Ask Ethan # 79: the smallest neutron star
What happens if you break a piece of a neutron star?
Sometimes the most interesting experiments in physics can be done only in your imagination. Despite the physical limitations that do not allow us to go anywhere, cut and study in detail any object of the Universe that interests us, our understanding of matter - in all its manifestations - and the laws governing it, has progressed far enough.
')
This week it was difficult for me to choose the most interesting question, but I settled on this brain-exploding question from Rui Carvalho, which sounds like this:
What are these stars - neutron?
Judging by the name, these are balls from neutrons, tied together due to the strongest gravitational attraction, with a mass approximately equal to the mass of a star like the Sun. But this, of course, is nonsense, since neutrons cannot exist for long. You can take any particle that interests you, isolate it and see what happens. For the three particles that make up most of the normal matter — protons, neutrons, and electrons — the results will be very different.
Electrons are the fundamental and the lightest stable particles with an electric charge. As far as we know, they are absolutely stable and do not disintegrate.
Protons are composite particles consisting of quarks and gluons. In principle, they can decay and we tried to investigate this issue. We built huge tanks filled with individual protons — each with 10 33 protons — and waited years to see if any of them would fall apart. After decades, we found that even if the proton is unstable, its half-life is at least 10 35 years, or 10 25 times the age of the Universe. Apparently, they are also absolutely stable.
But not neutrons! If you observe a free neutron, it will most likely disappear in 15 minutes, decaying into a proton, an electron and an antineutrino (its half-life is 10 minutes).
And then how do we get an object like a neutron star?
There is a difference between a free and a bound neutron, which is why many elements and isotopes do not decay: when a nucleus is connected, there is a certain amount of binding energy in it that is sufficient to keep neutrons stable.
In the case of stable elements, some configurations are more stable than others, and there are approximately 254 stable variants in total. It is possible that on a large time scale some of them will be unstable - we just haven’t discovered this yet. But they are not particularly heavy and do not contain too many neutrons. The heaviest stable element is lead, number 82, with four known stable isotopes: Pb-204, Pb-206, Pb-207 and Pb-208.
So, of all known stable elements, the nucleus with 82 protons and 126 neutrons is the heaviest.
But this is if we assume that nuclear forces bind the particles. And in the case of a neutron star, something else is responsible for this. To understand, let's understand how a neutron star arises.
The most massive stars - the brightest and bluest, born in young star clusters - synthesize helium from hydrogen in their nuclei, like all young stars. But unlike stars like the Sun, it takes them to burn all the fuel, not billions of years, but several millions, or even less, since ultrahigh temperatures and densities lead to a very high synthesis rate.
When they run out of hydrogen, the insides begin to shrink and warm the star. Upon reaching the critical temperature in the star, carbon begins to be synthesized from helium, which leads to an even more active energy release.
In just a few thousand years, both helium and the entrails end up shrinking even more, heating to temperatures that the core of the Sun will never reach. In such extreme conditions, the synthesis of oxygen from carbon begins, and then, in subsequent reactions, silicon and sulfur are obtained from oxygen, silicon from iron, and then we have a problem.
Iron is the most stable element. With 26 protons and 30 neutrons, it receives the highest ratio of binding energy per nucleon, which means that all other combinations are less stable. (By some parameters, nickel-62 is a bit more stable, but for simplicity, we will focus on hardware-56). Of course, there are elements heavier than iron, but they are not created by synthesis from iron. When the core is filled with iron, it begins to shrink under the influence of gravity, and the fuel for combustion no longer exists. And there is an incredibly hot and dense plasma, which is constantly heated and compacted.
Upon reaching the critical value - a surprise - the synthesis from electrons and protons begins, which leads to the appearance of neutrons, neutrinos and energy!
This reaction produces so much energy that in the supernova explosion the entire upper layer of the star is destroyed, since the synthesis of electrons and protons into neutrons and neutrinos takes only a few seconds.
The outer layers will take weeks and months to expand from the star, and the nucleus is compressed to a ball of neutrons by a huge force, not gravitational, but gravitational.
The neutron star weighs about the same as the Sun, and all this mass is concentrated in a volume with a radius of only a few kilometers. The star's density is 10 19 kilograms per cubic meter and it is the densest three-dimensional object known in the Universe.
