If the Earth doesn't end in a helium flash, what will end in it?

The science fiction novel "The Wandering Earth" sets the sun to have a helium flash in a few hundred years, so humans wander with the earth. The movie "The Wandering Earth 2" "advances" the time of the sun's helium flash to 2078. In reality, will a helium flash occur in a few hundred years? If not, will the earth always be safe? If the earth is no longer safe, what can humans do to deal with it?

Written by | Wang Shanqin

In the science fiction novel "The Wandering Earth", the sun will have a helium flash in a few hundred years and destroy the earth. In order to survive, humans use planetary engines to propel the earth to wander. The movie "The Wandering Earth 2" directly "advances" the time of the sun's helium flash to 2078. Will the sun really have a helium flash in a few hundred years? If not, will it happen in the future? If the helium flash does not happen in the short term, will we encounter other challenges?

Image of the Sun with sunspots taken by the Solar and Heliospheric Observatory (SOHO) on October 28, 2003. Image credit: ESA

Helium flash: billions of years from now

The sun's energy comes from the fusion of hydrogen in its core. Hydrogen fuses into helium, releasing gamma rays that propagate outward, heating the matter they pass through, causing the sun to emit light and heat. The ignition temperature of hydrogen fusion inside the sun is about 15 million degrees.

Many people misunderstand the helium flash as the process of hydrogen fusion into helium in the sun. But in fact, the helium flash is the process of helium fusion into carbon in the sun, and the temperature required for helium fusion is about 100 million degrees.

The core of the sun is still in the process of hydrogen fusion. It must wait until the core hydrogen fusion process is completed before the core helium fusion will start. Then there will be a helium flash.

According to calculations, hydrogen fusion in the core of the sun will last for about 10 billion years, while the sun has only been fusion for 4.6 billion years. Therefore, it will take at least 5 billion years for hydrogen fusion in the core to end, for helium fusion to start, and for helium flash to occur.

Therefore, if we want to wait for the solar helium flash, more than 50 years will not be enough, nor will several hundred years. Instead, we will have to wait for at least 5 billion years.

Even if we wait for more than 5 billion years, we will not immediately see helium fusion and helium flash, because there is still a gap of about 1 billion years between the end of core hydrogen fusion and the start of core helium fusion.

During these 1 billion years, the sun's own gravitational compression has fused helium into a core. 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, and at some point the hydrogen outside the core will fuse, and the energy generated will heat the external matter, causing the outer layer to expand.

As this process continues, the core shrinks, the mass increases, and the temperature increases. On the contrary, the material layer outside the core expands and becomes colder. Therefore, the sun first becomes an orange subgiant, dozens of times brighter than the sun, and then gradually becomes a red red giant, hundreds of times brighter than the sun.

Schematic diagram of the Sun (left) and an artistic conception of the Sun after it becomes a red giant (right). Image credit: Department of Physics, NCKU

The highly compressed helium core also begins to undergo significant changes: because the electrons inside the core are compressed too close together, a strong repulsive force is generated. This repulsive force is not caused by the repulsion of charges of the same type, but by quantum mechanical effects. [Note 1] This repulsive force is called "degenerate pressure". The helium nucleus at this stage becomes a degenerate helium nucleus.

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 starts a fusion reaction, and the combined effect is: three heliums fuse into one carbon. Before the sun's helium fusion starts, it is already in a degenerate state, which causes helium fusion to immediately run away.

The reason for runaway fusion is that the pressure of a degenerate helium nucleus is mainly the degeneracy pressure, which is very insensitive to temperature. Helium fusion causes the temperature of the helium nucleus to rise rapidly, but the total pressure of the helium nucleus only increases slightly, so it will not be effectively cooled by expansion; the increased temperature causes the helium fusion rate to increase, which in turn causes the temperature to continue to rise...

This forms a positive cycle, just like when you are praised by the teacher for studying hard, you work harder, which leads to more praise from the teacher, so you work harder...

This cycle causes the power of helium fusion to increase sharply to more than 100 billion times the normal state, burning 6% of the helium core - 2.7% of the entire mass of the sun - into carbon within a few minutes. This waste of a few seconds to a few minutes is the helium flash.

Therefore, it will take at least 6 billion years for the Sun to have a helium flash. So, regarding the assumption that the Sun will have a helium flash in the next few tens to hundreds of years, the Sun will directly deny it three times: I am not, I have not, don't talk nonsense.

After a helium flash occurs in the sun, humans cannot observe a violent flash. This is because the energy released by the helium flash causes the helium core to expand, and the expansion process consumes this energy, leaving no remaining energy to be transferred to the surface of the sun. On the other hand, after the helium core expands, it changes 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 there are still humans and animals on Earth when the sun becomes a red giant. Therefore, there is no possibility that a helium flash will immediately destroy humans. Humans have not yet observed short-term changes caused by helium flashes in the cores of other stars. [Note 2]

However, the helium flash will still cause a long-term change effect. The rapid expansion of the helium core will cause the temperature to drop, which in turn will cause the temperature and pressure of the shell hydrogen to drop, causing the red giant to shrink as a whole. After about 10,000 years, the radius of the red giant will shrink to 2% of its original radius, and the brightness will drop to 2% of its original brightness. [Note 3] The surface temperature will also rise slightly, and the color will turn orange-yellow. The sun will once again become an orange subgiant.

