(ORDO NEWS) — Take a colossus with a mass of one and a half to two Suns. Compress it to a diameter of several kilometers, so that a cubic centimeter of matter weighs hundreds of millions of tons. Add the most powerful magnetic field in the universe. Wrap yourself in an atmosphere of particles moving at nearly the speed of light.
Recently, astronomers discovered the brightest pulsar of all located outside our galaxy, and in general one of the brightest pulsars. Surprisingly, this object was in front of astronomers for many years, but no one knew about its nature. An unusual method of searching for pulsars helped to establish the truth.
Below we will describe in detail what pulsars are and what they are eaten with. These are some of the most exotic objects in the universe and definitely worth talking about!
In the meantime, let us briefly recall that a pulsar (more precisely, a radio pulsar) is a neutron star with a powerful magnetic field. The reasons, which we will discuss later, turn it into a space radio beacon transmitting a strictly periodic signal.
Neutron stars are indispensable laboratories, arranged for physicists by nature itself. Mankind is unable to reproduce either the density of a substance of hundreds of millions of tons per cubic centimeter, or magnetic fields of billions of Tesla. So astronomical observations are the only way to check how matter behaves under such conditions.
But it was not there. Neutron stars, with rare exceptions, are practically invisible to conventional (optical) telescopes. The fact is that these are real crumbs: their diameter is measured in a few kilometers. For comparison: the diameter of the Sun is almost 1.4 million kilometers! So observing pulsars is one of the very few ways to study neutron stars.
Pulsars can also be used “in the national economy“, for example, for navigation and keeping time.
According to theorists, there should be tens or even hundreds of thousands of pulsars in our galaxy. Our radio telescopes are not yet good enough to detect them all. Observers know only a few thousand pulsars. It is even more difficult to look for such objects in other galaxies, and so far very few have been found.
The newly discovered pulsar PSR J0523-7125 is located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. In terms of flux density (roughly speaking, brightness), it is more than twice ahead of the previous “record holder” from this galaxy.
Of course, such a bright object has long been known to observers. However, no one suspected that it was a pulsar: it was unanimously mistaken for a distant background galaxy. So far, astronomers working with the Australian ASKAP radio telescope have not used an unusual method to search for pulsars.
They did not take advantage of the most obvious property of pulsars – the periodicity of their signal. Instead, scientists looked for sources of radio waves with strong circular polarization . This is another sign of pulsars, which, however, is rarely used to search for them.
After discovering PSR J0523-7125, astronomers observed it with the South African MeerKAT telescope. So they discovered the coveted periodic impulses. Those were unusually long. For the vast majority of pulsars, the pulse lasts only 1–5% of the period (the interval between pulses).
And for PSR J0523-7125, this share is as much as 35%. Figuratively speaking, this is not “peak … peak … peak …”, but “pi-and-and-and-and-and-ik … pi-and-and-and-and-and-ik … pi-and-and- i-i-i-ik … “. Perhaps that is why conventional methods did not recognize this object as a pulsar.
The authors hope that their method will make it possible to find many more unusual pulsars, including those in neighboring galaxies.
Little green men
The very history of the discovery of pulsars is very curious. It was the rarest case when scientists were seriously worried about whether they had stumbled upon the message of the brothers in mind.
Radio telescopes, which receive radio waves from the depths of the universe, work in general in the same way as household radios. You can even connect a speaker instead of a recording computer and try to listen to the “music of the spheres.” True, in fact it will turn out to be a meaningless crackle and hiss. But not in the case of pulsars.
In July 1967, Jocelyn Bell, a graduate student at the University of Cambridge, was processing data from a radio telescope.
The girl noticed a short radio signal, repeating exactly every 1.337 seconds. It wasn’t a random crack, but a regular, regular tick. It was as if somewhere in space a giant metronome was counting the beats – as it turned out later, comparable in accuracy to an atomic clock. To say it looked strange is an understatement.
At first, Anthony Hewish, Bell’s supervisor, thought the signals were interference from some kind of technology. They very much resembled the work of a radio beacon or a locator.
But the graduate student proved that the mysterious tick is born in space. And yet its regularity seemed defiantly artificial. The discoverers named the mysterious object LGM-1. This is an abbreviation for little green mans, that is, “little green men.”
It wasn’t a joke. The researchers were not in the mood for jokes. At that time, radio astronomy as a science was practically the same age as the young Bell. Mankind has just begun to “listen” to space in radio waves.
And it seemed quite possible that the galactic ether was teeming with transmissions from extraterrestrial civilizations. Not surprisingly, the discovery of pulsars (the so-called cosmic “beacons”) amazed and excited the scientific world.
