Old Age

What happens when the main fuel is running out? The star has burnt almost all the available hydrogen, which accounts for about 10% of the total, in the nuclear fusion, and the nucleus is composed almost exclusively of inert helium. The energy production that counterbalanced the gravitational collapse is no longer sufficient to oppose it and the balance is lost. Even if at first no change is noticeable on the star’s surface, the nucleus starts to contract under the pressure of its mass, gradually increasing its density and temperature. The first consequence is that fusion reactions occur out of the nucleus which by now is spent involving the thin layer of surrounding hydrogen. The displacement of fusion reactions towards the outside causes an increase in gas pressure of the superficial regions. As a result, while the nucleus of the star contracts, the external areas expand, cooling off because the same amount of heat is now released from a larger surface. We have already said that a star’s color is linked to its surface temperature and that the cooler stars tend to be red.
Therefore, as the star swells it becomes red, and enters a developing phase known as the red giant. Red giants can have diameters ranging from 50 to 2000 times larger than the sun and can be as much as three times cooler than our star.

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