Pulsars
The pulsar Powered Crab.
Credits : J. Hester and P.Scowen (ASU), (NASA)
Pulsars were discovered in the 1967 by graduate student Jocelyn Bell and her advisor Anthony Hewish; they were noticed because they emit short pulses in the radio spectrum. The pulses reach us at very regular, short intervals (usually around a second, but some are down to a thousandth of a second); pulsars can be considered as very accurate natural clocks. Now they are understood to be neutron stars. A neutron star is the final step in the evolution of a star three times more massive than the Sun.
After all the nuclear fuel is used up, the star explodes (supernova explosion). What is left is a very compact star, a white dwarf. However, in this case (i.e., parent star with greater than 3 solar masses) the dwarf is not stable. The density in a white dwarf is so large that it keep collapsing; now there is no fuel to burn and produce thermal pressure to sustain the star against further collapse. Electrons get expelled and protons convert into neutrons by the reaction: p + e+ à n + ne , where ne is the electron neutrino. The density of a neutron star is comparable to the density of a nucleus of an atom. Inside a neutron star the density approaches 1014 times the density of water. At these densities, neutrons, which otherwise decay into a proton and an electron plus a neutrino, are stable.
A neutron star posses magnetic fields that can be as much as 1011 - 1013 stronger than the magnetic field on Earth (which is a fraction of a Gauss, i.e., a thousand times smaller than the magnetic field of a household magnet; however, Earth's magnetic field is not small for a planet). In a pulsar the axis of rotation is tilted with respect to the magnetic axis, as it is on Earth, although the mis-alignment is greater. To understand where the pulses come from, we mention that charged particles that surround the neutron star spiral along the lines of the magnetic field, as particles coming from the Sun do near the Earth (producing, among other things, Auroras). As charged particles get accelerated at relativistic speeds, electromagnetic radiation is emitted. It turns out that this emission is concentrated along the magnetic pole axis. As the star rotates, the polar axis rotates too (precession), describing a circle in the sky. The electromagnetic radiation is emitted along this axis, and thus it rotates wit the star. The pulsar looks like a lighthouse, and if we are in the path of the light beam, we can record it.