The death of a star
A supernova is a stellar explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light (300,000 km/s), driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe. Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova (though the news for Earth still isn't good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf).
There are two ways in which a star can go supernova:
The star can accumulate matter from a nearby neighbour until a nuclear reaction ignites
The star runs out of nuclear fuel and collapses under its own gravity
For a supernova to occur in the event of the second example above, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here's what happens next:
Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter towards the outside of the star.
Once the star's core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode (for this reason, these supernovas are also known as core-collapse supernovas).
The core heats up and becomes denser.
Eventually the implosion bounces back off the core, expelling the stellar material into space.
This image shows the remnants of a supernova. Supernovas can be so bright that they remain visible for several weeks or even months. It is possible to even see them during daylight. About 400 years ago people on Earth, including the famous German astronomer Johannes Kepler, saw the light from a supernova.
The explosion was so bright that it was visible with the naked eye even though it occurred about 13,000 light-years away. Scientists have been studying what became known as the Kepler supernova remnant for about 30 years, but the formation had left them baffled about the type of explosion that created it. So far, scientists know of two types of supernovas: core-collapse and thermonuclear. Previous images suggested that the Kepler remnant is surrounded by dense material, as expected from a core collapse. But the formation also appeared to contain copious amounts of iron, a signature of a thermonuclear explosion.
The image, made with NASA's Chandra X-ray Observatory, clearly shows abundant iron (yellow) and sparse oxygen, proving the Kepler blast was thermonuclear.
The above information was sourced from Wikipedia
Sometimes other nearby groups are included in the Local Group and then we talk about the Extended Local Group of Galaxies. These other groups may have played an important role in the Local Group dynamics, or may still do so.