The first step in figuring out what a pulsar is requires finding its distance by making use of a phenomenon known as dispersion. The pulsar signals are modified as they pass through interstellar space, in particular, when they pass through regions of ionized hydrogen between the stars. Interstellar ionized hydrogen—in particular the thermal electrons that coexist with other components in space—causes radio waves to be slightly slowed down compared to the speed of radio waves in a vacuum. A pulse produced at the pulsar will arrive at earth at slightly different times depending on the frequency. This dispersion is measured by observing the arrival times of the same pulse at different frequencies. The amount of dispersion depends on the total number of electrons between the pulsar and earth. Astronomers know the average interstellar electron density is about 0.03 particles per cubic centimeter, and so the distance to a pulsar can be derived.
Most of 1,600 pulsars found to date are located throughout the disk of the Galaxy and tend to concentrate in spiral arms where the majority of stars are also found. Nine pulsars have been found in a nearby galaxy, the Large Magellanic Cloud, but none in other galaxies—so far at least. The farther away they are the weaker the signal is upon arriving at earth.
The majority of pulsar beams do not happen to flash in our direction, and taking this into account suggests there may be 500,000 pulsars in the Galaxy. They should therefore be born at the rate of about one every 10 years, which is odd, because supernovae, the likely source of pulsars, appear to be born at the rate of about one every 50-150 years.
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