********* Stars *********** A star is a massive, luminous sphere of plasma held together by its own gravity.There are billions of them — including our own sun — in the Milky Way Galaxy. And there are billions of galaxies in the universe. So far, we have learned that hundreds also have planets orbiting them.A star develops from a giant, slowly rotating cloud that is made up entirely or almost entirely of hydrogen and helium. Due to its own gravitational pull, the cloud behind to collapse inward, and as it shrinks, it spins more and more quickly, with the outer parts becoming a disk while the innermost parts become a roughly spherical clump. This collapsing material grows hotter and denser, forming a ball-shaped protostar. When the heat and pressure in the protostar reaches about 1 million degrees C, atomic nuclei that normally repel each other start fusing together, and the star ignites. Nuclear fusion converts a small amount of the mass of these atoms into extraordinary amounts of energy — for instance, 1 gram of mass converted entirely to energy would be equal to an explosion of roughly 22,000 tons of TNT.The life cycles of stars follow patterns based mostly on their initial mass. These include intermediate-mass stars such as the sun, with half to eight times the mass of the sun, high-mass stars that are more than eight solar masses, and low-mass stars a tenth to half a solar mass in size. The greater a stars mass, the shorter its lifespan generally is. Objects smaller than a tenth of a solar mass do not have enough gravitational pull to ignite nuclear fusion — some might become failed
Posted on: Sat, 13 Dec 2014 15:22:44 +0000
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