernova 1,000 years later. If it is 2,000 light years away we would see it 2,000 years later.

It is not currently known how a supermassive black hole forms, or how long it takes. However, since they are present in early galaxies, they must have formed soon after the galaxy (at the lastest), in a fairly short time. I would guess they took no more than a few hundred million years to form, perhaps even less.

the nebula stage in a stars life cycle lasts for 25,000 years before turning into a white dwarf and then into a black dwarf.

A near Earth supernova [See Link] is a supernova that occurs close enough to the Earth (less than 100 light-years away) to have noticeable effects on its biosphere. Gamma rays from a supernova induce a chemical reaction in the upper atmosphere, converting molecular nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation. This has been proposed as the cause of the Ordovician extinction - [See link], which resulted in the death of nearly 60% of the oceanic life on Earth. Type Ia supernova [See Link] are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernova occur from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. The closest known candidate is IK Pegasi [See Link] Recent estimates predict that a Type II supernova would have to be closer than 26 light-years to destroy half of the Earth's ozone layer.

SN 2006gy was the brightest and largest supernova ever discovered, scientists announced. The star was in the NGC 1260 galaxy, in the same direction as the constellation Perseus and may be a type of supernova previously predicted by theory but not observed. The conclusion was reached after extended observations of the supernova by both optical telescopes and X-ray telescopy. Currently, there are two possible explanations for the supernova's brightness, but both require the star in question to have been at least a hundred times as massive as the sun. Although the supernova is brighter than SN 1987A, which was bright enough to be seen by the naked eye, SN 2006gy is too far away to be seen by the naked eye.

The name is a bit of a give away really. A neutron star is a lump of matter composed entirely of neutrons, thus containing a large mass (typically between 1.4 and 3.2 solar masses in a very small volume. This matter is so dense that typically one teaspoon of neutron star (5 ml) would have a mass of about 5 and a half billion tons.

they are made from millions of kitfoxes

Yes and No. White dwarfs generally form from stars that have too low a mass to become supernovae. However, if the white dwarf has a companion star, it is possible for the white dwarf to accrete additional fuel from the neighboring star and then it can explode as a nova {See Link} or a Supernova type La [See link]

Your question does not make sense. A supernova does not die. A supernova is what happens when a supergiant or a hypergiant star becomes unstable and cannot maintain stability and explodes cutting away half of it's mass. I hope this helps you.

Most stars are dimmer than our sun (intrinsic brightness), and thus we cannot see them. However, most of the stars that we can see are actually quite a bit brighter than our sun. Some of them are exceedingly bright.

A supernova is when a massive star explodes.

The first possible supernova was after the death of the first stars, which would have been about 13 billion years ago. The last supernova could have been today.

A neutron star or a pulsar, or a black hole.

Several supernovas have been observed, in different years.

Several supernovas have been observed, in different years.

Several supernovas have been observed, in different years.

Several supernovas have been observed, in different years.

After a supernova explodes all of the remaining particles that have been released into space slowly start to compact together. After many years the particles once again combine with the other particles in space and create another star.

Neutron Star

Nebula.

Some nebulae are formed as the result of supernova explosions. The material thrown off from the supernova explosion is ionized by the supernova remnant. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova, SN 1054, in the year 1054 and at the centre of the nebula is a neutron star, created during the explosion.

When you think of a star think of something like the sun, while a supernova is the explosion of a star, while its at the ending of its life span. Star: "A star is a massive, luminous ball of plasma that is held together by its own gravity." Supernova: "A supernova (plural: supernovae) is a stellar explosion." Stellar: "Stellar is an adjective referring to one or more stars." Quotes are from: http://en.wikipedia.org/wiki/Supernova

Yes and no.

For a nuclear explosion to produce an EMP of enough strength to do damage beyond the range of the blast, thermal flash, prompt radiation, etc. it must occur in the ionosphere, where it can push large amounts of ionized gas large distances through the earth's magnetic field, causing an induced current of billions of amperes in that gas. This EMP can affect an area 1000 miles or more in radius around surface zero. It has been calculated that 3 properly placed nuclear explosions of the right yield in the ionosphere over the continental US would kill the entire US power grid and all solidstate electronics.

Ordinary air or surface bursts do not produce enough EMP for it to even be included in nuclear effects calculations.

The sun will never be a supernova - it is much too small. Only extremely large stars become supernovas. The sun will gradually burn out someday, and the ballpark for that is 3.5 to 4 billion years from now, though it will start to expand and redden some millions of years before that.

Much, if not all of the material that makes up the star is expelled at extremely high speeds (up to 30,000 Km/s). The shockwave from this explosion destroys any planets or other stellar bodies within its region of influence. The shockwave also pushes all the gas and dust surrounding the star into a vast, roughly sphereical cloud, for example, 'Kepler's supernova remnant sn 1604'. This accumulation of gas and dust can can result in the growth of new stars as the gas and dust clumps together.

The expelled material contains many higher mass elements such as nickel, cobalt and silicon and the material is the main source of elements heavier than oxygen which is the 8th element out of over 100 in the periodic table.

No. There are two ways of generating a supernova: a star at least 8 times the mass of the sun collapsing and exploding or a white dwarf interacting or colliding with a companion star. Our sun is not massive enough to explode when it dies and does not have a companion star.