A pulsar is a neutron star which, as it spins (having been accelerated to unbelievably rapid speeds), shoots out a continuous stream of electromagnetic radiation. From any given vantage point (for example, a telescope on Earth), these stars seem to "pulse," flickering on and off. The effect is roughly analogous to a lighthouse: while we are in the path of the "beam," the star is easily spotted, but when we are not, it becomes difficult or impossible to see.
- Pulsars -
A pulsar is a neutron star - one of the possible outcomes of stellar death. Dying stars follow predictable paths depending upon how large they are. Smaller stars, like our sun, will eventually form large planetary nebulae, at the centre of which the former core of the star will glow faintly as a so-called "white dwarf." Extremely large stars explode in supernovae, and form black holes.
In between these lie a range of stars which become neutron stars: stars which, when nuclear fusion ceases, are forced by gravity into an incredibly densely packed mass in which the normal spaces within the atoms have collapsed together. A neutron star may thus end up with a radius of less than a hundred miles, despite weighing several times as much as our own Sun. As it shrinks, however, it will conserve its angular momentum (according to the law of conservation of momentum, which may also be seen in figure skating, where skaters spin faster as they draw their arms closer to their body). The result is a small, extraordinarily heavy mass which completes its full spin in a matter of seconds.
In new neutron stars, this spinning is accompanied by the emission of an extraordinarily powerful "beam" of light and radiation, forming what we call pulsars. Eventually, over a hundred million years or so, the dying neutron star will run out of energy to sustain this beam, and, to continue with the lighthouse analogy, will seem to simply switch off, or burn out, forever. In the meantime, the beam continues to sweep around the sky once every few seconds, or sometimes even several times per second.
- The Danger -
Scientists have found pulsars in which the "beam" being emitted takes the form of various parts of the electromagnetic spectrum: radio waves, microwaves, infrared light, visual light, ultraviolet (UV) rays, X-rays, and gamma rays. The lower two-thirds of this spectrum is entirely harmless to humans: we use visual, infrared, and radio light on a regular basis in everyday life. Too much exposure to UV, X-rays, or gamma rays, however, can have dangerous consequences for life. Even just a relatively small increase causes a risk of cancer; being subjected to a constant, large exposure to these rays would have devastating consequences for all life on Earth.
Fortunately, however, the risk from pulsars is relatively low. All known pulsars are so far away from Earth that, while we can detect their "pulses" with the appropriate telescope equipment, they do not actually expose us to a meaningful increase in radiation. At least in theory, however, if a pulsar were to form as a star collapsed much closer to our own solar system, that could result in increased radiation, and therefore increases in cancer, birth defects, radiation sickness, and the various other consequences of exposure to radiation.
Even this, though, is relatively unlikely. The much more substantial risk posed to the Earth from dying stars is that of the gamma ray burst - a far more powerful beam of radiation which is emitted for just a few seconds from the north and south pole of a very large star as it dies and explodes into a supernova. Gamma ray bursts shoot out, in the space of those few seconds, more heat and radiation than our Sun will give off in its entire life. If one of those happened relatively nearby, and the beam happened to be pointing directly toward Earth, the consequences would be devastating. A sufficiently strong gamma ray burst would simultaneously disrupt much of the atmosphere (including our precious ozone layer) and deliver a severe or lethal dose of radiation to most life on the surface.