Evolution of the Sun

Similar to humans also stars have differents periods of their evolution. But this periods last for such a long time so that wie could not observe them on the Sun . However you can observe other stars in different periods of their evolution and conclude at this way the past and future of the Sun .

Formation

In space there are not only stars, but also huge clouds of gas and dust. Under certain conditions these clouds may contract and evolve to stars, as it was the case about 4.5 billion years ago when our Sun was formed. When the cloud of gas and dust contracts, its temperature and pressure rise. Two kinds of forces are acting: the gravitational force tries to further contract the cloud, whereas the pressure tries to expand it. Inhomogeneities in the cloud lead to a fragmentation process, yielding thousands of clouds which are further contracted. One of these clouds became our Sun. When the temperature in the interior reached some Million degrees, nuclear fusion set in and the Sun began to shine. A star was born.

The figure shows the Eagle Nebular M16, a candidate for star formation. © Wilfried Langer from » www.planetengrund.com

Sun in equalibrium

At present, the Sun is in its most stable phase of its evolution, it is a so called main sequence star, where gravity and gas pressure of the hot solar plasma are in equilibrium. This condition will remain for about another 4.5 billion years. In the core of the Sun, hydrogen atoms fuse to helium atoms, so-called hydrogen fusion. This process continuously produces energy, which is emitted on the solar surface.

Final Stage

When after billions of years, all the hydrogen in the solar interior will be consumed, nuclear fusion will start in outer shells of the Sun. The core will then contract, whereas the outer shells will expand. The temperature in the core will increases to 100 million degrees, and nuclear fusion of higher elements (carbon, oxygen) will set in. Finally, the Sun will develop to a Red Giant Star, which may expand up to the orbit of Earth. The figure shows a comparison of the size of the Sun now and as a Red Giant.

After about 100 million years as Red Giant, the Sun finally expels its outer shells and the core contracts to about the size of the Earth. Because of the extreme high temperature (≈100 000 degrees) the remaining core shines in white color, and is thus called a white dwarf. It consists of ionized carbon and oxygen. It has an enormous density, which is about one ton per cubic centimeter. The remnant outer shell is called a planetary nebula. It consists of ionized gas which absorbs and colorfully re-emits the light of its central star. A white dwarf has no possibility to produce further energy in its interior, and therefore slowly cools down. After several billions of years it is no longer visible and ends as a black dwarf (Image by NASA).