As a big star runs out of fuel to burn, it begins to shrink. And as it shrinks, all its atoms are crushed together into a smaller and smaller area.
Then, even the particles that make up the atoms are squashed together too. The result is a small, very heavy ball of atoms called a neutron star.
A neutron star with the same number of atoms as the sun may be less than five miles across! A thimbleful of this star would weigh 74 billion tons.
Even a piece of a neutron star no bigger than your fingernail would weigh 2,000,000,000,000 pounds!