After comparing photographs he took from a specific area of sky between August and September he noticed a tiny black dot that hadn't been there before. The explosion which caused the tiny black spot will outshine the combined light of billions of stars in the galaxy.
A supernova is a rare phenomenon which involves the explosion of most of the material contained in a star. The explosion results in an extremely bright object which emits vast amounts of energy. Mr Grennan spent the the last year surveying almost 3,000 galaxies before finding the supernova. In the last few years Mr Grennan also discovered two asteroids one of which he named 215016 Catherinegriffin, after his late mother.