Our sun is an undeniably impressive celestial body. Compared to Earth, its radius is 109 times larger, and it has a surface area equal to 12,000 Earths. Using the Kelvin scale, the sun’s temperature is around 5,772 K in the photosphere, 5,000,000 K in the corona, and 15,700,000 K in the center of the star.
But when it comes to density, the sun isn’t nearly as impressive. Its average density is around one-quarter the density of Earth's—perhaps not surprising considering that the mass of its photosphere is composed almost entirely of hydrogen (73.46%) and helium (24.85%), with very small amounts of oxygen, carbon, iron, and other elements.
Yet even the sun’s core, which is estimated to be around 12.4 times denser than the Earth’s, can’t come close to comparing with the density of a neutron star. These incredibly dense objects result when a massive supergiant star runs out of fuel and undergoes a supernova explosion and gravitational collapse. The high pressure causes the protons and electrons to fuse into neutrons as the core becomes compressed to a density comparable to that of atomic nuclei.
A neutron star starts as a supergiant the size of 10 or more suns, yet collapses into a relatively tiny object only 10 miles (16 km) across. It is prevented from further collapse due to repulsive nuclear forces and neutron degeneracy pressure (two neutrons cannot be in the same place at the same time). Even though neutron stars lose a huge amount of mass during the exploding and collapsing process, they still have around 1.5 times the mass of the sun. Under normal circumstances, a star with that amount of mass would have a diameter of more than one million miles (1.6 million km).
Neutron stars are affected by immense gravity that prevents them from rapidly expanding, which is what would happen if even a tiny amount of neutron star material was theoretically transported to Earth. Thankfully, such an occurrence (and the resulting explosion and vaporization of part of our planet) is firmly in the realm of science fiction. After all, a matchbox-sized amount of neutron star material would weigh more than 3.3 billion tons, around the same as a large mountain!
Massive, dense, and mysterious:
- Neutron stars have the highest stable density of any object and are the densest directly observable material in the universe (after all, we can’t look inside black holes). Imagine something close to twice as massive as the sun having a diameter the size of a city.
- Cold, non-rotating neutron stars will collapse into black holes if they are more massive than the Tolman-Oppenheimer-Volkoff limit, which is estimated to be between 2.2 and 2.9 solar masses.
- Astronomers have a hard time estimating the number of neutron stars in the Milky Way, as older neutron stars emit very little electromagnetic radiation. Estimates range from several hundred million to one billion neutron stars in our galaxy.
