A supernova is literally the biggest exploding star that humans have ever witnessed. Every exploding star is the brightest, most powerful explosion of a normal star known to the universe. One kind of supernova occurs when a dying large star goes out in a great bang! This spectacular event happens when a super exploding star approximately 5 times the mass of our sun goes super with a bang! This is called a superluminous supernova and can cause many problems for those on Earth.
The most common type of supernovae is made up of a core of hydrogen that spins fast in a black hole. There are other variations with these types of explosions but the main ones are made up of hydrogen and helium (sometimes with neutrons too). These types of explosions are very rare and astronomers are constantly looking for them with telescopes. Some scientists believe that there are only a few dozen such stellar explosions in the entire history of the universe and they occurred only once every ten billion years or so.
Another type is made up of two extremely hot metals, one in the outer shell and one inside the core. The outer shell is generally made up of beryllium, while the inner shell is made up of a highly metallic element. These types of explosions, which are also very rare, are often associated with extremely hot stars. One of the reasons that astronomers are able to detect these explosions is that they emit gamma rays - radio waves - and are very bright.
What causes a supernova? Usually, a supernova happens when an Astronomical body hits a white dwarf - a small, compact star like a dwarf planet or a brown dwarf - or a black hole. In the case of the white dwarf, this happens when the white dwarf is being swallowed by a black hole. When an Astronomical body like a black hole or a pulsar goes near a star like a supernovae, the supernova will hit the star and blow it to high heaven. The debris from such explosions then falls into a very dense core of the star, which is what we know as the nebula.
So, let us go back to our question: what causes a supernova? Well, if we take a long enough time to calculate the chances of a supernova occurring in a given year (not just the average because a supernova can happen at any time), we can estimate with some accuracy the chance of an explosion happening at any given time. Moreover, if you ever want to try to calculate this yourself, there are good programs online which can do this for you. Once the supernova happens, it leaves behind a shock wave and is very destructive, making it very difficult to locate and study in any detail. This is why it's very important to find out the precise location of the explosion (s) before it happens so that scientists (or people who are interested in space) can find and study it more accurately in the future.
The last question we would like to ask is - how does a supernova affect my chance of finding other similar explosions or supernovae? The reason is that a supernova is a very fast explosion, and a gamma-ray burst can travel through space for a very long time, passing many stars before slowing down and dying down. This means that any kind of companion star system (like a brown dwarf or a gas giant) could also be very much affected by a supernova and therefore blast up too. Alternatively, a much slower explosion might not be affected, but it could take a very long time to reach the point where it would cause such a gamma-ray burst in another system.