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Red giants are enormous, cool stars with high luminosity that forms after a star has used up the fuel at its core, which then collapses under its own weight (See fig. 1.5). After the star has exhausted the hydrogen in its core, it will contract, due to the lack of a source of heat needed to support it against gravity. When almost all the hydrogen is used up, the remaining available fuel is converted to helium. Eventually, a helium core forms. The temperature is not high enough to burn this heavier element, so nuclear reactions move outward. The result is a constant radiation pressure in the upper layers, but no support for the core. Therefore, the core collapses.
The collapse of the core generates more energy and increases the radiation pressure outward. This pushes material outward, expanding the star. Eventually, the balance between radiation pressure and gravity is restored, but the star has expanded many times from its original size. Also at this time, the star changes colour as a result of the drop in temperature from the increased surface area. Thus, the red giant may have a diameter 10 to 100 times that of our sun, but will consequently have a lower temperature of 2000-3000ºC. Large red giants are called supergiants, and have diameters up to 1000 times of that of the Sun and luminosities 1 million times greater.
Figure 1.5 (above) An expanding red giant star
Death of the Sun
When the sun becomes a red giant, its atmosphere will cover Earth. Furthermore, it will have an outer shell reaching out towards Jupiter. When the last of the hydrogen gas in the outer shell is forced out, it will form a ring around the carbon core, leaving a planetary nebula.
Figure 1.6 (above) One example of a planetary nebula.
Did you know... that the biggest stars are known as red supergiants 400 times larger than the sun, or 300 million miles across. The star Betelgeuse is a red supergiant that is so large, that if it were placed in the middle of the Solar System, it would fill the orbit of Jupiter!