How to Know the Distance Two Stars Collide?

We know how bright these things are, which means, when we see them here on earth, we can tell how far away they are by measuring how bright they appear in our telescopes. So that if they become a really, really useful way to measure distances to other galaxies, which are otherwise very hard to estimate weright yet. And cataclysmic collisions, like between two neutron stars, actually happen out there in the universe, and we have seen it.