It was August 1967 when a young doctoral student at the University of Cambridge in the United Kingdom detected a signal never seen before . This signal consisted of an intense pulse of radio waves that repeated itself exactly every 1.33 seconds . She was Jocelyn Bell and what she had discovered was the first pulsar , an entirely new class of astronomical object.
However, during the first months after this surprising discovery, the possibility that its origin was extraterrestrial was strongly valued. After all, no astronomical phenomenon was known that could emit a signal with such an exact or short periodicity . In fact, this signal received the informal name of LGM-1, as a result of the initials of “little green men” (“little green men” in English), in reference to the popular conception of what extraterrestrial beings could look like. In December of the same year, another signal with the same characteristics was detected, which made it possible to rule out an alien origin .
But then the question arose: what was the astrophysical origin of these peculiar signals? It had been proposed for some years that neutron stars , which are formed after the death of massive stars in the form of supernovae, could harbor intense magnetic fields, which would emit large amounts of radiation . In addition, these stars could rotate very quickly , with periods of one second or less, due to the conservation of angular momentum.
This conservation of angular momentum is the same thing that makes it so that if you start spinning with your arms outstretched, you speed up when you bring your arms in and close. Today we know that this is indeed the origin of these signals discovered by Jocelyn Bell in 1967. The explanation of the origin of this phenomenon was given to Anthony Hewish , the supervisor of Bell’s doctoral thesis during which he made his important observations, part of the 1974 Nobel Prize in Physics . Award that was unfairly not shared with the true protagonist of this discovery, by decision of the selection committee of the Swedish Academy of Sciences.
During the supernova explosion, temperatures and pressures are reached so high that the electrons and protons present in the core of the star are able to combine , thereby overcoming the barrier created by the strong interaction, to give rise to neutrons , emitting neutrinos in the process. This accelerates the compression of the star in a cascading effect that ends up creating a compact core made entirely of neutrons and a rebound of the outer layers , which is properly known as a supernova explosion . The object that remains behind, what we call a neutron star, is a star barely ten kilometers in diameter , but with densities similar to those of an atomic nucleus (and one billion times greater than those of a white dwarf) and a mass comparable to that of the Sun.
The name neutron star can be confusing, because although this object is indeed made up of neutrons, it is not really a star . Stars are characterized by nuclear fusion reactions taking place inside them that keep the star alive, in a delicate balance between gravity, which tries to compress it , and the heat generated, which tries to expand it . In neutron stars there is no nuclear fusion process going on and their brightness and high temperature are remnants of the violent process that created them , but nothing more.
As we mentioned before, the conservation of angular momentum will make us go from a star of hundreds of thousands or millions of kilometers in diameter with a relatively slow rotation, to an object of just a few kilometers in size with very fast rotations . This compression and rotation also creates magnetic fields billions of times stronger than that of our planet Earth. It is precisely these magnetic fields that are capable of accelerating particles to very high speeds, being emitted in two conical jets that are fired in opposite directions . If the rotation of the neutron star causes one of these jets to point directly at the Earth, we will be able to observe the radiation emitted with a period equal to that of the star’s rotation. This is the origin of the hundreds of pulsars that we know of today in our galaxy, and of that first pulsar observed by Jocelyn Bell more than 50 years ago.
The period of a pulsar is so stable in time, that they can be considered as the clocks of the universe, becoming more precise than our atomic clocks . In fact, it is believed that the period of a pulsar will only slow down on the order of one second for every million years . This allows them to be characterized in great detail according to their period of rotation, and was the basis for using the period and position of fourteen nearby pulsars to indicate the position of the solar system on the plates traveling aboard the Pioneer and Voyager probes. Plates intended, ideally, for possible alien beings.
