Where does the sun's energy come from, and will the fuel run out, causing it to become smaller and smaller and finally go out?

In a sense, the energy of the sun is inexhaustible. Scientists say that the lifespan of the sun is about 10 to 11 billion years. This is based on the speed of nuclear fusion caused by the core pressure and temperature of a star of this mass, which is used to calculate the speed of fuel consumption and how long it will take for the hydrogen fuel in the core to be consumed, thus deriving its lifespan.

All stars will stop fusion of hydrogen when they have consumed the hydrogen fuel in their cores, thus ending the main sequence stage and entering an unstable late stage of evolution.

Note that what is being said here is the exhaustion of energy in the "core of the sun", not the exhaustion of energy in the entire sun. Research suggests that the sun's nuclear fusion occurs in the core, which is one-quarter of the sun's radius, which only occupies a small part of the core of the giant sun, and only uses the fuel in that area.

Source of solar energy The energy of the sun, like all stars, comes from the fusion of hydrogen nuclei in the core. That is, due to the gravitational centripetal pressure formed by the huge mass of the star, the core part forms a high temperature and high pressure state of about 15 million degrees and 300 billion Earth's sea level atmospheres. In this state, the electrons outside the hydrogen nuclei are driven away, and the exposed hydrogen nuclei collide violently and fuse with each other, and the process of 4 hydrogen nuclei fusing into one helium nucleus continues.

This is what is called nuclear fusion. In this process, a mass loss of about 0.7% will occur. It is this mass that is converted into energy, which is transmitted from the core through radiation and convection in the form of electromagnetic radiation to the surface of the sun, and then released into space in the form of light and heat. How much energy is this? After observing and analyzing the spectrum of the sun and calculating it, scientists have concluded that a star with the mass of the sun consumes about 600 million tons of hydrogen per second, which is converted into about 595.8 million tons of helium. A mass loss of 0.7% is 4.2 million tons/s (second).

How big is this energy? According to Einstein's mass-energy equation E (energy) = M (mass) * C^2 (speed of light squared), 4.2 million tons of mass can be converted into energy of about 3.78*10^26J (joule)/s. The earth in the vast space can absorb one 2.2 billionth of this energy, about 1.72*10^17J/s, which is equivalent to the total amount of electricity generated by 10 million Three Gorges Dams.

The huge energy radiation pressure of the sun's core resists the centripetal gravitational pressure formed by its own huge mass, forming a balance, allowing the sun to burn stably and continuously for 4.6 billion years, and it can burn for another 5.4 billion to 6.4 billion years. This is the main sequence stage of the sun. So how much hydrogen can the sun burn in its lifetime? How much mass is lost in total, and what is the proportion of the total mass of the sun?

We can do a simple calculation: the total volume of the sun is about 1.41*10^18km^3 (cubic kilometers), and the nuclear fusion of the sun is only carried out at 1/4 of the solar radius core, which is about 2.2*10^16km^3. Therefore, the volume of fuel used only accounts for about 1.6% of the total volume of the sun. Since the density of the core is much greater than that of the surface under high pressure, the actual mass ratio is much greater than the volume ratio.

The sun burns 600 million tons of hydrogen every second, with a mass loss of 4.2 million tons. One year is 31,557,600 seconds, and if we calculate it based on 11 billion years, it is about 3.471336*10^17s, burning about 2.08*10^29kg of hydrogen, with a mass loss of about 1.46*10^27kg.

The total mass of the sun is 1.9891*10^30kg. After 11 billion years of burning, the hydrogen burned accounts for 10.46% of the total mass of the sun, and the loss of mass accounts for 0.0734% of the total mass of the sun. Hydrogen accounts for about 75% of the total mass of the sun. Even if calculated based on the total mass of the sun's hydrogen, only less than 14% of the total hydrogen of the sun has been used in 11 billion years.

The rate and amount of energy consumed by the sun during its 11 billion years of life are not exactly the same. They are slower in the early stages and faster in the later stages, but they can be roughly calculated based on this average. This shows that the sun has not consumed all its energy until its death, but still has plenty of energy.

Since there is still a lot of fuel, why do stars die? This is due to the laws of stellar evolution. The main sequence of all stars is the longest, accounting for more than 90% of the total lifespan of stars. Because the star formation period and the end of evolution are extremely unstable and the time is relatively short, the general lifespan refers to its main sequence period.

The lifespan of a star is limited by its mass. The larger the mass, the shorter its lifespan, and vice versa. Red dwarfs have the longest lifespan, which can reach tens of billions, hundreds of billions, or even trillions of years. The universe was born only 13.8 billion years ago, so no red dwarf has entered middle age, let alone died. Some massive stars have a lifespan of only a few million years. For example, R136A1, which is considered to be the most massive star known to date, has a mass of more than 200 times that of the sun, and its predicted lifespan is only about 3 million years.

Stars of different masses all die because the hydrogen in their cores is completely burned out. In fact, there is still a lot of hydrogen in the periphery, but this hydrogen will not go to the center to participate in nuclear fusion. When they die, they will slowly dissipate into space through expansion, or be blown into space in a supernova explosion, returning to nature and becoming a regenerated nebula, the raw material for the birth of the next star.

The debris left behind by stars of different masses after death is also different. Some are exploded to nothing, while others will leave a corpse. The sun will leave a white dwarf after death, and some massive stars will leave a neutron star or a black hole. This is not the scope of this article. Those who are interested can refer to my previous articles, so I will not go into details here.

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