"Hubble observed what kind of explosion created by two neutron stars merging?"
In another galaxy two stars that were orbiting one other, a binary system, ended up merging together.
That event also spawned an explosion that we call a kilonova.
That event was so traumatic in the world of spacetime that it sent a pulse of gravitational waves through the universe.
They were detected, not by Hubble, but by gravitational wave detectors on the ground.
When we knew there was a gravitational wave event, and we pointed Hubble in the direction of that event,
we pointed it at that galaxy, we saw a beautiful picture of this explosive smudge in that galaxy
which was basically the pulse of light that came from that kilonova.
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