system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Scientists studied remains of a kilanova (when two neutron stars collide) and measured one of the neutron stars to be 12.4 miles in diameter.

How could they have known it was 12.4 miles in diameter just by examining the explosion aftermath? I saw this on “How the Universe Works” but they never explain the reasoning.
gpt
Basically they are able to estimate the masses of the two neutron stars based on the gravitational wave signature. Since we know the density of the stars (they are made of nuetronium, the densest substance in the universe, or what we theorize anyway as we haven't ever created it)  we can calculate the volume of the star and then the radius. As other users pointed out, the actual estimation is 20km, and the conversion to 12.4 miles implies a presision that is incorrect, it would probably be more correct to say something like 12-13 miles in radius.