The Orion nebula is large and full of gas and stars that are lightened by young stars. The cloud contains mainly hydrogen and helium. source: US naval obsorvatory | In the enormous space between the stars large dust nebulae exist, that consist of gas molecules and dustparticles. These clouds are very thin but still stars are formed of them. The birth of a star. A star is formed when matter in the clouds clump together, through which the clouds gets more dense. This packing occurs under the influence of stars in the neighborhood, these bring about strong ionization, as well as shockwaves. The matter is compressed by this and slowly kernels are formed of more packed up dustparticles of which the gravitational forces increase. While the density of the dustkernels get higher, the clouds collaps more and more. This proces generates energy, that heats up the dust and (mainly hydrogen) gas. Finally so much heat is generated that the gas and dustparticles start to glow. Now a protoster is formed. Inside the nucleus of the protoster the matter is heated up. The temperature goes slowly up to millions Kelvin. When it reachess a temperature of 10 million Kelvin, nuclearfusion reaction are started in the gas. The hydrogen atoms are fused to helium, and from this lots of energy is released. |
Lifetime depends on the size.
The size, the surface temperature and the energy the star emits remains the same for a very long period. A star remains in the steady stade as long as the gases contain hydrogen, after it runs out of this the star will change again.
The length of the steady stade depends on the size of the star. Heavy stars transform their hydrogensources must faster into helium and other elements than light ones, because their internal pressure and temperatures are much higher. Therefor the nuclearenergy of the larger is faster depleted. They have a much shorter lifetime, about some tens of million years (a factor 10 shorter!) in stead of the ten billion years that smaller stars, like our sun, are alive.
Stars more or less of the size of our sun.
Smaller stars, about the size of our sun come rather peaceful to their end.
When all hydrogen is burnt, the gaspressure from the inside of the star and the gravity forces pressing from the outside get out balanced. The outer layers collaps and the star shrinks. This causes a rise in temperature inside the star and a new nuclear reaction starts.
The remaining helium starts to fuse to carbon, oxygen of neon. The energy generated from this reaction causes a expansion of the star. Because of its very large surface the outer layers cool down and emit red light. The star is now called a red giant.
As the star is very large it cools down rapidely, and now the star shrinks again. The shrinking makes the star more compact, the density increases, and so the inner parts are heating up again which causes new swelling. The luminosity of the stars will rise when the star shrinks and get less when it swell, the star now became a cepheid. During this proces lots of matter are swung into space under influence of the large quantities of energy that come from the nucleus, these are called sterreblizzards????.
When all helium is depleted, the inner layers do not have sufficient energy to remain swollen, so they collaps. The atoms are pressed together and get very densely packed. The mass of the star is more or less equal to that of our sun, while it collapses to a size of aproximately the Earth, so the star turns into a much smaller compact sphere. It now is a white dwarf, its mass is very dense, matter of a few grams on Earth weighs on a white dwarf several tonnes. It is mainly made out of carbonatoms, sometimes even in the cristalform of diamond, and it will slowly extinguish to a black dwarf. A black dwarf is not observed yet because it takes a very long time before the white dwarf is completely extinguished.
The larger stars.
The end of larger stars is much more sensational. After their much shorter lifetime, these become red supergiants. They sometimes reach diameters of over 1000 times that of our sun. 
A supergiant contains many different elements, that are created by nuclearreactions. Here also the starblizzards??? peel off the cooler outer layers of the star. These expand into space and are known as planetary nebula. From this material full of more complex atoms and molecules planets may be formed around a second generation star.
The star remains as a smaller object that emits blue light, because of its high temperature. The red giant turned into a blue one. When finally iron is formed, the star comes to its end. Most large stars end their red or blue giant phase with an enormous explosion and go supernova, after which it finally is transformed into a neutronstar or, in case it was a super massive star, it turns into a black hole.
resouce:
http://library.thinkquest.org/18188/english/universe/main.htm
