How astronomers opened space factories to produce gold and nuclear fuel
What are gravitational waves?
As we already wrote, gravitational waves are ripples of space-time that occur when two superdense bodies begin to accelerate next to each other. Imagine a stretched canvas, onto which one steel ball is thrown - it will slightly push the canvas. If we put a second ball next to it, it will also push the canvas. But if we begin to quickly move the balls in a spiral, closer to each other, then the "pressed" places will begin to overlap each other and the fabric will go in waves. Something similar happens in space.
Waves weaken sharply with distance from the source. It follows from this that they are generally very difficult to detect. The mutual acceleration of two supermassive bodies occurs only before merging. And black holes rarely merge. Neutron stars - another candidate for mergers and acquisitions - may do this more often, but they are dozens of times lighter. That is, it is possible to “see” such an event only at much smaller distances than for black holes.
Everyone around is talking about gravitational waves and merging of neutron stars
Neutron stars - space factories of gold and uranium
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Moreover, the observation of mergers of such stars is extremely important. Astrophysicists have long calculated that without such a process, the picture of the surrounding Universe “does not add up”. Take our planet or our solar system - we have relatively large amounts of gold, platinum, iridium and uranium. This is good for jewelers and nuclear scientists, but completely contradicts all calculations of how such heavy elements should be formed. Stars like the Sun almost do not "produce" anything heavier than carbon - their mass is too small, the pressure in the center is also relatively low, and the fusion of the nuclei of such atoms in the center of our star does not go.
There are also supernovae. They are massive stars that explode at the end of their life. But they shouldn't give too many heavy elements. To get a lot of uranium or gold, it is necessary that more free neutrons "fly" into the nucleus of a lighter atom - and very quickly, because otherwise the nucleus will decay before it accumulates the required number of neutrons with which it can exist for a long time. And the process of recruiting neutrons in supernova explosions (s-process), as luck would have it, is too slow.
Therefore, a hypothesis was proposed for the so-called r-processes, or a rapid collection of neutrons by atomic nuclei. The problem is that it needs a lot of free neutrons around the atoms. The best candidate for this is a neutron star. Its diameter is usually less than the length of an average Russian city, but its mass is greater than that of the Sun. Therefore, there is a monstrous density of matter, and the gravitational field is 200 billion times stronger than the Earth's and seven billion times stronger than on the surface of the Sun.
Black holes rarely merge
From such gravity the atoms "flatten" each other, and part of the neutrons "fly out" from them. If two neutron stars collide, then atomic nuclei will begin to actively mix with neutrons at tremendous pressure and temperature. And this is exactly what is needed for the formation of gold, platinum, uranium and other cesium. It is believed that this is how about half of all elements heavier than iron that surrounds us arose. Yes, yes, the wedding ring on your finger carries the substance from the merger of a pair of neutron stars!
Gravitational waves as a gunner. Telescope as a gold digger
It was a great hypothesis, but it had a drawback - neutron stars are very "dark". When you have gravity 200 billion more powerful than Earth's, photons have a hard time leaving the surface. They are practically extinct, their radiation in the visible range is not very strong. Neutron stars are difficult to see for hundreds of light years. And mergers don't happen that often, and most are pretty far away. Before the registration of the first gravitational waves the year before last, it was very difficult to find traces of such an event.
On August 17, 2017, astronomers recorded fluctuations in space-time that lasted 100 seconds. They immediately suspected that it happened when two neutron stars approached and merged. For the first time there is an opportunity to prove old hypotheses!
However, gravitational waves are not all. Yes, the GW170817 wave recorded by the American LIGO detector (built, by the way, according to the scheme proposed in the USSR back in the 1950s) showed that this time bodies of 1.1-1.6 solar masses merged. Which is too small for black holes. But on the other hand, this is exactly the mass range that neutron stars can have. However, how to understand if gold, uranium and other elements with an unclear origin were formed there?
For this, telescopes and spectrometers of more than 70 observatories around the world were used. They saw both gamma radiation from the decay of heavy radioactive elements and spectral traces of cesium, tellurium, platinum, gold, and other elements. More importantly, they saw a kilonova flash. This is the name for an outburst in "a thousand new" stars, which, at the same time, is weaker than a supernova. Until now, they have only been seen through telescopes. And although there were suggestions that this is the merger of two neutron stars, it was impossible to verify this before the registration of the gravitational wave GW170817.
More gold needed, my lord
Observing traces of heavy metals is good. But it would be much better to make more of them, not to be limited to the current discovery. It's great that now humanity has LIGO and the ability to further search for kilonova using gravitational waves.
The point is that until we understand the frequency of such mergers, it will be unclear how much of the heavy elements originated in neutron stars. Besides, the merger is a dangerous event. When one hyperdense object with a diameter of Perm falls on another, the formation of heavy elements is accompanied by a powerful gamma flash. Astronomers have long been raising the question that such an event with its gamma radiation can sterilize the Earth. At least if it happens very close and our planet is "in focus" of the outbreak. Some researchers believe that this has already happened, which is why there have been mass extinctions on the planet. To understand how serious the threat is, and whether it is necessary to fight it, it would be a good idea to first find out how often such murderous "gold factories" break out.
ALEXANDER BEREZIN
