Pu-244 is a relatively long-lived isotope of plutonium, with an 80 million year half life. Once upon a time,someone suggested that the Cretaceous extinction might have been caused by a nearby supernova: if so, we should see some Pu-244 around, or so it was thought. We don’t.
But that needs to be revised: it now looks as if Pu-244 is _not_ made in a supernova. Instead, it is probably generated by colliding neutron stars. So nearby supernova over the past couple hundred million years are not ruled out, although they are a priori unlikely.
The asteroid/comet explanation for the Cretaceous extinction is still good.