Anthropologist Stanislav Drobyshevsky once said: “One of the main principles of any science is incredible tediousness.” Come up with an amazing hypothesis, and colleagues will make a list of one hundred and twenty-seven reasons why it is too early to accept it as a proven fact. It’s annoying, but this is the only thing that allows science to seek the truth without getting bogged down in heaps of fantasies.
As tempting as the alien theory may be, astronomers have been looking for a natural explanation for the strange properties of pulsars. It is clear that a periodic radio signal is generated by a periodic process in space, but what kind?
Hardly any formless cloud of gas can work with the precision of an atomic clock. Such a strict regularity suggested that we were talking about the motion of a rigid body. And what is this movement? Rotation around its own axis? Orbiting? Ripple?
I was also puzzled by the period of this movement – about a second. Whatever the space beacon was, it turned out to be very small. If the Earth, with its radius of 6400 kilometers, rotated around its axis in a second, the speed of a point on the equator would exceed 40 thousand kilometers per second, reaching 13% of the speed of light!
However, the nature of pulsars did not remain a mystery for long. All the pieces of the puzzle were already in the hands of the researchers. As early as 1934, just two years after the discovery of the neutron, Walter Baade and Fritz Zwicky suggested that neutron stars form in supernova explosions.
And shortly before the discovery of pulsars, Nikolai Semenovich Kardashev and Franco Pacini showed that a neutron star must rotate rapidly and have a powerful magnetic field. Based on these ideas, Thomas Gold unraveled the nature of pulsars soon after their discovery, although competing hypotheses were considered for some time.
At that time, neutron stars were known only to theorists, not to observers. The discovery of pulsars for the first time confirmed that neutron stars exist in reality, and not just in the calculations of astrophysicists. For this achievement, Hewish (but for some reason not Bell!) received the Nobel Prize in Physics in 1974.
Neutron stars are, so to speak, the afterlife incarnation of some luminaries. Let’s talk about this in more detail.
Any star would shrink into a tiny ball under the influence of its own gravity, if it were not for the pressure that prevents compression. Moreover, the decisive contribution to this pressure is made not at all by matter, but by radiation. The star is literally saved from death by the power of light its own light.
Throughout its life, the star “loses weight”: the mass is carried away by both the stellar wind and radiation. But still, the luminary remains quite massive until the very end. And when the thermonuclear fuel runs out, the rest of the star is left alone with gravity. It doesn’t end well for him.
If the original luminary at birth had a mass of more than ten suns, its death is accompanied by an impressive show.
The outer layers of the star, deprived of the support of radiation, rapidly fall on the dense core and bounce off it like a ball. The energy of this impact is such that the expanding shell of the star flares up like a whole galaxy. This phenomenon is known as a supernova explosion.
Meanwhile, the core of the star is rapidly contracting under the influence of gravity. Even atoms cannot withstand the growing pressure. In the center of a celestial body, electrons combine with protons, and a continuous mass of neutrons is obtained, denser than an atomic nucleus. And only then the monstrous pressure stops the compression.
If the core of a star is more than 2.7 times more massive than the Sun, then even the pressure of neutron matter is not enough. Then the core of the dead luminary turns into a black hole. But this is a completely different story, and here we are talking about neutron stars.
Let us now recall the law of conservation of angular momentum. A simple circumstance follows from it: if a body rotating around its axis is compressed, it begins to rotate faster. A figure skater who presses his arms to his body to perform a sheepskin jump will understand what this is about.
The compression of the core of a dead star stops only at a matter density of hundreds of millions of tons per cubic centimeter. This means that it shrinks to a size of several kilometers. According to the law of conservation of angular momentum, the speed of its rotation increases … to about one revolution per second.
In the star’s autobiography, one can imagine the chapter “How I Became Neutron.” “It was a difficult time for me. The time when I lost almost everything (of course, such mass loss! – Naked Science ). I had to become much stiffer and spin much faster. And no one calls me the sun anymore.
Rigidity is mentioned for a reason. The matter in neutron stars is perhaps the toughest and most durable material in the universe. Therefore, the celestial body does not fall apart from such a rapid rotation.
And if a colossus with a mass of 1.5–2.7 suns is spinning, it is very difficult to slow down or speed up such a flywheel. In other words, the speed of its rotation will be almost perfectly constant.
That is why a pulsar is a very stable clock. Strictly speaking, their course is still slowing down, but very slowly: less than a second in 100 million years. This happens as the energy of the rotation of the pulsar is spent on radiation.
