This is an isekai because there's more than one world involved, but I'm still not clear whether the protagonist is originally from Earth or a fantasy world, since he knows what a neutron star is. It's not impossible for denizens of another world to discover those given enough advancement in science, but his world didn't seem so advanced.
Anyway, everything so far is pretty bog-standard stuff; except for the possibility the protagonist is a fantasy world native transported to another fantasy world.