After a few years the center of a runs out of ( of ). What is left is a core or central region made of helium nuclei. The outer layers of the star still contain hydrogen, but they are not hot enough to .
Because it has run out of fuel, the star begins to cool, and contract. The outer layers of the star fall inwards under gravity, and as they fall they heat up. A shell surrounding the central core becomes hot enough to fuse protons into helium nuclei. This outer layer is called the helium fusion shell.
So the star has gained a new source of energy.
The core of the star is now hotter than it was during its and this heat causes the outer parts of the star to swell. The star becomes a giant. The radiation from the fusing shell has grown weak by the time it reaches the surface of the star. Weak radiation is red, so the star becomes a red giant.
Diagrammatic representation of structure of large Red Giant star
Meanwhile inside the helium fusion shell, the core of the star shrinks and heats up enough to fuse the together into even heavier ones. Among the commonest nuclei are , and . Heavier and heavier nuclei are created inside a red giant, the heaviest nearest the middle. At its center are nuclei.
These fusions release only a little more , so they keep the red giant burning for a little longer. But they do not produce as much energy as the fusion of . Iron nuclei cannot be used as fuel because they need to be given energy to make them fuse. So nuclei collect in the heart of a red giant star.
The Sun will run out of fuel and become a red giant in about 4 billion years. What do you think will happen to the Earth then? Click for the answer.
For every million atoms found in the Universe today, the commonest types are listed in the following table, along with the number of protons each contains. Note that one type of atom is often called an element.
It is perhaps no surprise that these common atoms will be the main ones in our story. In particular, hydrogen, oxygen, nitrogen and carbon will be the most important. We will learn more about how they join together in the following pages.
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