However, the Universe is a well-known entertainer. Sometimes she manages to spin a neutron star and make an exotic millisecond pulsar out of a respectable second pulsar. Such an object, as its name implies, rotates around its axis in just a few milliseconds.
The mechanism of “promotion” is still not entirely clear. The most popular hypothesis is this. Millisecond pulsars are produced from neutron stars that form close pairs with ordinary luminaries. Neighborhood with a neutron star does not lead to good.
The powerful gravity of this monster literally sucks the substance out of the “respected partner”. Together with the matter, the neutron star is also given the angular momentum, and it begins to rotate faster.
In general, everything is almost like in life. In our country, who spins fast, he eats well, and in the world of neutron stars, cause and effect have changed places.
However, millisecond pulsars are still an exception. Most neutron stars rotate in a second or so.
A very stable periodic process with a period of about a second would definitely remind something of Jocelyn Bell. But how does the rotation of a neutron star generate a radio signal so powerful as to be picked up on Earth?
The fact is that neutron stars have another reason to be called the most-most. These are the most powerful magnets in the universe. The magnetic fields of pulsars are measured in billions of Tesla. For comparison: the most powerful field created by experimenters (and then for brief fractions of a second) is “only” 2800 Tesla.
Where do such impressive numbers come from? It’s about the law of conservation of magnetic flux. Yes, yes, again these conservation laws.
They are able to mess things up when the huge core of a star shrinks to a few kilometers. With a decrease in the size of a celestial body, its magnetic field thickens, as it were, concentrates. So it turns out a compact magnet of incredible strength.
Wait a minute, the reader familiar with physics will ask. Magnetic fields are generated by currents, and what kind of currents can there be in a neutron star? Neutrons have no electric charge, there is simply nothing to flow!
The fact is that only the central layers of a neutron star consist entirely of neutrons. Closer to the surface, where the pressure is less, there are still protons and electrons.
The movement of protons, according to theorists, creates currents of incredible strength in a neutron star. Moreover, these protons are paired like electrons in a superconductor. In fact, a neutron star is a superconducting magnet. Only the currents in it are not electronic, but proton.
A rotating magnet is actually a generator. The rotation of a neutron star creates a powerful electric field that pulls charged particles out of the crust. Therefore, despite the huge gravity, the pulsar is surrounded by a peculiar atmosphere of protons and electrons.
These particles move in a magnetic field at near-light speeds, generating a variety of radiation, from gamma rays to radio waves. At the same time, the strength of the magnetic field is such that energetic gamma quanta decay into electrons and positrons, replenishing the atmosphere of the pulsar. Matter is literally born from radiation!
In a seething cocktail of a superstrong magnetic field and matter accelerated to near-light speeds, many amazing and seemingly impossible things happen. This is not the place to talk about everyone, so we refer those who wish to the excellent popular science book “On the Pulsar” by B. B. Kadomtsev.
We will only tell how a periodic signal of a pulsar, or rather a radio pulsar, is formed. There are also X-ray and gamma-ray pulsars, these are also neutron stars with strong magnetic fields, but their radiation mechanisms are somewhat different.
Light the beacon
So, the radio waves of a pulsar are emitted by electrons and positrons circling around the magnetic field lines. This is the so-called synchrotron radiation.
However, near-light speeds do funny things with space. The particle radiates radio waves in all directions – from its own point of view, or, as physicists nerds say, in its frame of reference. But for an external observer like you and me, these “all sides” of it are compressed to a narrow cone. It turns out that the neutron star emits a thin radio beam.
The pulsar rotates around its axis, and the beam rotates with it. When this beam covers the Earth, the radio telescope “sees” a bright flash of radiation. This is the momentum of the pulsar. Then the beam turns away and returns on the next turn. So it turns out: peak… peak… peak…
Unless, of course, the Earth is on the ray path at all. It can also be directed in such a way that it passes by our planet all the time. In this case, we will not notice the pulsar, even if it is under our noses.
By the way, a neutron star does not remain a pulsar for long. Particles in its atmosphere flash their energy in just a few million years. After that, the former luminary goes into radio silence. According to theorists, there are about a billion neutron stars in our galaxy, and pulsars from them, we repeat, tens or hundreds of thousands. So each of them is a rare bird.
There is still much we do not know about neutron stars in general and about pulsars in particular. For example, sometimes mysterious interruptions occur in the operation of the cosmic clock, and the pulsar misses a pulse.
Why? A neutron star can’t miss a turn! And some impulses are so intense that they are called gigantic. It is still unclear where they come from. Questions without answers can be listed for a long time. They are waiting for their explorers.
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