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Monday, October 17, 2005

The birth of stars

A star is a body that at some time in its life generates its light and heat by nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density. The Sun is powered by hydrogen fusion, as are many of the other stars you see at night. The fusion does not take place throughout the star, but only in its deep interior, in its core, where it is hot enough. To create the conditions for such "thermonuclear fusion," stars must be massive. The Sun has the mass of 333,000 Earths. Stars can range up to about 100 times the mass of the Sun (at which point nature stops making them) down to around 8% that of the Sun, at which point the internal temperature is not high enough to run the full range of nuclear reactions (which requires at least 7 million degrees Kelvin).

The space between the stars is filled with dusty gas. If the gravity in a dust cloud is great enough they can form into one or more stars. Contraction causes more rapid spin, which creates a disk around the birthing star, from which it can draw matter. Further condensation within the disk can create planets (or even stellar companions). The contraction of forming stars raises the internal temperature, finally to the point of ignition of hydrogen fusion. (I haven't written any of this. It's been abridged from here)


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