Imagine a star that is one and a half times the mass of our sun packed together inside a sphere ten kilometers in diameter, the size of a medium-sized city. Now, we set it to rotate so that it rotates on the order of a thousand times per second. It is difficult to conceive, but that exists in our universe: it is a neutron star . And although its existence has been known for ninety years, its deepest nature remains shrouded in mystery.
What we do have evidence is that matter there is so highly concentrated – with a density equivalent to a thousand billion times that of water – and subjected to such high pressures that it does not appear in the form of atoms: it would form a kind of soup of neutrons and other subatomic particles with names as peculiar as pions. Its structure would collapse under the action of gravity, but it is maintained due to the so-called degeneration pressure. To understand it, let’s think about what happens in bars and nightclubs during parties: they are so crowded that there is no room, as is vulgarly said, not even a pin. If we wanted to enter, we should overcome the pressure exerted by other people, who seem to be practically stuck. Well, the same thing happens inside these peculiar stars: their weight, which tends to concentrate all the mass in the center, does not prevail because two particles of matter cannot occupy the same place at the same time.
What is a neutron star made of?
So far everything is correct, but what kind of matter are we talking about? Or, in other words, what is at the heart of a neutron star? Although theorists have proposed different models to explain it, it is not yet known with certainty what it hides inside.
To shed light on this point, the Neutron Star Interior Composition Explorer (NICER) telescope was installed on the International Space Station in June 2017, which took less than a year to make an exceptional discovery: a neutron star making a complete revolution. around his partner in just 38 minutes. Christened IGR J17062–6143, its orbit is less than the distance between Earth and the Moon, making this binary star system the most closed so far known. With a rotation period of 9800 revolutions per minute, it is what astronomers have dubbed a millisecond pulsar.
But why is it called a pulsar? Acronym in English for ‘pulsating star’, the story of the discovery of these stars contains all the necessary ingredients to make a good movie. Its first scene would start in 1930, when a young Hindu physicist, Subrahmanyan Chandrasekhar, was traveling by boat from his country to England to do a doctorate at the University of Cambridge. The long journey gave him time to study one of the research topics of who was to be his tutor: the evolution of the stars. This is how he calculated that if they have a mass less than one and a half times that of the Sun, they should end their days as white dwarfs.
With the same mass as the king star, these celestial bodies are composed exclusively of helium and contract until they reach a size similar to that of the Earth. Its matter is so compressed that a single teaspoon of white dwarf weighs more than a ton. But if the mass of the star is greater than 1.5 times the solar mass, then gravity prevails, the helium nuclei shatter, and the collapse continues. These calculations earned him the Nobel Prize in Physics in 1983.
What happens then if the star exceeds what is now known as the Chandrasekhar limit? The first clue was given by physicists Walter Baade (Germany) and Fritz Zwicky (Bulgaria) in 1933, when they proposed the existence of stars composed of an extremely dense soup of neutrons. They would be the remnant of a supernova, the final megaburst with which a massive star dies. In 1939, the one who a few years later would be known as the father of the atomic bomb, Robert Oppenheimer, and his student George Volkoff found that every star that ends its days with a mass located between the Chandrasekhar limit and about 3.5 times the mass of the Sun ends up becoming a neutron star. In these stellar corpses, the degeneracy pressure of the neutrons would stop the gravitational collapse. But this was still a theoretical exercise, and such objects disappeared from radar for almost thirty years.
Towards the end of 1967, a peculiar radio telescope erected in the English countryside near Cambridge, consisting of a series of poles in a row on which 2,000 mini-antennas sat, intercepted a completely unknown message: a series of very brief impulses, of a few hundredths seconds long, and spaced 1.3 seconds apart. Its discoverer was a doctoral student named Jocelyn Bell, who was dedicated to measuring the size of some radio emission sources. To do this, he patiently examined the records printed on strips of paper that the observatory spit out twenty-four hours a day. Bell and his thesis supervisor, Antony Hewish, concluded that such a precise signal must have a terrestrial origin. But they soon realized that it could not be, because it appeared every night about four minutes before the previous one, the same thing that happens to the stars. They christened the issuer LGM 1, the acronym for Little Green Men.
A crazy discovery
What had they found? They were not the emissions of an alien civilization, as might be thought. It was undoubtedly a space object, albeit of a totally unknown type. Then a rather iconoclastic Austrian astronomer named Thomas Gold took up the matter. He proposed that the newly discovered pulsars were rapidly rotating neutron stars. Too crazy?
That’s what the astronomical community thought, because they were not even allowed to defend their idea in a congress. But two years later, in 1969, after a pulsar was identified in the Crab Nebula, his unique proposal was accepted. Gold was very hurt by the criticism of his colleagues: “After that, I would never again compromise with the opinions of others,” he said.
The most fascinating part of his idea was that electromagnetic radiation released by a rapidly rotating neutron star is not thrown from its surface everywhere, like our sun or a light bulb, but in two specific directions, coinciding with its poles magnetic. That is why we observe a flash that turns on and off up to five hundred times a second, just like the lighthouses on the coast.
Since then, neutron stars have not stopped giving us surprises. The first was in 1974, when Joseph Hooton Taylor Jr. and Russell Hulse discovered a binary pulsar, the PSR B1913 + 16, whose two stars took only eight hours to orbit each other. This discovery made it possible to verify one of the most extraordinary predictions of the general theory of relativity: gravitational waves. Einstein conjectured that such a system must lose energy, because it emits strong gravitational radiation, a phenomenon that causes the orbit to get smaller. The observations of Taylor and Hulse confirmed this prediction and thus provided the first proof of the existence of gravitational waves; Detecting those space-time wrinkles directly was confirmed by the LIGO experiment, which announced the news in 2016.
