Shulou(Shulou.com)11/24 Report--

As the first stars born in the dawn of the universe, the third star family has always been mysterious. These stars are formed by the collapse of primitive gas clouds in the early days of the universe, so the elements are almost only hydrogen and helium, so they are also called metal-free stars. Unlike the stars we see today, metal-free stars are thought to have very large masses, usually hundreds of times the mass of the sun, so their lifespans are also extremely short.

However, with regard to the characteristics of metal-free stars, these are only theoretical and have not been confirmed by observation until now. The search for the first stars in the universe has also become a dream for astronomers.

Why are the first generation stars so hard to find?

First of all, people are born early. About hundreds of millions of years after the Big Bang, the first stars began to form. To see the scene of that period, we need to look as far away as possible, because the farther the photons arrive, the later they arrive. The distance means that not only the number of photons reaching us is very small, but the wavelength at the same time is also stretched very long, that is, the redshift of the spectrum will become very large. In this way, the stars located there are very difficult for us to find.

Even the Weber (Webb) telescope, which is designed to observe the infrared band, is basically able to see quasars at this distance. It can be seen that even the brightness of the galaxy scale is only a very dim little red dot at this distance. It is conceivable that if it is a star, no matter how bright it is, it is very difficult for us to find it.

Unless you encounter a very coincidental gravitational lens effect, as in the case of the "Earendel" introduced earlier. With the help of this "magnifying glass", it is possible for us to find such distant and ancient stars.

In addition to being born too early, because of its huge mass, the lifespan of the primary star is also very short, perhaps only a few million or even hundreds of thousands of years. This is an "instant" thing for the universe, so they are even less likely to be discovered.

Because it is difficult to see directly, astronomers at this stage are more likely to indirectly confirm the existence of primary stars through traces of their existence.

We know that with the death of the first stars, there are a small amount of metal elements in the universe, that is, elements other than hydrogen and helium. These elements were then mixed with primitive gas to form a second generation of stars.

Although these stars contain some metal elements, they contain very little, so they are called "metal-poor stars". For these metal-poor second-generation stars, although the metal content is not high, it contains some of the chemical characteristics of the previous generation of stars (that is, primary stars), just like DNA. So in theory, we can look for clues of the primary stars by observing the chemical characteristics of the elements of this metal-poor star.

But the lifespan of the primary star is only "instant", so the second-generation star should have appeared very early, so isn't it equally difficult to find this metal-poor second-generation star?

In fact, for second-generation stars, they don't have to be as big as primary stars. In this way, for some small second-generation stars, they can live for a very long time and are still alive today.

And the biggest advantage of observing such stars is that we no longer have to bother to find the "edge" of the universe, and there may be such stars near us (that is, in the Milky way).

Although many metal-poor stars have been found later, and even some of them are very metal-poor stars, so far none of them is a clear second-generation star.

However, not long ago (June 2023), an article published in the journal Nature said that the research team of the National Astronomical Observatory of the Chinese Academy of Sciences seemed to have found the legendary second-generation star through the Guo Shoujing Telescope (LAMOST).

Guo Shoujing Telescope (LAMOST) because Guo Shoujing telescope is mainly used to survey the sky, researchers searched the spectra of tens of thousands of stars for metal-poor stars, and finally found a star with very low sodium content-J10102358.

Usually a star is located in the galactic disk of the Milky way, but this star is in the halo of the Milky way. The galactic halo is a globular region of sparse stars and interstellar matter on the periphery of the Milky way, where many globular clusters are distributed. The stars here are generally very old, almost as old as the Milky way.

Obviously, it's not surprising to find metal-poor stars here, and by analyzing the chemical characteristics of the star, the researchers found that the star seems to have been born in a very special supernova explosion-an unstable pair of supernovae, also known as a pair of unstable supernovae (PISN).

For primary stars, ordinary supernova explosions generally occur on stars that are not particularly massive (about dozens of times the mass of the sun). After their internal fusion reaction went all the way to iron, the core further collapsed into a black hole and caused a supernova explosion because it could no longer generate radiation pressure to counter gravitational collapse.

But for supermassive stars that are hundreds of times the mass of the sun, their endings are different. In the interior of these supermassive stars, the photon energy is so high that it interacts with atoms to produce a large number of positive and negative electron pairs. As a result, the radiation pressure inside the star will decrease rapidly, causing further collapse, and eventually the interior will explode violently due to thermal runaway. The explosion is so violent that the outer shell of the star will be completely blown up, and even the entire star will be completely blown up, leaving even the black hole too late. The supernova produced by this explosion is called an unstable supernova.

For the second generation stars born in unstable supernovae, they will show obvious parity characteristics in terms of metal element content: that is, the element content with odd atomic number is low, while the element content with even number is relatively high.

The more massive the star, the more obvious the parity effect, but these are just theoretical predictions that have never been observed in reality before. This time, the researchers found that the amount of elements in the outer layer of the star coincided with the parity. In addition, there are few elements above iron, and there are signs that the supernova explosion before the formation of this star is likely to be an unstable supernova predicted by the theory that only the first generation of stars will have.

This means that evidence of the existence of the first generation of stars has been found for the first time, and the relevant theories about the first generation of stars have been verified.

To sum up: although this discovery does not discover the "real body" of the first generation of stars, but only traces of its existence, it is still a milestone in the evolution of stars and galaxies.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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