How many supernovas occur each year?

Answer 1

On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way.

However, there are an estimated 100 billion galaxies.

The reason we don't see them all the time, is the shear distance from Earth and the time it takes light to reach us.

See related for information about a supernova

Answer 2

Very rarely.

Stars last billions of years (e.g. our Sun [a G star] will last another 4 billion years).

Even though there are 400 billion stars in our Universe.

Less than 20 have ever been witnessed in History (dating back thousands of years)

Answer 3

On average, about 1 or 2 supernovae are observed in our galaxy each century. In the entire observable universe, it is estimated that a supernova occurs roughly once every 50 years. Supernovae are rare astronomical events that mark the explosive death of a massive star.