In order for a neutron to be stable and not decay, its binding energy must exceed the mass difference between the neutron and the proton, of the order of 1 MeV, or 0.1% of the mass of the neutron. The neutrons in the nucleus are quite simple to restrain, and those on the surface will be in the most rarefied conditions. If the mass of the neutron star is equal to the solar, and the radius is 3 km, then the neutron bound at the surface will have 400 MeV of binding energy: enough to prevent its decay.
But what if we pull out a cubic centimeter of such matter, as Rui asks, from a neutron star? What will we get?
Unfortunately, the gravitational binding energy of neutrons on the surface will be only 0.07 eV - too little to keep them from decay!
In the real universe, there are similar situations: when neutron stars collide with each other. Most of the matter merges and forms a black hole, but about 3% of the mass is thrown away. Instead of the formation of exotic matter, it quickly disintegrates and leads to the emergence of a large part of the heaviest elements of the periodic table. If you were wondering where such elements as gold came from on Earth, it was from there: from the union of neutron stars!
So, if you extract a small mass of neutrons, it will quickly decay into stable (or long-lived) elements and isotopes from the periodic table, in a time not exceeding the lifetime of the neutron, and most likely much faster.
And if we wanted to break off a piece sufficient to ensure that the neutrons on its surface remained stable? To do this, it must be a radius of at least 200 meters.
The mass of such a piece will be comparable to the mass of Saturn and this is only the lower limit of the mass necessary to achieve your goal. Anything that will have less mass will disintegrate.
So, even if you wanted to believe that the hammer of Thor is made of the matter of a neutron star ...
Physics will not allow this. It is too small and the gravitational binding energy will be too small, as a result of which it will undergo rapid and catastrophic radioactive decay.
Thank you for the wonderful question, Rui, and I hope that if you dream of creating the smallest neutron star, you will start thinking on a large scale! Send me your questions and suggestions for the following articles.
Imagine what it feels like to fall asleep and not wake up ... Now imagine what it feels like, wake up if you didn't fall asleep.
- Alan Watts
Sometimes the most interesting experiments in physics can be done only in your imagination. Despite the physical limitations that do not allow us to go anywhere, cut and study in detail any object of the Universe that interests us, our understanding of matter - in all its manifestations - and the laws governing it, has progressed far enough.
')
This week it was difficult for me to choose the most interesting question, but I settled on this brain-exploding question from Rui Carvalho, which sounds like this:
If we broke off a piece of a neutron star (cubic centimeter) and removed it from it, what would happen to it?
What are these stars - neutron?
Judging by the name, these are balls from neutrons, tied together due to the strongest gravitational attraction, with a mass approximately equal to the mass of a star like the Sun. But this, of course, is nonsense, since neutrons cannot exist for long. You can take any particle that interests you, isolate it and see what happens. For the three particles that make up most of the normal matter — protons, neutrons, and electrons — the results will be very different.
Electrons are the fundamental and the lightest stable particles with an electric charge. As far as we know, they are absolutely stable and do not disintegrate.
Protons are composite particles consisting of quarks and gluons. In principle, they can decay and we tried to investigate this issue. We built huge tanks filled with individual protons — each with 10 33 protons — and waited years to see if any of them would fall apart. After decades, we found that even if the proton is unstable, its half-life is at least 10 35 years, or 10 25 times the age of the Universe. Apparently, they are also absolutely stable.
But not neutrons! If you observe a free neutron, it will most likely disappear in 15 minutes, decaying into a proton, an electron and an antineutrino (its half-life is 10 minutes).
And then how do we get an object like a neutron star?
There is a difference between a free and a bound neutron, which is why many elements and isotopes do not decay: when a nucleus is connected, there is a certain amount of binding energy in it that is sufficient to keep neutrons stable.
In the case of stable elements, some configurations are more stable than others, and there are approximately 254 stable variants in total. It is possible that on a large time scale some of them will be unstable - we just haven’t discovered this yet. But they are not particularly heavy and do not contain too many neutrons. The heaviest stable element is lead, number 82, with four known stable isotopes: Pb-204, Pb-206, Pb-207 and Pb-208.
