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Pulsars: Mighty Mice

© Wesley Colley

Pulsars were discovered in the early '60s by Jocelyn Bell (as a graduate student), although her professor received the Nobel Prize for doing so. That discovery, it was quickly realized, confirmed a strange prediction of Chandrasekar (for which he later received the Nobel Prize), which was that burned-out stellar cores, if sufficiently massive, would have gravitational acceleration so strong, that it would break atoms.

Pulsars are really a sub-class of neutron stars. Neutron stars are the remnants of very massive stars (several times as massive as the sun) Chandrasekar predicted. Such stars have exhausted their nuclear fuel, and can no longer produce enough heat to balance the intense gravity. A star in this situation has burned hydrogen into helium on the "main sequence" for maybe 100 million years. That star would have been very bright, like Rigel, for instance. After the nuclear engine in the stellar core has used up all the hydrogen it could find, the core collapses until it is sufficiently dense to fuse helium into carbon and oxygen, then silicon, and so on, until the only available reactions left involve burning iron, which requires more energy than it produces. At this point, there is no nuclear energy left in the stellar core, and the core collapses under its own weight.

During the collapse, the core reaches a density where every atom is very tightly packed against its neighbors; now, the only thing left balancing gravity is the structure of the electron shells around the atoms - this halts collapse in less massive stars, which become white dwarves (as will the sun). The gravity in massive stars, however, is so strong that the electron shells collapse, and inverse beta decays begin, as atomic nuclei swallow their electrons. Within seconds, nearly all the nuclei in the core have eaten their electrons. These beta decays produces a phenomenal amount of neutrinos, because for every single electron that is eaten, an electron is emitted. Since the core mass is about 1.5 solar masses, the number of neutrinos emitted is on the order of 10⁵⁷ neutrinos, carrying a punch of 10⁴⁶ Joules, about a 100 times the total energy output of the sun over its entire lifetime. This phenomenon is known as a Type II supernova.

The remnant from this explosion is the core of the star, now at nuclear density (a teaspoon weighs as much as a large skyscraper) and is about 10km across. This is called a neutron star, because by eating electrons, protons became neutrons.

A pulsar is a neutron star, but one with an intense magnetic field, and rapid rotation. The field is typically a trillion times as strong as that of the