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This article comes from the official account of Wechat: ID:fanpu2019, by Wang Shanqin

The science fiction "wandering the Earth" assumes that helium flashover will occur in the sun hundreds of years later, so humans wander with the earth. The movie wandering Earth 2 "advances" the timing of helium flashover in the sun to 2078. In reality, will helium flashes occur hundreds of years later? If not, will the earth always be safe? If the earth is no longer safe, what can mankind do to deal with it?

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In the science fiction "wandering the Earth", the sun will destroy the earth with helium flash hundreds of years later. In order to survive, human beings use planetary engines to drive the earth to wander. The movie wandering Earth 2 directly "advanced" the time of helium flashover in the sun to 2078. Will the sun really have helium flashover in hundreds of years? If not, will it happen in the future? If helium flashover does not happen in the short term, will we encounter other tests?

An image of the sun with sunspots taken by the Solar and heliospheric Observatory (SOHO) on October 28, 2003. Photo Source: ESA Helium Flash: billions of years later, the Sun's energy comes from the fusion of hydrogen in its core. Hydrogen is fused into helium, and the gamma rays released spread outward, heating the passing matter, causing the sun to shine and heat. The ignition temperature of hydrogen fusion in the sun is about 1500 degrees.

Many people misunderstand helium flashover: helium flashover occurs when hydrogen in the sun is fused into helium. But in fact, helium flashover occurs when helium inside the sun is fused into carbon, and helium fusion requires a temperature of about 100 million degrees.

At present, the solar core is still in the process of hydrogen fusion, and the helium fusion of the core will not be started until the end of the core hydrogen fusion process. And then there's a helium flash.

According to calculations, the hydrogen fusion in the solar core lasts about 10 billion years, while the sun only fuses for 4.6 billion years. Therefore, it will be at least more than 5 billion years before the core hydrogen fusion ends, helium fusion can be started, and helium flashover can occur.

Therefore, if you want to wait for the helium flash of the sun, it is not enough to wait more than 50 years, and it is not enough to wait for hundreds of years, but at least more than 5 billion years.

Even if we wait for more than 5 billion years, we will not immediately wait for helium fusion and helium flash. Because there is an interval of about 1 billion years from the end of the core hydrogen fusion to the start of the core helium fusion.

During these 1 billion years, the gravitational compression of the sun itself has fused into the core of helium. The contraction of the core causes part of the gravitational potential energy to be converted into heat, causing the core to become hotter and hotter, causing hydrogen outside the core to be fused at some point, and the resulting energy heats up the external matter, causing the outer layer to expand.

This process continues, the core is getting smaller and smaller, the mass is getting bigger and higher, and the temperature is getting higher and higher. In contrast, the layer of matter outside the sun's core is getting bigger and colder. As a result, the sun first becomes an orange subgiant, dozens of times brighter than the sun, and then gradually becomes a red giant, hundreds of times brighter than the sun.

The schematic diagram of the sun (left) and the artistic imagination after the sun becomes a red giant (right). Image source: Department of Physics, NCKU's highly compressed helium core also begins to change significantly: the electrons inside the core are compressed too close, creating a strong repulsive force. This repulsion is not caused by the charge repulsion of the same charge, but by the quantum mechanical effect. [note 1] this repulsion is called "degenerate pressure". The helium nuclei at this stage become degenerate helium nuclei.

When the mass of the helium core reaches 0.45 times the mass of the sun and the temperature at the center of the helium core reaches about 100 million degrees, the helium core initiates the fusion reaction, and the comprehensive effect is that three helium are converged into one carbon. Before the sun's helium fusion starts, it is already degenerate, which causes helium fusion to get out of control immediately.

The reason for out-of-control fusion is that the pressure of degenerate helium nucleus is mainly degenerate pressure, but the degenerate pressure is very insensitive to temperature. Helium fusion leads to a rapid increase in helium core temperature, but the total pressure of helium core increases only slightly, so it will not be cooled effectively by expansion. The higher temperature leads to an increase in helium fusion speed, which in turn leads to a further rise in temperature.

This forms a positive cycle, just like you work harder because you are praised by the teacher because you study hard, so you get more praise from the teacher, so you work hard crazily.

This cycle leads to a sharp increase in the power of helium fusion to more than 100 billion times the normal state, burning 6% of the helium core-2.7% of the sun's mass-into carbon in a matter of minutes. The profligacy of a few seconds to a few minutes is helium flash.

Therefore, it will take at least more than 6 billion years for the sun to have helium flashover. So, for the assumption that the sun will have helium flashover in the next few decades to hundreds of years, the sun will directly deny the triple company: I am not, I am not, don't talk nonsense.

After the helium flashover occurs in the sun, humans cannot observe a violent flash. Because the energy released by the helium flash causes the helium nucleus to expand, the expansion process consumes this energy, and no remaining energy is transferred to the surface of the sun. On the other hand, helium nucleus expands from degenerate matter to ordinary matter, and helium fusion in the sun proceeds smoothly thereafter.