Later, in 1982, a team led by Don Backer of the University of California (USA) discovered the first millisecond pulsar. It was the PSR B1937 + 21, named for its extremely short rotation period. In particular, this celestial object takes 1.6 milliseconds to rotate on itself; or what is the same, it makes 38 500 laps in a minute. And we don’t know why. Its magnetic field is usually much weaker than that of the others, although there are exceptions, such as the one discovered by Backer. It is believed that they are old neutron stars recycled thanks to the presence of a companion from which it steals matter, which causes its rotation speed to increase. Now, this theory does not explain the existence of pulsars like Backer’s. In addition, astrophysicists Bulent Kiziltan and Stephen E. Thorsett of the University of California (USA) recently showed that there must be an entirely different formation process.
Neutron stars and exoplanets
One of the great surprises that these peculiar objects have given us was when it was found that they had planets orbiting them. The first of these was discovered in 1992 by Aleksander Wolszczan, and revolved around the PSR B1257 + 12 millisecond pulsar. In reality, there are three ex-worlds there: one with a mass equal to one fifth that of Earth and two with about four times that of our planet. Nobody could believe it: the first extrasolar planets discovered were not around a normal star – they would be found three years later – but around a stellar corpse.
Since then, other exoplanets have been discovered in neutron stars, such as the one orbiting PSR J1719-1438 every two hours and ten minutes. It has the mass of Jupiter, is rich in carbon and oxygen, and is no more than 55,000 kilometers in diameter. With a density of about 23 grams per cubic centimeter – similar to that of a foam rubber or platinum mattress – it is assumed to have formed from the remains of a white dwarf. And the most fascinating thing about this object is that a large part of it is probably an immense diamond.
However, and despite all the enigmas that surround neutron stars, the greatest of all is found within them. What is it made of? Finding out is the main objective of the aforementioned NICER mission, which is dedicated to collecting X-ray emissions originating from pulsars. It is equipped with 56 gold-coated detectors that also record the moment of arrival of the pulse with an accuracy of one hundred nanoseconds, which is essential to determine its period of rotation.
Thanks to this magnificent observatory and the combined measurements of other instruments, the experts begin to collect precise data of the mass, the radius, the magnetic field … A whole panoply of essential information to understand what happens inside. “We are at the beginning of a golden age of neutron star physics,” declared with excitement the theoretical physicist Jürgen Schaffner-Bielich, of the Goethe University in Frankfurt, Germany.
Knowing the interior of the stars is complex, because we only have information about what happens on the surface. In the case of neutron stars, the task is complicated by the fact that we have little idea of what happens when matter is so compressed. Still, theoretical astrophysicists have made a rough atlas of its morphology.
Beneath a thin atmosphere of hydrogen and ice lie several layers made up of atomic nuclei, free electrons, and neutrons. But then we come to his heart, unknown territory. It is thought that it is divided into two zones: the outermost can consist of a quantum liquid rich in neutrons, while the inner … Well, here the physicists only venture to say that it is ultra-dense matter. As the astrophysicist from the University of California Jocelyn Read says, “It is one thing to know the ingredients and another is to understand the recipe and how those ingredients are going to interact with each other .”
Some think that there are quarks and gluons – the particles that make up protons and neutrons – roaming freely. Others believe that there may be other particles, like hyperons, made up of more exotic quarks. These are very unstable and disintegrate quickly, but perhaps they can withstand such inconceivable pressures. Another possibility is that in the center we have a Bose-Einstein condensate, a state of aggregation of matter that occurs at temperatures close to absolute zero (- 273.15 º C) and that is below the solid. In it, all the particles behave as if they were a single super atom.
In the latter case, the star should have a smaller radius than if it were made of ordinary material. And if it consists of hyperons, the nucleus could be even smaller. This is why knowing its size accurately is so important to science. Unfortunately, these types of measurements are very complicated and lead to many inaccuracies: they have only been calculated on a dozen pulsars, and the margin of error reaches 20%.
Similarly, measuring mass is also crucial. Today it can only be estimated in the case of pulsars in binary systems, since it is calculated from their orbital elements; even so, the estimates suffer from an experimental error of almost one solar mass.
For this reason, many hopes are placed on NICER: astronomers are convinced that from their observations it will be possible to obtain both the mass and the radius with sufficient precision and it will be possible to refine much more the so-called equation of state of neutron stars, that describes how the matter is inside. The first important advance occurred in 2018, when two teams of scientists, from the universities of Helsinki and Indiana, improved the existing one based on observations made with the LIGO and Virgo gravitational wave detectors. Both recorded the last minutes of the life of two neutron stars that were in full orbital collapse. Thanks to the data collected then, a much more precise equation was obtained, something that had not happened since 2012. Optimists, physicists think that the NICER findings will allow us to clean up the different models that populate the scientific literature.
And it seems that they are not wrong. The first objective of the observatory was a pulsar that rotates about two hundred times per second and is located 1,100 light years from Earth: J0030 + 0451. After analyzing 850 hours of observation, the scientists concluded that it has a mass between 1.3 and 1.4 times that of the Sun and a radius of approximately 13 kilometers.
But the most important thing about the NICER measurements is that we will be able to better understand what is characteristic of pulsars: their intense magnetic field, billions of times stronger than that of the Earth. Early data suggest that they may be much more complex than previously thought. Here, the theoretical effort is also arduous: understanding how a huge magnet interacts with ultra-dense matter is not available to everyone.
NICER, LIGO, Virgo and the Kamioka gravitational wave detector (KAGRA) –which became operational in February 2020– will allow us to delve into the mystery of neutron stars based on their gravitational footprints in space-time tissue. And they will be joined by space X-ray observatories that are in the design phase, such as the European-Japanese eXTP or the North American STROBE-X. Will we finally know what they hide inside? and
