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The Life of Stars

Simplified description of stellar life:

Stars are - as our Sun - huge glowing balls of gas. Some stars are bigger, but most of them smaller as the Sun, which itself has a mass 332 830 times more than the Earth.

At the beginning of their life stars contain about two thirds hydrogen and one third helium. Heavier elements are less than one percent. During the birth of a star a cloud of gas collapses until in the center pressure and temperature are high enough to start the nuclear fusion. The cloud begins to shine, first due to the set free gravitational energy, later because of the nuclear fusion. A star is born.
In its center the star now fusions at several million degrees hydrogen to helium. This it does for the longest part of its life as a main sequence star. The more massive a star is indeed the more fuel it has got, but it fusions it much quicker than a small star. The more luminous it is then.

If in a small star the hydrogen in its center is spent, it can't continue with the fusion. But the universe with its maybe 14 billion years is still too young as that small stars could already have spent their fuel. When this time comes, these stars will simply slowly become cooler and dimmer.

Bigger stars continue by fusioning hydrogen in their outer layers. Therefore the hull heats up and is driven out into space. The star expands and increases its brightness massively. It leaves the main sequence, a red giant is evolved. This happens to all stars with up to three times the mass of the Sun (except the very small ones). The hull is driven further away and we can see them in telescopes often as a very beautiful planetary nebula (it looks like a planet but it has nothing to do with it). The core is left behind as an inactive white dwarf with about the size of Earth (but much more massive), which slowly fades.
Stars with three to eight solar masses behave similar, but create further elements like carbon, oxygen, nitrogen and neon, at first in their core, later in the outer layers.

Massive stars with more than 8-10 solar masses fusion even more heavy elements, up to iron. The star becomes bigger and bigger and becomes unstable. It pulsates and can erupt heavily. This last phase is relatively short compared to the main sequence phase of the star.
When the star has created an iron core then it has reached a dead end. It can't produce any more energy. The core with more than 1.44 solar masses cools down and therefore can't handle its own gravitation any more. It collapses in one fell swoop to an only some kilometers big neutron star or a black hole. The thereby arising shockwave pushes against the hydrogen and helium hull, which is about to crash inside. The complete hull now fusions immediately - a supernova explosion (of type Ib or II).
Only a few stars have the required mass for a supernova. During the explosion the star shines for a few days several billion times brighter and then leaves a nebula and a tiny spot in its center - a neutron star or a black hole.


Stars very rarely appear single without a companion. This is because the cloud from which a star evolves rotates slowly. By contracting the rotation becomes quicker (pirouette effect) and a single star can't handle the angular momentum. Therefore often a multiple system evolves or a single star with planets.
So called exoplanets, planets of other stars, aren't easy to discover for us and even more difficult to see, because they don't shine by their own. But there have been discovered already more than a hundred exoplanets because of the proper motion of their mother stars, most of them huge gaseous planets like Jupiter.

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The center of our Milky Way.
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