Sun

The Sun is a typical star of relatively low mass. Stars consist mainly of hydrogen and helium and about 0.01-2% of heavier elements. In comparison to this the light elements are missing nearly completely in earthlike planets. Stars are shining because in their deep interior lighter elements are converted to heavier ones by way of nuclear fusion. In the case of the Sun and most other stars, hydrogen is converted to helium.

Stars are shining because of the internal fusion of light elements to heavy elements. By this huge amounts of energy are set free, which allows stars to shine for up to billions of years. Nuclear fusion has not been realized in a nuclear reactor on Earth, yet. In most stars, as in the Sun, four hydrogen nuclei (protons) produce a helium nucleus (alpha-particle) in several steps. This happens in the core, where temperature is highest; this stage in a star's life is called the main sequence phase. Because of the nuclear fusion the composition of the Sun and the stars changes, which results in a change of their structure. For example, they become red giants with time.

When stars have converted all their central hydrogen into helium by nuclear fusion, hydrogen is still burning in a shell around the core. This shell shifts outwards and leaves a growing core of helium behind. In this phase the Sun and other stars are about hundred times bigger than the Sun is now (700 million km). At the same time the surface is cooler (3500 degrees; compared to the present 5800 degrees of the Sun). In this phase of their life stars are called red giants. The helium core of such stars is very compact and resembles the final stage of sun-like stars, so called white dwarfs.

At the end of a star's life the source of energy, nuclear fusion, will be exhausted. Stars with as much mass as the Sun lose a great part of their mass by stellar winds while being red giants. Only the central regions, consisting either of helium or of carbon and oxygen, remain. They are extremly dense and matter exists in a strange quantum stage named "degenerated electron-gas". These stellar cores have nearly half the mass of the Sun, but only the radius of the Earth (about 6000 km). They are called white dwarfs because they are very hot and very small. In the course of millions of years they get cooler and dimer. Most of the stars will finaly float through the universe as cold dwarfs.

Our Sun is more than four and a half billion years old. During this time it slowly got larger and its brightness increased steadily. Here we show its size in comparison to the orbits of Mercury and here in comparison to the whole solar planet system. From a distance of initially 20 billion km we begin our flight into the Sun. We are now crossing Earth's orbit at 150 million km, now reaching the Sun, whose radius is 700.000 km Three important quantities are shown on the slices of the opened Sun. The production of energy: the brighter, the more effective. The Sun is shining because hydrogen is converted into helium in the core. The chemical composition: the brighter, the more helium is present. In the centre, already about half of the hydrogen has been burnt by nuclear fusion. The distribution of mass: the lines indicate, how much of the solar mass is lying within this radius. Presently, about 70% are contained within half of the solar radius or 12% of the total volume. We are now holding our position at 500.000 km in the interior of the Sun and observe its evolution into the future. During the next 3 billion years the brightness will be continue to increase slowly; the solar radius is hardly changing during this time. More and more hydrogen is burnt in the centre, until it will be consumed completely after another 5 billion years. Outside the center, however, enough hydrogen is still available and the production of energy is shifting there. We are going to fly deeper into the centre, where the action is. When hydrogen-burning is ceasing in the centre, the whole core is contracting and mass is getting more concentrated. The more exterior layers are expanding simultaneously: the Sun is getting larger rapidly. To follow the further evolution, we back up beyond Jupiter's orbit. In connection with the swelling of the Sun its brightness also increases strongly. Because of the increase of its size, the Sun is engulfing the nearest planets: Mercury first, then Venus. Earth is spared at this time, but life has been extinguished long ago due to the strong increase of the solar brightness. Meanwhile, almost half of the solar mass is concentrated within the innermost 15.000 km, while the solar radius has grown to almost 150 million km. The Sun now is about 5000 times brighter than today and has become a so-called Red Giant. The centre is dense and hot enough to ignite the fusion of helium to carbon. This new source of energy is temporarily stabilizing the Sun, which becomes smaller and dimmer again. The light-blue regions indicate carbon freshly produced by helium fusion. All biological life is carbon-based - here is the site where it is produced. After helium, too, has been used up at the centre, the Sun is producing its energy in two shells around the core. As earlier the Sun is rapidly increasing in size and brightness. The further evolution of the Sun is now becoming very complicated and takes place within only a few million years. Therefore we have to resolve the luminosity evolution much better, and retreat to a safe distance at the orbit of Uranus. The further evolution of the Sun is now becoming very complicated and takes place within only a few million years. Therefore we have to resolve the luminosity evolution much better, and retreat to a safe distance at the orbit of Uranus. Within a mere few ten thousand year strong and regular brightness outbreaks occur, connected with huge variations in the solar size. In this phase, Earth will be devoured. The variations in the interior we can hardly follow. We therefore restrict ourselves to consider only the rather regular brightness variations. About every 100.000 years strong energy outbreaks occur, during which the Sun is inflates to a gigantic balloon of hot gas within a few thousand years only. Mars is devoured as well. These eruption recur regularly. From the solar surface a strong stellar wind is blowing matter into space. In particular during the quiet phase between outbreaks the Sun is going to loose a large fraction of its mass. Because the solar mass is decreasing, the planets' orbits get larger. Finally, the Sun has lost half of its initial mass. No further energy production by nuclear fusion is possible, and therefore the Sun begins to contract slowly. At the end it will become a White Dwarf, consisting mostly of carbon and oxygen. It fades away, becoming cooler and smaller; its size will finally be comparable to that of Earth, which vanished long ago together with all demo CDs. This terminates the life of our Sun. Here we once more have an overview of the incredible size variations the Sun will be going through: The Sun today, as a Red Giant, and as a White Dwarf. Although it will be hardly larger than Earth, the Sun even then will have 200.000 times more mass.