A star so dense that a single teaspoon of its material could weigh more than an entire mountain—that's the mind-boggling reality of a neutron star. These stars are the densest, most extreme objects in the universe, aside from black holes, and their weight is truly mind-boggling. So, why are they so incredibly heavy?

Neutron stars are the remnants of massive stars that have exploded in a supernova. When a star much bigger than our Sun runs out of fuel, its core collapses under its own gravity. This collapse is so intense that protons and electrons in the core are crushed together, forming neutrons. Now you have a star made almost entirely of neutrons—hence the name!

But the amazing part comes next. These neutrons are packed so tightly that a sugar-cube-sized piece of a neutron star would weigh about a billion tons—that’s the combined weight of all the humans on Earth! The reason they’re so heavy is that in a neutron star, there’s virtually no empty space between the particles. They’re crammed together at a density about 100 trillion times greater than anything on Earth.

This super-density also means neutron stars are incredibly small compared to their mass. While a neutron star might weigh more than our Sun, it would only be about 12 miles across—the size of a city.

And they don’t just sit there being heavy—they spin like crazy. Some neutron stars, known as pulsars, rotate hundreds of times per second, emitting beams of radiation like a cosmic lighthouse. If you were standing on the surface of a neutron star (though you wouldn’t survive, obviously), you’d experience gravity a billion times stronger than Earth's—so strong it would flatten you instantly!