Why are there not many detectable supernovae?

Question

Astronomers estimate that there are between 200 billion to 400 billion stars contained within the Milky Way, and that the Andromeda galaxy probably has 1 trillion stars. There may be around 500 billion galaxies in the observable universe.

So my question is, statistically speaking, because of the number of stars, shouldn't there be lots of stars going supernova, each at times glowing brighter than a whole galaxy? Why then do we not see many supernovae, for example like the 1987A? Why have we not been able to see one in our own galaxy since the SNR G1? Should there not be more supernovae in surrounding galaxies and even our own one as it has around 400 billion stars?

I appreciate that there are different types of stars with varying life span, but our Galaxy being almost as old as the Universe, surely there should be stars dying all the time?

Answer 1

Supernovae do occur all the time, if you are thinking on cosmic time scales. The number I have typically seen in the Milky Way is that supernova occur once per century, roughly. Maybe a little bit more often, maybe a little bit less. Relative to a the lifetime of a typical star, that is a very short timescale indeed.

Now, the relevant number of stars is not actually 400 billion. There are two basic ways of producing a supernova. One is with a very massive star. Massive stars, meaning stars at least about 10 times as massive as the Sun, are quite rare, relative to stars like the Sun and more low mass stars. The other is when a white dwarf experiences some sort of mass transfer (the details of which are still not understood) from another star, exceeds its maximum mass, and explodes. While binary star systems are quite common, not all binary stars are close enough that sufficient mass will be transferred from one star to the other.

You are correct that stars are dying all of the time, much more frequently than supernova are observed. The key is that when stars like our Sun die, they do not produce supernova.

One other thing worth noting is that we do not necessarily see all supernova. This seems on, on the face of it, ridiculous. Supernovae are extremely bright! However, since very massive stars produce supernovae, and also have very short lifetimes, some supernovae may be hidden because they occur while still surrounded by the remnants of the molecular cloud from which they formed. With more and more advanced telescopes, the likelihood of completely missing a supernova in the Milky Way now is pretty low, but there have been supernovae in the Milky Way in the past 400 years since SN 1604 that were not visible to the naked eye as a result of being blocked out by dust. The best known example is Cassiopeia A, which from its supernova remnant is known to have occurred in the mid to late 1600s, but was not recognized at the time. Looking around some more, it appears that another young supernova remnant has recently been identified (Youngest Stellar Explosion in our Galaxy Discovered) that was not seen by the naked eye.