Giants and Super-giants

There are two ways that a star can become a giant or super-giant. First, if a star is lucky enough to amass enough matter prior to the beginning of fusion in its core, it can become a very massive star or super-giant. Typically, the life of a super-giant is short because it's core temperature is extreme. This is because the high mass of the star keeps the core temperature very hot since gravity is so strong in the core. Even though the core may have plenty of hydrogen to fuse, it can have run away nuclear reactions that occur simultaneously. Eventually the core is exhausted of fusible matter and the star collapses to become either a neutron star or a black hole. In a since the collapse of the super-giant is the final victory of gravity over matter.

The other method by which an ordinary star can become extraordinary is by the beginning of helium fusion. As illustrated on the fusion page, as the star runs out of hydrogen in its core, in a since it prolongs it's life by beginning to fuse larger nuclei. Helium begins to accumulate in the core. As the amount of heat given off by the core decreases the outer areas of the star collapse slightly which puts pressure on the core. The core's temperature and density increase. When the temperature of the helium core gets high enough, the hydrogen on the outside of the helium core begins to fuse. This shell of hydrogen fusing is called a hydrogen fusion shell. As the hydrogen fuses on the outside of the helium core, the helium it produces is left behind. This helium falls into the core causing the and the hydrogen fusion shell moves outward. The fusion moves outward and the heat is produces causes the outer layers of the star to expand.

Since the core does not produce it's own energy, it continues to collapse and heat up. Eventually, the core reaches 100,000,000°F and helium fusion can initiate. Stars more than 2 sun masses begin helium fusion gradually since they have such a large helium core. Stars the size of the Sun or smaller begin helium fusion with a massive explosion in the core. The extreme density of the core causes the explosion. If you were to place a teaspoon of this material on earth, it would weigh as much as a small car. This explosion does not destroy the star, but the distending outer envelope of the star absorbs the energy. It can be so intense that the energy released in a few minutes in the core of the star may rival the star's mother galaxy's total output of energy. After the helium flash, the temperature in the core is high enough to stable sustain helium fusion.

Eventually the star is bloated tremendously. Betegeuse and Aldebaran are examples of stars that have gone through these changes. If Betelgeuse were placed where the Sun is located, it's surface would extend out to the orbit of Mars.

. It is believed that the Sun will go through these changes many millions or even billions of years from now.