Can you imagine a world without the sun? Astronomers’ research shows that if there is no sun, the average surface temperature of the earth will drop to below-17.78 degrees Celsius in a week, and it will drop to-73.33 degrees Celsius in a year. Most plants will die in a few weeks, and then most life will disappear because of the disappearance of the sun, and everything will be dead.
Now we know how important the sun is to us! However, something unfortunate happened. Recently, foreign scientists’ research on the metal abundance of the sun shows that we may overestimate the life of the sun by one billion years, and the sun may die one billion years earlier than we expected.
Elemental abundance controls the sun’s life gate
It is understood that scientists have previously deduced that the life of the sun is about 10 billion years based on the standard solar model that ignores the rotation effect, which is a theoretical value simulated by the stellar evolution program, not a real value. And this value only refers to the central hydrogen combustion life in the theory of star evolution.
"This is not to say that the sun really can only exist for 10 billion years. The accurate statement should be called the main sequence life of the solar model. In theory, the sun will become a red giant in about 10 billion years, but it still exists and will emit more sunlight. " Associate Professor Yang Wuming from the Astronomy Department of Beijing Normal University told the Science and Technology Daily reporter.
He said that there are many preconditions to get the theoretical value of 10 billion years, and it is necessary to assume the initial element abundance of the sun and the physical process inside the sun. The life of the solar model is different with different initial element abundances. Because it is impossible to really get the initial element abundance of the sun and determine its internal physical processes, the life of the sun and even other stars cannot be accurately estimated.
Twenty years ago, astronomers’ direct and indirect measurements of the sun showed that the metal abundance of the sun was around 1.8%; However, the latest solar spectrum measurement shows that the metal abundance of the sun is only 1.3%, which is far lower than the previous measurement results, and correspondingly, the life of the sun will be reduced by 1 billion years.
The so-called solar metal abundance is the general term for the abundance of solar elements except hydrogen and helium. This metal is different from what we usually call metals. Because the contents of hydrogen and helium in the sun account for the vast majority, astronomers call all the heavier elements in the sun "metals". Although the content of these metals is small, it affects the fate of the sun.
Because the electrons of metal elements easily absorb photons, which hinders radiation, the higher the metal abundance of the sun or star, the higher the opacity. The opacity of the sun or star is closely related to its life, temperature, brightness and other properties. Sani Vaniotch, a physicist who studies solar metal abundance at Stockholm University in Sweden, said that a very small amount of metal is enough to completely change the behavior of a star, and metal abundance can basically tell you how the star will die.
Influence of atmospheric model on solar lifetime estimation
The solar spectral measurement used in the latest research usually means that astronomers get the spectral lines of all measurable elements such as hydrogen, carbon, nitrogen, oxygen and iron through solar spectral analysis, and then compare them with the spectral lines given by the solar atmospheric model. When the two are consistent, the abundance of elements such as hydrogen, carbon, nitrogen, oxygen and iron used in the atmospheric model is considered as the element abundance of the solar photosphere. "However, this method cannot obtain the relative abundance of inert gases such as helium, neon and argon. The abundance of helium on the surface of the sun can only be inferred by helioseismology. " Yang Wuming said.
After the latest measurement results come out, many people are wondering: Why is there such a big difference between the solar metal abundance measured before and after?
In this regard, Yang Wuming explained that the old solar metal abundance was derived from the one-dimensional atmospheric model of local thermal balance combined with the observed spectral lines; The new solar metal abundance is derived from the three-dimensional atmospheric model of non-local thermal balance combined with the observed spectral lines. The old one-dimensional atmospheric model is considered to ignore many physical factors. Some scientists think that the new atmospheric model is closer to the real situation, and it is the improvement of the atmospheric model and the update of the transition probability data of atoms and molecules that cause the difference in the abundance of old and new metal elements.
In the evolution of stars, if other conditions remain unchanged, the decrease of metal abundance will lead to the energy generated by the central nuclear reaction radiating outward more easily. However, the energy tends to radiate outward, which will cause the star to contract more easily and increase the density and temperature of the center of the star. More importantly, the increase of density and temperature will accelerate the hydrogen combustion rate, which will lead to the shortening of the total time of central hydrogen combustion, that is, the main sequence life of the solar model will be shortened. However, if other conditions change, such as the increase of hydrogen abundance or the influence of the sun’s rotation on its evolution, then the aforementioned life reduction does not exist.
So, how to verify which method is more correct in the future?
In Yang Wuming’s view, the answer to this question lies in which metal abundance measurement is more accurate and whether the helioseismology inversion result is reliable. Only by giving the correct abundance of solar elements can we determine how long the life of the sun is. If we can give a new solar model of gold abundance consistent with the helioseismology inversion results, all related problems will be solved.
Life geometry is related to the theory of stellar evolution.
Chen Pengfei, a professor at the School of Astronomy and Space Science, Nanjing University, said that he was inclined to think that the initial life span of the sun was correct, and the life span of the sun would not be reduced by one billion years. "For several reasons, I prefer to think that the original model is correct: first, the measurement results of metal abundance in the solar wind by satellites support 1.8%; Secondly, the density of photosphere on the surface of the sun is high enough, so it is reasonable to adopt the approximation of local heat balance model in the past; Thirdly, the new model takes into account the inhomogeneity of the solar atmosphere, which is indeed an improvement, but it also ignores other factors, such as the influence of magnetic field; Fourth, to take a step back, even if the results of the new model are correct, because the distribution of metal abundance may be uneven, the metal abundance on the surface of the sun may be different from that inside. " Chen Pengfei said.
At the same time, Yang Wuming also agrees that the initial solar life is correct. He said that the life of the sun has not been overestimated by 1 billion years, and that the life of the sun is overestimated because only the reduction of metal abundance is considered. When the influence of hydrogen abundance increase and rotation on the evolution of the sun is considered, the life of the sun will not be reduced.
However, the astrophysicist Martin asp Lund of the Australian National University insists that the 1.3% solar metal abundance is correct. He said that all aspects of astronomy will be affected, and an accurate understanding of the evolution of stars has laid the foundation for almost everything.
So, if astronomers do overestimate the life of the sun by one billion years, what theories need to be revised?
In Yang Wuming’s view, if the life of the sun is really overestimated by 1 billion years, it shows that the theory of stellar evolution established by astronomers before is not complete, so they need to re-examine the theory and model of stellar evolution established before. For example, the standard theory of star evolution ignores the influence of star rotation, but in fact all stars are rotating. We have reached the time when we need to seriously consider the influence of rotation on the evolution of stars. Astronomical research on star clusters and galaxies depends on the theory of stellar evolution to a certain extent. If the theory of stellar evolution is wrong, the previous understanding of star clusters and galaxies needs to be adjusted.
