Solar evolution:
Poplar was formed in a collapsed hydrogen molecular cloud about 4.57 billion years ago. The formation time of the sun is determined by two methods: the age of the sun on the main sequence belt, confirmed by the computer model of star evolution and primitive nuclear synthesis, is about 4.57 billion years. This is very consistent with the radioactive dating method that the oldest material of the sun is 4.567 billion years.
The sun has reached middle age in the evolution stage of its main sequence. At this stage, nuclear fusion is to condense hydrogen into helium at the core. Every second, more than 4 million tons of matter are converted into energy in the core of the sun, producing neutrinos and solar radiation. At this speed, up to now, the sun has converted about 100 earth mass matter into energy, and the total time spent by the sun on the main sequence belt is about 100 billion years.
The sun doesn't have enough mass to explode into a supernova. Instead, it will enter the red giant stage after about 5 billion years, and the helium nucleus will shrink against gravity and become hot at the same time. The hydrogen cladding next to the core accelerates the fusion due to the increase of temperature, and the heat generated increases continuously, which is conducted to the outer layer and expands outward. When the core temperature reaches 1 100 million k, helium fusion will start and burn to generate carbon.
Because the helium core at this time is equivalent to a small "white dwarf star" (electron degeneracy), runaway helium fusion will lead to helium flash, and the huge energy released will greatly expand the solar core, release electrons degeneracy, and then the remaining helium in the core will undergo stable fusion.
From the outside, the sun will suddenly light up by 5 ~ 10 like a new star (compared with the previous "red giant" stage), and then its volume will be greatly reduced, and it will become much darker than the original red giant (but it will still be brighter than the current sun), until the core carbon gradually accumulates and enters the core contraction and external expansion stage again. This is an asymptotically huge bifurcation stage.
The fate of the earth is uncertain. When the sun becomes a red giant, its radius will be about 200 times that of the present, and the surface may extend to the present orbit of the earth-1au (1.5×10m).
However, when the sun becomes a star branch of an asymptotic giant, it has lost about 30% of its mass due to the stellar wind, so the earth's orbit will move outward. If so, the earth may survive, but new research shows that the earth may be swallowed up by the sun due to the interaction of tides.
But even if the earth can escape the fate of being burned by the sun, the water on the earth will still boil and most of the gas will escape into space.
Even if the sun is still in the present stage of the main sequence area, the brightness of the sun is still increasing slowly (about every 1 100 million years 10%), and the surface temperature is also rising slowly.
In the past, the brightness of the sun was dark, which may be the reason why life appeared on land 1 100 million years ago. If the temperature of the sun rises at this rate, the earth may become too hot in the next 654.38 billion years, so that water can no longer exist on the surface of the earth in liquid state, and all living things on the earth will be extinct.
After the red giant stage, intense thermal pulsation will cause the outer gas of the sun to escape and form a planetary nebula. After the outer layer was stripped off, the only thing left was the white dwarf star, the hot core of the star, which gradually cooled and darkened over billions of years. This is a typical feature of the evolution of low-mass and medium-mass stars.