What remains after a supernova explosion? Is the entire progenitor star blown to bits and dispersed throughout interstellar space, or does some portion of it survive? For a Type I (carbon-detonation) supernova, most astronomers regard it as quite unlikely that any central remnant is left after the explosion. The entire star is shattered by the blast. However, for a Type II supernova, involving the implosion and subsequent rebound of a massive star's iron core, theoretical calculations indicate that part of the star may survive. (Sec. 21.2) The explosion destroys the parent star, but it may leave a tiny ultracompressed remnant at its center. Even by the high-density standards of a white dwarf, however, the matter within this severely compressed core is in a very strange state, unlike anything we are ever likely to find (or create) on Earth.

Recall from Chapter 21 that during the moment of implosion of a massive star—just prior to the supernova explosion itself—the electrons in the core violently smash into the protons there, forming neutrons and neutrinos. (Sec. 21.2) The neutrinos leave the scene at (or nearly at) the speed of light, accelerating the collapse of the neutron core, which continues to contract until its particles come into contact. At that point, neutron degeneracy pressure causes the central portion of the core to rebound, creating a powerful shock wave that races outward through the star, violently expelling matter into space.

The key point here is that the shock wave does not start at the very center of the collapsing core. The innermost part of the core—the region that bounces—remains intact as the shock wave it causes destroys the rest of the star. After the violence of the supernova has subsided, this ball of neutrons is all that is left. Researchers colloquially call this core remnant* a neutron star, although it is not a star in any true sense of the word—all its nuclear reactions have ceased forever.

*(Astronomers commonly use the term remnant to mean whatever remains of a star's inner core after evolution has ended. Such objects are small and compact—no larger than Earth. They should not be confused with supernova remnants, which are the aftermath of supernova explosions: glowing clouds of debris scattered across many parsecs of interstellar space. (Sec. 21.3))

Neutron stars are extremely small and very massive. Composed purely of neutrons packed together in a tight ball about 20 km across, a typical neutron star is not much bigger than a small asteroid or a terrestrial city (see Figure 22.1), yet its mass is greater than that of the Sun. With so much mass squeezed into such a small volume, neutron stars are incredibly dense. Their average density can reach 10¹⁷ or even 10¹⁸ kg/m³, nearly a billion times denser than a white dwarf. A single thimbleful of neutron-star material would weigh 100 million tons—about as much as a good-sized terrestrial mountain. For comparison, the density of a normal atomic nucleus is about 310¹⁷ kg/m³. In a sense, we can think of a neutron star as a single enormous nucleus, with an atomic mass of around 10⁵⁷.

Figure 22.1 Neutron stars are not much larger than many of Earth's major cities. In this fanciful comparison, a typical neutron star sits alongside Manhattan Island.

Neutron stars are solid objects. Provided that a sufficiently cool one could be found, you might even imagine standing on it. However, this would not be easy, as a neutron star's gravity is extremely powerful. A 70-kg (150-pound) human would weigh the Earth-equivalent of about 1 billion kg (1 million tons). The severe pull of a neutron star's gravity would flatten you much thinner than this piece of paper!

In addition to large mass and small size, newly formed neutron stars have two other very important properties. First, they rotate extremely rapidly, with periods measured in fractions of a second. This is a direct result of the law of conservation of angular momentum (Chapter 15), which tells us that any rotating body must spin faster as it shrinks. More Precisely 15-1 Even if the core of the progenitor star were initially rotating quite slowly (once every couple of weeks, say, as is observed in many upper main-sequence stars), it would be spinning a few times per second by the time it had reached a diameter of 20 km. Second, newborn neutron stars have very strong magnetic fields. The original field of the progenitor star is amplified by the collapse of the core because the contracting material squeezes the magnetic field lines closer together, creating a magnetic field trillions of times stronger than Earth's field.

In time, theory indicates, our neutron star will spin more and more slowly as it radiates its energy into space, and its magnetic field will diminish. However, for a few million years after its birth, these two properties combine to provide the primary means by which this strange object can be detected and studied.