Radio Astronomy of Pulsars Pre-lab
Introduction to Pulsars
Many of the most massive stars, astronomers believe, end their lives as neutron stars. These are
bizarre objects so compressed that they consist mainly of neutrons, with so little space between
them that a star containing the mass of our sun occupies a sphere no larger than about 10 km. in
diameter, roughly the size of Manhattan Island. Such objects, one would think, would be
extremely hard, if not impossible, to detect. Their surface areas would be several billion times
smaller than the sun, and they would emit so little energy (unless they were impossibly hot) that
they could not be seen over interstellar distances.
Astronomers were therefore quite surprised to
discover short, regular bursts of radio radiation coming from neutron stars—in fact it took them a
while before they realized what it was they were seeing.
The objects they discovered were called pulsars, which is short for “pulsating radio sources.”
The discovery of pulsars was made quite by accident. In 1967, Jocelyn Bell, who working for her
Ph.D. under Anthony Hewish in Cambridge, England, was conducting a survey of the heavens
with a new radio telescope that was designed specifically to look for rapid variations in the
strengths of signals from distant objects. The signals from these objects varied rapidly in a
fashion due to random motions in the interstellar gas they pass through on their way to
earth, just as stars twinkle randomly due to motions of air in the earth’s atmosphere.
surprised one evening in November, 1967 to discover a signal that varied
, not in a random fashion. It consisted of what looked like an endless series of short
bursts of radio waves, evenly spaced precisely 1.33720113 seconds apart.
The pulses were so regular, and so unlike natural signals, that, for a while, Bell and Hewish tried
to find some artificial source of radiation—like a radar set or home appliance—that was
producing the regular interference. It soon became clear that the regular pulses moved across the
sky like stars, and so they must be coming from space. The astronomers even entertained the idea
that they were coming from “Little Green Men” who were signaling to the earth. But when three