Stellar astronomy

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Life of Stars
The life of a star starts in molecular clouds containing 75%H, 24%He and 1% other trace elements. The interstellar cloud collapses, and starts to pull inwards. Certain places in the cloud, the collapse goes faster especially in the center. The temperature in center becomes so great that it turns into a (himmellegeme) At this stage before the fusion starts, the star is called a proto-star. At about 10 million K nuclear fusion stars by fusing hydrogen into helium. The gravitational pull inwards, and the photon pressure outwards balance each other. Most of the stars life is now on the main sequence, for about 10 billion years. When most of the hydrogen is burned up, the core gets pushed together. A shell of hydrogen around the core is still fusing, and the shell moves outwards in lack of more fuel, leaving the helium in the core. The outer layers of the sun grows very much, and we get a red giant. At 100 million K, helium fuses to carbon. The helium"Wave" (blaffet) is an explosion that make the temperature and size increase a lot. The core consist now of carbon, and a layer of helium is burning outwards, behind the hydrogen burning. If the stars mass is very big, the star will expand outwards to a super giant. The giant throws it's outer layers away in a violent explosion, and makes a planetary fog.

Stellar Classification
Stellar classification is a classification of stars based on their spectral characteristics. There are 7 main classifications, and each classification has an index from 0-9 indicating where it is between two classifications.