Neutron Stars

Neutron stars are the endpoints of stellar evolution for the cores of massive stars that become supernovae if the collapsed core does not exceed about 3 solar masses. The adjacent figure shows one of the first observations of the orbiting Chandra X-ray Telescope. The tiny, faint dot in the very center of this supernova remnant may be the neutron star formed from the core of the star in the explosion.

Not Just Neutrons
The term "neutron star" is slightly misleading since we shall see below that neutron stars have a complex structure involving more than neutrons. However, in the deep interior it is a reasonable first approximation to view them as star cores where the pressure has become so large that the electrons have been crushed into the protons to form a pure fluid of neutrons (see the box).

Electron Capture and Neutronization

The formation of a neutron star results from a process called electron capture, which is a form of beta decay that we mentioned in conjunction with stellar energy production in Chapter 18. The process is also called neutronization, because its effect is to destroy protons and electrons and create neutrons. The basic reaction is

e - + p + --> n + electron neutrino

Speaking loosely, the negatively charged electron and positively charged proton are crushed together to form a neutron and a neutrino, neither of which carry electrical charge. This reaction is slow under normal conditions, but very fast in the high density and temperature of a core collapse. The neutrinos escape, leaving behind the neutrons. Because neutrons carry no charge, there is no electrical repulsion as in normal matter and the core can collapse to very high density once it has become mostly neutrons.

We think there are some 108 neutron stars in our galaxy. About 1000 of these have actually been observed by astronomers so far. Neutron stars typically have masses of around 1-2 solar masses and diameters of approximately 10-20 km. Thus, they have enormous densities that are similar to those encountered in the nucleus of the atom (the density of a neutron star is estimated in the box below). In fact, in certain ways, neutron stars are similar to giant atomic nuclei the size of a city.

Neutron stars had long been conjectured theoretically (see the right panel). However, the first clear detection of neutron stars was in the 1960s discovery of radio pulsars, which are spinning neutron stars that emit radio frequency pulses. We shall discuss the discovery of pulsars in the next module. Although most neutron stars have been discovered as radio pulsars, the vast majority of the energy emitted by neutron stars is in very high energy photons (X-rays and gamma rays, with the highest energies exceeding 100 million electron-Volts), rather than radio waves. Typically only about 1/100,000 of their radiated energy is in the form of radio waves.

Estimating the Density of a Neutron Star

We may estimate quickly the average density of a neutron star. The density is the mass divided by the volume and the volume scales as the cube of the radius. Within factors of about two, neutron stars have the same mass as the Sun. Therefore, the ratio of densities between a neutron star and the Sun is given by the inverse of their volume ratios, which is the inverse cube of the ratio of their radii. The ratio of the radius of the Sun (about 700,000 kilometers) to the radius of a neutron star (about 10 kilometers) is 70,000 and the cube of that is a little over 1014 (100 trillion). Therefore, the density of a neutron star must be about 100 trillion grams per cubic centimeter since the average density of the Sun is a little over 1 gram per cubic centimeter.