Neutron Star

When a star that is between about three to eight times as massive as the Sun dies, it goes in spectacular fashion. The star's core temperature is in excess of half a billion degrees kelvin, and must remain this hot for thermonuclear fusion reactions involving its last reserves of fuel to take place.

Then the fuel runs out. No longer able to produce fusion reactions to sustain it, the star collapses. The core—a few million trillion trillion tons of it—falls in on itself, and in the ensuing cataclysm the star's outer layers are flung outward in a supernova explosion.

Intuition might tell you that the collapsing core will keep falling, squeezing itself together until it becomes so dense that it can fall no farther. This is like when you crumple a sheet of paper into a ball. You can only squeeze it to a certain point, after which it is too tightly packed to reduce its size any further.

In the case of a collapsing star, a law of physics known as the Pauli exclusion principle describes this phenomenon. Atoms are composed of a nucleus surrounded by electrons. Electrons do not "orbit" the nuclei in the sense that planets orbit the Sun; rather, electrons exist in what are called "energy states," meaning that they have only certain amounts of energy. The Pauli Exclusion Principle states that two identical electrons may not share the same energy state. It is therefore possible for the energy levels of an atom to become completely filled with electrons, in the same way that an auditorium can only hold as many people as it has seats. Matter with its energy levels filled like this is called degenerate.

The Pauli exclusion principle will come into play when the Sun dies and its core collapses. The carbon and oxygen atoms will become squeezed together until the atomic levels are filled and the whole core becomes a ball of degenerate matter. At this point, the resistance of the electrons to gravity, or electron degeneracy pressure, will halt the contraction. This ball of hot, degenerate, carbon and oxygen atoms is called a white dwarf, and it is the fate of the Sun.

If the collapsing core is between 1.4 and about 2.5 times the mass of the Sun, however, the gravity will be so strong that the electron degeneracy pressure will fail. Unable to resist the weight of their own gravity, the atoms will be crushed into a ball made mainly of neutrons about 32,810 ft (10 km) across. This object is called a neutron star.