Step Two: Neutron Stars

Astronomers have determined that the ultimate fate of average-sized stars those having up to 1.4 times the mass of the Sun (or 1.4 solar masses) is to become white dwarfs. But what about stars that start out with more than 1.4 solar masses? They obviously have stronger gravities. So it is only logical that their end will be more violent and result in the formation of an object even more dense than a white dwarf.

Indeed, a star possessing between 1.4 and perhaps 8 solar masses bypasses the white dwarf stage and proceeds to the next stop on the road to the black hole. (Scientists still differ on the mass of stars that will become neutron stars. Other estimates include 1.4 to 3.2 and 1.4 to 5 solar masses.) This heavier star goes through the same initial steps as a Sun-sized star depletion of hydrogen, expansion into a red giant, and the burning of helium. But after that, things happen very differently, as Begelman and Rees explain:

Massive stars are powered in later life by a sequence of nuclear reactions involving heavier and heavier elements. As each nuclear fuel is exhausted hydrogen fused into helium, then helium into carbon and oxygen, etc. the inner part of the star contracts becoming even hotter. . . . This process would proceed all the way up to iron. At every stage up to this point, the creation of heavier atomic nuclei releases energy that staves off gravitational collapse. But there are no nuclear reactions that can release energy from iron; iron is the end of the nuclear road for a star. What happens next is one of the most spectacular events known in astronomy. . . . Since there are no nuclear reactions that can extract energy from iron, the supply of fuel is shut off and the core suffers sudden and catastrophic collapse . . . in a fraction of a second. . . . The density of the collapsing core becomes so great that the protons and electrons [the charged particles of its atoms] are fused together to form neutrons, electrically neutral subatomic particles.

Because such an object is made up almost entirely of neutrons (forming a substance many scientists call neutronium), it is called a neutron star.

The collapse that creates a neutron star is so violent that it triggers a secondary catastrophe a stupendous explosion. In this spectacular outburst, called a supernova, significant portions of the star’s outer layers blast away into space. This material forms a gaseous shell, often referred to as a supernova remnant, that expands outward for thousands or even millions of years, growing increasingly thinner. (It grows fainter, too, except when lit up by the glow of any stars it passes.)

The bright object at the center of the Crab Nebula is the pulsar discovered in 1968. Such objects are actually neutron stars.

The rest of the star’s original mass is now concentrated in a ball of neutronium about ten to twenty miles across, roughly the size of a large city. So dense is the material in a neutron star that a tablespoon of it weighs at least several trillion tons. Furthermore, such a star’s escape velocity is nearly 125,000 miles per second, about two-thirds the speed of light.

All of this sounds convincing in theory. But astronomers had no direct proof of the existence of neutron stars until the late 1960s, when objects called pulsars began to be found. In 1968, for example astronomers discovered a strange object at the center of the Crab Nebula. Located in the constellation of Taurus, the bull, this bright, rapidly expanding cloud of gases is the remnant of a supernova that occurred in 1054 and was recorded by Chinese and Japanese observers. Modern astronomers noted that the object at the center of the nebula gives off regular, intense bursts, or pulses, of radiation at the rate of thirty per second. Appropriately, they named this and other similar objects pulsars.

It soon became clear that pulsars are neutron stars, which rotate (spin) at incredible speeds. This rapid rotation is caused by the enormous inward rush of energy that occurs during the star’s collapse into a superdense ball. As for why a neutron star pulsates energy, noted astronomer Herbert Friedman writes:

When a neutron star collapses, it also drags with it the original stellar magnetic field until it is concentrated one billion-fold at the surface of the neutron star. In the tight grip of such a strong field, plasma [hot gases] at the magnetic poles would be whipped around with the spinning star. This whirling plasma could generate [a] highly directional radio emission [i.e., radiation shooting out of a specific location on the star] that would beam into space like the light of a rotating searchlight beacon atop a lighthouse. As the radio beam sweeps over the Earth, our radio telescopes record repeated flashes.

Instant Death on a Neutron Star

Neutron stars have enormous gravity, which would cause a living creature to be crushed out of existence in a fraction of a second, as explained by the great science explainer Isaac Asimov in his book. The Collapsing Universe.

Suppose that an object with the mass of the sun collapses to the neutron-star stage and is only 14 kilometers [8.7 miles] in diameter. An object on its surface will now be only 1/100,000 the distance to its center as it would be if it were on the surface of the sun. The tidal effect on the neutron star’s surface is therefore 100,000 × 100,000 × 100,000 times that on the sun’s surface, or a million billion times that on the sun’s surface and a quarter of a million billion times that on the Earth’s surface. A two-meter-tall human being standing on a neutron star and immune to its radiation, heat, or total gravity would nevertheless be stretched apart by a force of 18 billion kilograms in the direction toward and away from the neutron star’s center, and of course the human being, or anything else, would fly apart into dust-sized particles.