In January 1932, British physicist James Chadwick made a discovery that revolutionized the scientific conception of atomic structure. It had long been known that atoms were composed of positively-charged protons and negatively-charged electrons, but Chadwick had found the first direct evidence of something else: a neutral particle with no electrical charge of its own residing in the nucleus and containing slightly more mass than a proton. This mysterious counterweight had been previously theorized, but never before observed. Fittingly, Chadwick dubbed his find the “neutron.”
As it turns out, the neutron facilitates the energy-producing chain reactions that occur deep within stars. Stars are the veritable furnaces of the universe. They smash together prodigious amounts of hydrogen atoms in order to produce helium, creating thermonuclear energy in the process. A star can power itself for billions of years using this time-tested formula.
But eventually, an aging star’s supply of hydrogen fuel runs out. Desperate to sustain enough heat to push back against crushing gravitational pressure, it will begin fusing its helium atoms as a stopgap while its super-hot core contracts, almost as if entering a defensive crouch against the forces that would seek to tear it apart.
These last ditch efforts ultimately prove futile. Gravity always wins. The star enters its death throes, but what happens next depends largely on its mass. A sun the size of our own will see its outer layers expand to create a bloated husk of great size but diminished density. During this “red giant” phase, the star’s core contracts further, growing hotter even as it runs on fumes. Finally, as the last of its wispy gases are cast off into space to become nebulous clouds, only the shrunken core remains. It will slowly cool, turning into a far less luminous “white dwarf.”
But what happens to stars just a bit more massive than our own? Take one, for instance, with several times the mass of the Sun. It will have more gravitational pressure and thus collapse more violently when its time is up. Due to the shock of this collapse, this star will go out with a bang, literally: it explodes in a spectacular supernova that expels stellar material clear across the galaxy.
Prior to the 1930s, little was known about the remnants of these detonations.
But just two years after Chadwick’s neutron breakthrough, a Swiss-American astronomer named Fritz Zwicky posited that a leftover star core could be composed entirely of densely packed neutrons. His idea stemmed from the notion that the intense pressure of the stellar collapse would be enough to force all of an atom’s electrons to combine with its protons, thereby removing all the empty space and transforming every atom into a neutron through the sheer strength of gravity.
The idea of such a “neutron star” was initially met with skepticism. After all, Zwicky, somewhat of a scientific gadfly in his day, was proposing a very strange alchemy. To get a sense of how strange, imagine squishing a peach and a handful of raisins together so hard that they transform in to an orange.
The mathematics of a hypothetical neutron star were eye-popping too. By compacting that much stellar mass that efficiently, it would have incredible density and contain the weight of half a million Earths while still fitting within the city limits of Boston. A single teaspoon from a neutron star would weigh around ten million tons, roughly the weight of 20,000 jumbo jets.
Owing to their greatly diminished luminosity, neutron stars would be difficult if not impossible to view directly with a telescope. They would also be relatively rare, for they would only form out of stellar cores that are between 1.4 and 3.2 times the mass of our own sun. If the star is any weightier than that, its gravitational mass during a supernova will be heavy enough to crush even those dense neutrons out of existence, resulting in a black hole—a gravity well out of which nothing (not even light) can ever escape.
It took more than thirty years for Zwicky’s theory to be vindicated. In 1967, Jocelyn Bell, then a graduate student at Cambridge University, was analyzing data from a radio telescope when she noticed an unusual blip in her records. She slowed down her recorders and realized that an odd signal was emanating from a single point in the sky with astonishing regularity. For a while, she labeled this anomaly LGM-1—for “little green men.”
It was, in fact, a neutron star. Bell had stumbled across one of its quirks. As a neutron star shrinks and pushes those atoms ever closer together, it retains the angular momentum of the original star. This spin intensifies as the star gets smaller, increasing its rotational speed in the same way a figure skater gains speed when she pulls her arms in during a turn.
Neutron stars can make hundreds of rotations per second. This creates electromagnetic energy that runs along the surface to the north and south poles. From there, it shoots outward in the form of light, radio waves, and even X rays. When the beams from these cosmic lighthouses sweep across Earth, they can be measured. And so with Bell’s findings came a new term in the astrophysical lexicon: “pulsar.”
Today, there are hundreds of known pulsars with rotations ranging from 1.6 milliseconds to 9.6 seconds (an eternity by comparison). These interstellar clocks are incredibly precise, making them useful benchmarks for studying other interstellar characteristics like the curvature of the universe. Some of them have pronounced “glitches,” or irregularities, in their signals, but even those are of interest. By monitoring faint disturbances in these normally reliable beacons, astronomers hope to one day observe a gravitational wave—an actual ripple in the fabric of space-time itself. From Chadwick’s discovery to Zwicky’s predictions to Bell’s eureka, the once-fantastical neutron star has become one of our greatest resources for probing cosmic mysteries.
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