Remarkably, because the Sun, just like every other object we encounter throughout our lives, is mostly empty space. The electric fields within the objects lend them a sense of ‘solidity.” Slap your hand down on your desk: the resistance you experience is a direct result of those fields. One cannot regard electrons as occupying points, but, instead, exist in “clouds.” The Pauli Exclusion Principle*** does not permit the electron clouds in one’s hand to occupy the same quantum state as the electron clouds in the table.

To explain the comparatively minuscule size of a neutron star and that of the enormous Sun, we need to examine the relative sizes of subatomic particles. Regard, for instance, the simple hydrogen atom. Were we to inflate the nucleus to the size of a basketball, the electron would be found about two miles away!* The intervening space would be empty.** The atom is almost entirely empty space.

Now, regard the Sun. Even though the core is extremely dense, 150,000 kg/cubic meter-about 10 times denser than lead-, the density of a neutron star is 100,000,000,000,000,000 kg/cubic meter. Despite the solar core’s high density, it isn’t nearly as dense as it could be. The Sun remains so large because of two contrary forces: the gravitational compression is counterbalanced by the energy pressure expansion. The balance of these forces produces a state known as hydrostatic equilibrium.

Eventually, when the Sun reaches the end of its life cycle, it will cast its outer layers away to become a planetary nebula while leaving a white dwarf core behind. The density of this hot, planet-sized stellar remnant will equal about 1,000,000,000 kg/cubic meter. The strong gravitational compression within a white dwarf is not counterbalanced by energy pressure because the thermonuclear fusion reactions have ceased. Instead, electron degeneracy: the electrons within the white dwarf resist further compression.*** Consequently, even though a white dwarf is far denser than even the solar core, some space still remains within it.

However, when highly massive stars -those at least eight times more massive than the Sun- end their lives, most of them explode as Type II supernovae. Prior to this explosion, the outer layers collapse onto the star at half the speed of light. This incomprehensibly powerful force compresses the core so much that even electron degeneracy cannot prevent further size reduction. The resultant neutron star, about the size of a large city, is sustained by neutron degeneracy pressure. The protons and electrons are squeezed together to produce neutrons contained within the smallest possible volume. No inner space remains. A single teaspoon sized piece of a neutron star would weigh about one trillion kilograms!

So, a neutron star can be so small because all the empty space has been removed from it.

Consider the two facts below in reference to the amount of empty space within “ordinary objects.”

Remove all the empty space out of the moon and it could fit neatly inside a soup can.

Gather together all the human beings currently alive now. (8.004 billion). If you were to remove all the empty space from all those people, the resultant object would be the size of a sugar cube.

I hope this answer proves helpful.

*Yes, we know that electrons are not discrete and so do not occupy specific points in space. However, we’re trying to explain the relative proportions of atoms.

**Yes, we know about virtual particles and vacuum energy and the like, but if I have to keep writing footnotes, I’ll never get through this answer.

***This is an example of degeneracy pressure. The Pauli Exclusion Principle does not allow two electrons with the same “spin” to occupy the same energy state within the same volume.