Therefore, the helium flash of the sun itself is harmless to humans and animals-assuming that when the sun becomes a red giant, there are still people and animals on earth. So there is no possibility that helium flash will destroy mankind immediately. Short-term changes caused by helium flashover in the cores of other stars have not been observed. [note 2]

However, helium flash can still cause long-term change effects. The rapid expansion of the helium core will lead to a decrease in temperature, which in turn leads to a decrease in the temperature and pressure of hydrogen in the shell, causing the red giant to contract as a whole. After about ten thousand years, the radius of the red giant has shrunk to 2% of the original radius, the brightness has decreased to 2%, and the surface temperature will also rise a part, and the color will become orange. The sun is once again an orange subgiant.

Arcturus, the fourth brightest star in the night sky, is an orange subgiant. Photo Source: Greg Parker in other words, helium flash will not cause the whole star to expand and brighten, but will cause the whole star to contract and darken.

The real solar crisis, since the sun's helium has been flashing for at least 6 billion years, does mankind have peace of mind?

and be not so. This is because, except for the early hundreds of millions of years, the brightness of the sun has been slowly increasing over the past few billion years. When the sun was born, its brightness was about 85% of what it is now, then dropped to 75% over the next few hundred million years, and then rose to its present value for about 4 billion years to date. In the future, the brightness of the sun will continue to rise. When the sun is about 10 billion years old (6 billion years later), it will be twice as bright as it is now, as shown in the solid line below.

In most of the past and future time, the sun's brightness (solid line) gradually increased, the radius (long dash) gradually increased, and the temperature (dotted line) remained basically unchanged for 10 billion years, and began to decrease after 10 billion years-becoming a red giant. The Abscissa is the time, the unit is 1 billion years; the ordinate is the brightness, the unit is the present brightness of the sun. Image source: Ignasi Ribas,arXiv:0911.4872, doi:10.1017 / S1743921309992298, S2CID 119107400 the brighter and brighter sun makes the earth hotter and hotter. After comets and other celestial bodies brought a lot of water to the early earth, about 90% of the earth was once covered by water. As the sun's brightness continues to rise, this proportion has dropped to about 70% of its current level.

This is a long-term effect of global warming, but it is not caused by carbon dioxide emissions, but by changes in the brightness of the sun.

As the sun gets brighter in the future, the earth will get warmer. According to a rough estimate, in more than ten or two billion years, all the surface water on the earth will be evaporated into clouds, and it will no longer rain, and the sky will be full of clouds.

In the process of expanding into a red giant, the sun will scorch the earth directly because of its huge brightness. After the helium flash, the sun shrinks from a red giant to a subgiant, and after a short stay, it will re-expand into a red giant or even a red supergiant, engulfing Mars and Jupiter.

After that, the sun will produce several eruptions, and the ejected material will become a planetary nebula, leaving behind a core that emits mainly X-rays and ultraviolet rays. After a period of time, the core cools and becomes a carbon-oxygen white dwarf. At this stage, the sun is too dark in the visible band, and the whole solar system will be dim.

The dumbbell nebula captured by the very large Telescope (VLT) of the European Southern Observatory (ESO) is a planetary nebula. Billions of years from now, our sun should do the same. Photo Source: European Southern Observatory, where are the human beings going?

Before the sun expands into a red giant, the earth is already in danger because the rising brightness of the sun will lead to a rise in the earth's temperature, making the earth more and more uninhabitable.

However, this process of change will take at least hundreds of millions of years to become significant, so human beings can come up with various ways in those hundreds of millions of years. "We are responsible for imagination, and someone is responsible for achieving it."

First of all, humans can emigrate to Mars. Mars is still relatively cold now. But as the sun gets brighter, Mars will heat up as a result. Although the Martian atmosphere was thin, humans at that time should have been able to produce enough oxygen on Mars.

Second, build a number of space cities and make them orbit around the sun in a proper orbit.

Third, find habitable planets around other stars and migrate in batches.

Some of the above schemes cannot be done once and for all. Because, after the sun expands into a red giant, the earth will be scorched and Mars will be too hot. So before that, humans either moved the space city to a farther orbit around the sun, or moved to other planets.

After the sun becomes a white dwarf, humans may have to emigrate to other planets.

However, it will take at least 6 billion years for the sun to expand into a red giant and eventually become a white dwarf, when humans should be able to emigrate to other planets or come up with better plans.

Zhuangzi once said in "Xiaoyaoyou": "the morning fungus does not know the dark new, and the cricket does not know the spring and autumn." The short life of human beings is obviously much shorter than the age of various celestial bodies in the universe; in front of them, the length of our lives is no different from that of Korean fungi (a legendary plant that dies at the sight of the sun) and oratosquilla (a kind of cicada). However, it has been only a few thousand years since human beings began to have words, but they have roughly solved the mysteries of the evolution of the universe and stars. We should naturally have more confidence in the wisdom and survival skills of our future generations.

Annotation

[note 1] according to the "Pauli incompatibility principle" in quantum mechanics, the same kind of fermions will produce this quantum mechanical effect when they are too close to each other, thus repelling each other. And electrons, protons and neutrons are all fermions. For example, a white dwarf is a compact star supported by electronic degenerate pressure.

[note 2] although the core helium flashover cannot be observed, the "shell helium flashover" in the helium shell has been observed, causing the star to increase its brightness by 10-1000 times in a few days, and then decline rapidly.

Taylor, David. The End Of The Sun, https://faculty.wcas.northwestern.edu/ infocom / The%20Website / end.html

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