Thor's most renowned weapon is his hammer, Mjolnir, which was constructed in an astronomical manner, very literally. In the Marvel world, Thor's hammer was forged in the centre of a dying star. When a star dies in our universe, its core burns so hot that it creates a dense substance known as "neutronium." A single teaspoon of the substance, like Thor's hammer, is thought to weigh billions of tonnes. Some of the science in Marvel's Thor films is based on beliefs we know in this reality to make it more credible.