After a few billion years the center of a star runs out of protons (nuclei of hydrogen atoms). What is left is a core or central region made of alphas (nuclei of helium atoms). The outer layers of the star still contain hydrogen, but they are not hot enough to fuse.
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 alphas. So the star gains a new source of energy. The core of the star is now hotter than it was during its normal life 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.
Meanwhile inside the shell, the core of the star shrinks and heats up enough to fuse the helium nuclei together into even heavier ones. Among the commonest nuclei are carbon, nitrogen and oxygen. Heavier and heavier nuclei are created inside a red giant, the heaviest nearest the middle. At its center are iron nuclei.
These fusions release only a little more energy, so they keep the red giant burning for a little longer. But they do not produce as much energy as the fusion of protons. Iron nuclei cannot be used as fuel because they need to be given energy to make them fuse. So iron 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?
As already mentioned, the sgtar begins to swell as the core becomes even hotter than it was during its normal life. All planets closest the sun, including Earth, will burn up. This event will take billions of years to arrive so hopefully by then future humans will have figured out some way of escaping our solar system.
Inside a Red Giant
An international team of astronomers has for the first time figured out what goes on inside red giants. After examining what goes on below the surface of dozens of red giant stars, the scientists found that these objects have incredibly strong magnetic fields.
To peer inside these stars, they used a technique called asteroseismology, which uses waves on their surface to interpret the turbulence in the deeper layers. This is similar to medical ultrasounds which use sound waves to see within the human body. The scientists looked at two different types of wave: pressure waves from internal turbulence (akin to sound waves), and gravity waves which are driven by the buoyancy of the different layers. The different types can penetrate to different depths inside a star.
The findings will expand our understanding of the life and inner mechanisms of stars. The study focuses on a particular type of objects called red giant branch (RGB) stars.
RGB stars are objects in a late phase of stellar evolution. They form when a star of low to intermediate mass (from 0.3 to eight times the mass of the Sun) runs out of hydrogen to fuse in its core. When that happens, the star begins to contract under its own gravity. This free-fall contraction eventually generates enough pressure and heat to start fusion in a shell around the core. The sudden restart of fusion generates an outward force that makes the star swell. The newly formed red giant has a tenuous and inflated atmosphere, a dense helium core, and a radius even hundreds of times larger than the original star.
The dense core makes red giants perfect candidates for asteroseismology. The pressure waves do not bounce off the core, but they are actually transformed into gravity waves. These waves travel across the interior of the stars and cause the red giants to oscillate in different patterns. One of these patterns is called dipole mode and can be observed as one side of the star becomes brighter and the other becomes dimmer.