Why Do Pulsars Pulse?

Seven thousand years ago, a supermassive star in the constellation we now call Taurus collapsed in on itself and exploded into a supernova so bright that—when its light reached Earth in 1054 C.E.—it could be seen in broad daylight. What was left behind was the brilliant Crab Nebula, a well as the Crab Pulsar that illuminates it. This neutron star pulses out radiation across the entire electromagnetic spectrum at a rate of 30 times per second. But why does it pulse at all?

Only half a century ago, nobody knew that pulsars, short for “pulsating stars,” existed.In 1967, when astronomers Jocelyn Bell Burnell and Antony Hewish first discovered a pulsating source of emissions all coming from the same point in the sky,among the first hypotheses was that these pulses were radio waves emitted by an alien civilization. Burnell and Hewish even went so far as to name the object LGM-1, short for “Little Green Men.” Subsequent discoveries of new pulsars, including the Crab Pulsar, ruled out the alien emissions hypothesis.

Today, scientists know that pulsars are generated by rotating neutron stars. The stars rotate quickly due to the conservation of angular momentum: When a large rotating body collapses, the remaining matter spins at a much higher rate, akin to the effect spinning figure skaters experience when they hold their arms close against their body. Some of these neutron stars have strong magnetic fields—in the case of pulsars, about 1 trillion times as strong as Earth’s—and emit a beam of radiation that coincides with their magnetic poles. This radiation can be the result of the quickly spinning star’s slowing momentum, the accretion of matter as it falls into the star, or the twisting of the star’s magnetic field. This magnetic axis is not always the same as the axis of rotation. When they do not coincide, the beam of radiation wobbles about the rotational axis. The result of this wobbling is a beam of radiation that, when viewed from Earth, seems to be pulsating.

Since Burnell and Hewish first discovered pulsars, astronomers have identified nearly 2,000 more, emitting visible light, X-rays, and, in some cases, only gamma rays. And while we have a general idea of why pulsars pulse, astrophysicists believe that there is still much more to discover.

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