Arcturus, the fourth brightest star in the night sky, is an orange-yellow subgiant. Image credit: Greg Parker

In other words, a helium flash will not only not cause the star to expand and brighten as a whole, but will cause it to shrink and darken as a whole.

The real solar crisis

Since the sun's helium flash will take at least 6 billion years, can humans rest easy?

This is not the case. This is because, except for the first few hundred million years, the brightness of the sun has been slowly increasing over the past few billion years. When the sun was first born, its brightness was about 85% of its current brightness, then it dropped to 75% of its current brightness over the next few hundred million years, and then in the approximately 4 billion years since then, the sun's brightness has continued to rise to its current value. In the future, the sun's brightness will continue to rise. When the sun is about 10 billion years old (6 billion years from now), its brightness will be twice as bright as it is now, as shown by the solid line in the figure below.

For most of the past and future, the brightness of the sun (solid line) gradually increased, the radius (long dash line) gradually increased, and the temperature (dashed line) remained basically unchanged for 10 billion years, and began to decrease after 10 billion years - becoming a red giant. The horizontal axis is time, in units of 1 billion years; the vertical axis is brightness, in units of the sun's current brightness. Image source: Ignasi Ribas, arXiv:0911.4872,
doi:10.1017/S1743921309992298, S2CID 119107400

The increasingly brighter sun causes the Earth to get hotter. After comets and other celestial bodies brought a large amount of water to the early Earth, about 90% of the Earth was covered with water. As the brightness of the sun continues to increase, this proportion has dropped to about 70% now.

This is a long-term global warming effect, 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 also get warmer. It is roughly estimated that in a few billion years, all the water on the surface of the Earth will evaporate into clouds, and it will no longer rain, and the sky will be full of clouds.

As the Sun expands into a red giant, its enormous brightness will directly scorch the Earth. After the helium flash, the Sun shrinks from a red giant to a subgiant, and after a brief pause, it will expand again into a red giant or even a red supergiant, and engulf Mars and Jupiter.

After that, the sun will have several eruptions, and the ejected material will become a planetary nebula. The remaining core will emit mainly X-rays and ultraviolet light. After a period of time, the core will cool and become a carbon-oxygen white dwarf. At this stage, the sun is too dim in the visible light band, and the entire solar system will also be dim.

The Dumbbell Nebula, photographed by the Very Large Telescope (VLT) of the European Southern Observatory (ESO), is a planetary nebula. Billions of years from now, our sun should also look like this. Image source: European Southern Observatory

Where is humanity heading?

Even before the Sun expands into a red giant, the Earth's survival is already in danger because the rising brightness of the Sun will cause the Earth's temperature to rise, making the Earth increasingly uninhabitable.

However, this change process will take at least hundreds of millions of years to become more significant, so humans can come up with various solutions during these hundreds of millions of years. "We are responsible for imagination, and someone else is responsible for realization."

First, humans can migrate to Mars. Mars is still relatively cold now. But as the sun gets brighter, Mars will also warm up. Although the atmosphere of Mars is thin, humans should be able to produce enough oxygen on Mars at that time.

Secondly, build several space cities and let them revolve around the sun in appropriate orbits.

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

The above solutions are not permanent solutions. After the sun expands into a red giant, the earth will be scorched and Mars will be too hot. Therefore, before that, humans must either move the space city to a farther orbit around the sun or migrate to other planets.

After the sun becomes a white dwarf, humans may have no choice but to move 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. By then, humans should be able to migrate to other planets or come up with other better solutions.

Zhuangzi once said in "Xiaoyaoyou": "The morning mushroom does not know the new moon and full moon, and the cicada does not know 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 life is no different from the lifespan of the morning mushroom (a legendary plant that dies when it sees the sun) and the cicada (a kind of cicada). However, it has only been a few thousand years since humans began to have written language, but they have roughly solved the mysteries of the universe and the evolution of stars. Naturally, we should have more confidence in the wisdom and survival skills of our descendants.

Notes

[Note 1] According to the Pauli exclusion principle in quantum mechanics, fermions of the same kind will produce this quantum mechanical effect when they are too close to each other, thus repelling each other. Electrons, protons, and neutrons are all fermions. For example, a white dwarf is a dense star that relies on the degeneracy pressure of electrons to support it.

[Note 2] Although the core helium flash cannot be observed, the "shell helium flash" occurring in the helium shell has been observed. The shell helium flash will cause the star's brightness to increase by 100-1000 times within a few days and then drop rapidly.

[Note 3] Taylor, David. The End Of The Sun,
https://faculty.wcas.northwestern.edu/infocom/The%20Website/end.html

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Produced by: Science Popularization China

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