Neutron stars pack more mass than the Sun into a sphere the size of a city.

The longer version

A neutron star forms when the core of a massive star collapses during a supernova. Gravity crushes protons and electrons together into neutrons, leaving matter so dense that a city sized sphere outweighs the Sun. The collapse happens in a fraction of a second.

Rotation speeds up dramatically as the core shrinks, and magnetic fields become enormously strong, producing beams of radiation that sweep past Earth like a lighthouse. Some are observed as pulsars. Their surface gravity is so intense that a mountain there would be only millimeters high. A teaspoon of such material would weigh an enormous amount if brought to Earth.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.