A supernova was discovered in galaxy M82 during mid-January 2014. It was discovered by Steve Fossey of University College London.

On 31st January the supernova stopped brightening at a peak magnitude of 10.5, it is the closest Type 1a supernova to be discovered in the past 40 years.
Designated as SN 2014J is a type 1a supernova which occurs in a binary system (2 stars orbiting each other) one of the stars is a white dwarf. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion. White dwarfs are however capable of further fusion reactions that release a lot of enbergy as long as their temperatures rises high enough.

This is where the companion star of the white dwarfs plays an important role.
When this white dwarf accumulates matter from its companion star it rises the temperature of the core of the star and at a specific point the white dwarf ignites and nuclear fusion (carbon fusion) starts again.

Within a few seconds of starting the nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy to unbind the star in a huge supernova explosion.

The typical visual absolute magnitude of a Type 1a supernova is about 5.000.000.000 times brighter than our own Sun. This also allows us to see it over a distance (in the case of M82) of over 12 million light years.