When a massive star explodes in a supernova explosion, for a few weeks its luminosity becomes about as high as that of an entire galaxy. Supernova explosions occur about once every 50 years per galaxy. To get a reasonably large sample of supernova explosions, one therfore has to permanently monitor a large number of galaxies. In the explosions, most of the stars' mass is recycled back into the interstellar medium. However, some of it stays behind and collapses into either a neutron star (1.5-3.0 solar masses) or a black hole (>3.0 solar masses). The density of a neutron star is the same as that of atomic nuclei (about 109 tons/cm3).
Charged particles cannot cross magnetic field lines. Therefore, the matter surrounding a neutron star can get to its surface only by spiraling along the magnetic fields lines onto the magnetic poles of the neutron star. It generates synchrotron radiation which is emitted along the magnetic field axis. If spin and magnetic field axes are not aligned, the radiation beam from the neutron star will continuously change its direction. If the Earth happens to lie on the cone described by the beam, we will observe pulses of radiation. Such neutron stars are therfore called pulsars. The puls rate is the same as the spin rate and varies from one puls every few seconds to almost 1000 pulses per second. The energy which the pulsar radiates away in its beams comes from the rotational energy of the pulsar. Therefore, the spin rate of a pulsar slows down with time. In some pulsars which are memebers of a binary system, the spin rate actually increases with time as orbital enery is converted into spin energy of the pulsar.
Astronomy 201 Homepage
Astronomy Mac Lab Homepage