For those curious, the lethal range is thought to be 5,000--8,000 light years, at least for supernovae GRBs, if directed toward our solar system. (GRBs are directional rather than omnidirectional.)

<https://www.universetoday.com/118140/are-gamma-ray-bursts-da...> (Via Wikipedia: <https://en.wikipedia.org/wiki/Gamma-ray_burst#Rate_of_occurr...>)

The energy released, in a few tens of milliseconds to a few hours, exceeds that of the Sun over its entire 10+ billion year lifespan.

(I'm not sure if neutron star / black hole events might trigger larger emissions lethal at greater ranges.)

Sounds like a Fermi paradox related fact. Specially in the center of the galaxy, I suppose.

This is why there exists the concept of a galactic habitable zone. See: https://en.wikipedia.org/wiki/Galactic_habitable_zone

Some valid criticism at the end there.

I wonder about a merger of a pair of supermassive black holes, say a billion solar masses each. Would it sterilize the whole galaxy?

A galaxy is a pretty dangerous place to be. Probably we should get clear at first opportunity.

A pretty big pair is scheduled to merge sometime in the next three years. Probably we will get enough warning to be watching carefully when it happens.

It would be interesting to hang a number on that, like how far away you could be and receive a dangerous dose.