There's a common misconception that Thor's hammer was forged from a dying star. It wasn't! It was forged inside of one, ya dummy. But just for fun, what would happen if Thor's hammer actually had been forged from a neutron star's demise?
Well, according to the newest Vsauce video, we have our answer: nothing good. Just one bit of dying neutron star the size of a sugar cube would have the same mass as roughly all of humanity—or 400 million tonnes. Now combine a bunch of those chunks into a hammer sized to fit the hand of a Norse god, and you're looking at 4.6 trillion tonnes. Or as the video puts it, "97 million Titanics condensed into the size of an American football." In other words, you have a damned heavy hammer.
Just be glad Thor's got a strong grip. Because if he were to drop it, he would essentially be recreating the meteor impact that wiped out the dinosaurs. Not that you'd be OK otherwise; just standing near the hammer would create enough of a gravitational difference to rip your body in half. Bottom line? Be glad Mjölnir isn't made of star stuff. If it were, it'd destroy humanity way quicker than Loki ever could.