So, of all known stable elements, the nucleus with 82 protons and 126 neutrons is the heaviest.
But this is if we assume that nuclear forces bind the particles. And in the case of a neutron star, something else is responsible for this. To understand, let's understand how a neutron star arises.
The most massive stars - the brightest and bluest, born in young star clusters - synthesize helium from hydrogen in their nuclei, like all young stars. But unlike stars like the Sun, it takes them to burn all the fuel, not billions of years, but several millions, or even less, since ultrahigh temperatures and densities lead to a very high synthesis rate.
When they run out of hydrogen, the insides begin to shrink and warm the star. Upon reaching the critical temperature in the star, carbon begins to be synthesized from helium, which leads to an even more active energy release.
In just a few thousand years, both helium and the entrails end up shrinking even more, heating to temperatures that the core of the Sun will never reach. In such extreme conditions, the synthesis of oxygen from carbon begins, and then, in subsequent reactions, silicon and sulfur are obtained from oxygen, silicon from iron, and then we have a problem.
Iron is the most stable element. With 26 protons and 30 neutrons, it receives the highest ratio of binding energy per nucleon, which means that all other combinations are less stable. (By some parameters, nickel-62 is a bit more stable, but for simplicity, we will focus on hardware-56). Of course, there are elements heavier than iron, but they are not created by synthesis from iron. When the core is filled with iron, it begins to shrink under the influence of gravity, and the fuel for combustion no longer exists. And there is an incredibly hot and dense plasma, which is constantly heated and compacted.
Upon reaching the critical value - a surprise - the synthesis from electrons and protons begins, which leads to the appearance of neutrons, neutrinos and energy!
This reaction produces so much energy that in the supernova explosion the entire upper layer of the star is destroyed, since the synthesis of electrons and protons into neutrons and neutrinos takes only a few seconds.
The outer layers will take weeks and months to expand from the star, and the nucleus is compressed to a ball of neutrons by a huge force, not gravitational, but gravitational.
The neutron star weighs about the same as the Sun, and all this mass is concentrated in a volume with a radius of only a few kilometers. The star's density is 10 19 kilograms per cubic meter and it is the densest three-dimensional object known in the Universe.
In order for a neutron to be stable and not decay, its binding energy must exceed the mass difference between the neutron and the proton, of the order of 1 MeV, or 0.1% of the mass of the neutron. The neutrons in the nucleus are quite simple to restrain, and those on the surface will be in the most rarefied conditions. If the mass of the neutron star is equal to the solar, and the radius is 3 km, then the neutron bound at the surface will have 400 MeV of binding energy: enough to prevent its decay.
But what if we pull out a cubic centimeter of such matter, as Rui asks, from a neutron star? What will we get?
Unfortunately, the gravitational binding energy of neutrons on the surface will be only 0.07 eV - too little to keep them from decay!
In the real universe, there are similar situations: when neutron stars collide with each other. Most of the matter merges and forms a black hole, but about 3% of the mass is thrown away. Instead of the formation of exotic matter, it quickly disintegrates and leads to the emergence of a large part of the heaviest elements of the periodic table. If you were wondering where such elements as gold came from on Earth, it was from there: from the union of neutron stars!
So, if you extract a small mass of neutrons, it will quickly decay into stable (or long-lived) elements and isotopes from the periodic table, in a time not exceeding the lifetime of the neutron, and most likely much faster.
And if we wanted to break off a piece sufficient to ensure that the neutrons on its surface remained stable? To do this, it must be a radius of at least 200 meters.
The mass of such a piece will be comparable to the mass of Saturn and this is only the lower limit of the mass necessary to achieve your goal. Anything that will have less mass will disintegrate.
So, even if you wanted to believe that the hammer of Thor is made of the matter of a neutron star ...
Physics will not allow this. It is too small and the gravitational binding energy will be too small, as a result of which it will undergo rapid and catastrophic radioactive decay.
Thank you for the wonderful question, Rui, and I hope that if you dream of creating the smallest neutron star, you will start thinking on a large scale! Send me your questions and suggestions for the following articles.
Source: https://habr.com/ru/post/369539/
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