Why did the first stars light up?

Why did the first stars light up?
We are all made of matter born in the bowels of exploding stars. There would be no them, there would be no carbon, no oxygen, let alone iron. There would be no chemical element heavier than helium. But what caused the first stars to light up? It turns out that the two most important characters in their story are dark matter and molecular hydrogen. I’ll try to explain why.


Dark matter

All matter in the Universe consists of what we see in the form of stars or interstellar gas – it is also called baryonic or ordinary matter – and matter that is invisible to us, the existence of which we guess only by its gravitational effect on the motion of stars and galaxies. Modern estimates show that this mysterious dark matter accounts for 84% of the total mass of matter in the universe.

Both dark and ordinary matter, most likely, has always been in the Universe, since the Big Bang. When it expanded and cooled, particles of ordinary matter, electrons, neutrons and protons, merged together and formed nuclei and atoms. However, in the first seconds of the existence of the Universe, such nuclei existed for a few moments: space was filled with intense gamma radiation, which easily destroyed any nuclei. After a dozen seconds, however, the temperature dropped to some trillions of degrees, which, although too much for the comfortable existence of our species, is cool enough for helium nuclei to survive. Primary nucleosynthesis began.

It lasted about 20 minutes. After which the temperature of the Universe became too low to support thermonuclear combustion and the fusion of new nuclei. As a result, the Universe turned out to be composed of 76% hydrogen and 24% helium, and in addition contained a small amount of lithium. It was all ionized because the temperature was still too high for atoms to form. This composition was retained until the first stars were formed, in the depths of which the process of further nucleosynthesis was launched.

But stars cannot be created just like that. Their forerunners are clouds, which form as a result of self-gravity in regions where the gas density is slightly higher than in the rest of the universe. The question is, why did the Universe turn out to be so highly heterogeneous that the number of formed stars was large?

When we look at the modern night sky, we can see with our instruments that it is permeated with a relic microwave background that originated about 13.5 billion years ago, just 377,000 years after the Big Bang. This background is like an imprint of the Universe of that time: as if we were looking at a photo of an 81-year-old man, taken a day after his birth.

The temperature of the Universe at that moment was already only 3000 degrees, this is the temperature at which it becomes possible for electrons and nuclei to form atoms, which led to the disappearance of plasma, and light finally got the opportunity to spread freely. So it has been flying ever since, forming a microwave background that permeates the entire Universe.

So, the problem is that this snapshot of the early Universe turned out to be too smooth. Judging by it, the inhomogeneities in it were so small that gas clouds simply could not have formed so quickly, and the stars would not have lit up to this day.

This is where the role of dark matter is important. The high homogeneity of the Universe is associated with its high temperature and, as a result, its high luminosity. Emitting a large amount of light led to a rapid cooling of slightly hotter areas, and its absorption – to heating of slightly colder ones, as a result, a high degree of uniformity was achieved. But dark matter does not interact with light! Consequently, it was more heterogeneous. And where there was a little more dark matter, gravity was a little stronger and pulled all matter there: both dark and ordinary. It was in these areas that the first stars formed.

Molecular hydrogen

So, in the Universe, which has already become neutral, the gas began to contract into clouds in those places where there was a little more dark matter. The potential energy of gravitational interaction was converted into kinetic energy of particles, and the gas was heated to high temperatures. About half a billion years after the beginning of the universe, the temperature of these gas clouds reached 1000 degrees.

But temperature alone is not enough for a star to emerge. We also need a high density of gas. But if the gas particles move too fast, then the desired density may not be achieved. One of the mechanisms for the cooling of hot gas particles was the emission of radiation into the Universe, which at that time had already cooled down below 100 Kelvin.

But atoms cannot cool down on their own. In fact, they can only exchange energy with each other: if one atom loses some of its energy, another atom will acquire it, and the temperature will not change. Effective cooling requires a kind of catalyst.

Molecular hydrogen became such a catalyst: that is, hydrogen molecules formed from two hydrogen atoms. While atoms are point objects, molecules are like dumbbells that can rotate around their axis. Such a dumbbell, upon collision with another atom, began to rotate rapidly, and then emitted a low-energy photon, which was already freely leaving the hot gas. Individual atoms did not have the ability to emit photons of such low energy.

But where did the hydrogen molecules suddenly appear in the hot gas of the early Universe? It turns out that many chemical reactions could have taken place even then. The most sophisticated modern models take into account up to 500 different reactions! Fortunately, it will be enough for us to consider only two of the most important ones.

The first was called the reaction of associative alienation by chemists. Initially, most of the atomic hydrogen in the cloud was in a neutral state with one electron in orbit and one proton in the nucleus. However, a small fraction of the atoms randomly captured two electrons into their orbit and formed negatively charged hydrogen ions. Such ions can react with neutral hydrogen, lose (alienate) an extra electron and form a hydrogen molecule:

H + H⁻ → H₂ + e⁻

As a result of associative alienation, about 0.01% of atomic hydrogen is converted into hydrogen molecules, but this is already enough to start the process of efficient cooling of gas clouds.

In chilled clouds, a second important reaction comes into play, called the three-body association:

H + H + H → H₂ + H

If three hydrogen atoms collide at the same time, two of them can form a hydrogen molecule, giving extra energy to the third atom. This reaction eventually converts the entire cloud into molecular hydrogen, which results in the gas cooling to a temperature high enough to form a star.

Stars

True, it is still not understood in detail how exactly a thermonuclear reaction is ignited in a dense cold gas cloud and the star itself actually appears. The complexity of this process is dozens of times greater than the complexity of those processes that we have considered in this article. Even the most powerful modern supercomputers cannot cope with its simulation. The main problem is the last 10,000 years of evolution of the gas cloud. The first 200 million years are modeled in 12 hours of massive parallel computations, but when the gas density rises to the stellar structure of the cloud, it begins to change faster and faster, so if the initial stages can be modeled in increments of 100,000 years or so, then for the last 10,000 years it is required to take a step no longer than several days! Such a calculation would take over a year, even on the fastest machine in existence. And human life is not enough to model all possible parameters of the stars. For this reason, we still do not know, for example, how the primary stars were distributed in terms of masses, and since it is the mass of a star that determines its temperature and the number of various elements that are formed in its interior, therefore, we do not know the chemical composition of the Universe. after the era of primordial stars. And we have only one hope: Moore’s Law.


Source: Daniel Wolf Savin. Before There Were Stars // Nautilus

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