A star dies

When a star runs out of fuel

At its center, a star joins light atoms together (nuclear fusion) to make heat, and that heat holds the star up against its own weight. When the fuel runs out, nothing holds it up anymore, and the center collapses. A star like the Sun becomes a white dwarf about the size of Earth. A star much heavier than the Sun explodes, leaving behind a neutron star (about 20 km across) or a black hole.

Why?A neutron star is more mass than the Sun squeezed into a ball the size of a city. A single teaspoon would weigh about 2 billion tonnes. A clock on its surface runs at about 0.8 of the rate of a clock in empty space.

Learn moreStars more than about 8 times the Sun’s mass explode like this, in what is called a supernova. If the leftover core is very heavy (roughly more than 2 to 3 Suns), nothing can stop the collapse, and it becomes a black hole. Scientists do not yet know this limit exactly.

Clock we are watching

A clock on a neutron star’s surface

How much slower

11 min slower per hour

Clock rate

0.81

seconds this clock ticks while a clock far away in empty space ticks one second · calculated with the formulas of relativity