A teaspoon of neutron star would outweigh a mountain

When a massive star burns through its fuel, its core implodes in a core-collapse supernova, and the inward crush is so violent that electrons and protons are mashed together into neutrons packed as tightly as the particles inside an atomic nucleus. The result is a neutron star: roughly the mass of the Sun squeezed into a sphere only about 20 kilometers across, small enough to drop inside a single city.

The density defeats the imagination, so physicists reach for the kitchen. The US Department of Energy notes that a teaspoon of neutron-star material would weigh around 10 million tons here on Earth. Britannica puts the mean density near 100 trillion times that of water, essentially matching the interior of a nucleus — which is why a neutron star is sometimes called one gigantic atomic nucleus.

Surface gravity runs about 100 billion times Earth’s, so fierce that the entire star is sanded flat: its tallest “mountains” measure mere millimeters. Some neutron stars, the magnetars, carry the strongest magnetic fields known — trillions of times Earth’s, and a thousand times stronger again than an ordinary neutron star’s. Their crusts can fracture in starquakes that unleash giant flares of radiation across the galaxy.

There is a ceiling. Above roughly 2 to 3 solar masses — the Tolman-Oppenheimer-Volkoff limit — even the outward pressure of degenerate neutrons can no longer resist gravity, and the object collapses the rest of the way into a black hole.

A neutron star is the densest thing that can exist while still being a thing at all.