Nuclear fusion in stars converts hydro gen and helium into the other elements

Nuclear fusion in stars converts hydro gen and helium

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the ejected matter of long-dead stars. (Nuclear fusion in stars converts hydro- gen and helium into the other elements found in the universe.) Nearly 5 billion years ago this huge cloud of gases and minute grains of heav- ier elements began to slowly contract due to the gravitational interactions among its particles (Figure 1.24). Some external in- fluence, such as a shock wave traveling from a catastrophic explosion ( supernova ), may have triggered the collapse. As this slowly spiraling nebula contracted, it ro- tated faster and faster for the same reason ice skaters do when they draw their arms toward their bodies. Eventually the in- ward pull of gravity came into balance with the outward force caused by the rota- tional motion of the nebula (Figure 1.23). By this time the once vast cloud had as- sumed a flat disk shape with a large concentration of material at its center called the protosun (pre-Sun). (As- tronomers are fairly confident that the nebular cloud formed a disk because simi- lar structures have been detected around other stars.) During the collapse, gravitational en- ergy was converted to thermal energy (heat), causing the temperature of the inner portion of the nebula to dramatical- ly rise. At these high temperatures, the dust grains broke up into molecules and excited atomic particles. However, at dis- tances beyond the orbit of Mars, the tem- peratures probably remained quite low. At - 200°C, the tiny particles in the outer por- tion of the nebula were likely covered with a thick layer of ices made of frozen water, carbon dioxide, ammonia, and methane. (Some of this material still re- sides in the outermost reaches of the solar system in a region called the Oort cloud. ) The disk-shaped cloud also contained ap- preciable amounts of the lighter gases hy- drogen and helium. The formation of the Sun marked the end of the period of contraction and thus the end of gravitational heating. Tempera- tures in the region where the inner planets now reside began to decline. The decrease in temperature caused those substances with high melting points to condense into tiny particles that began to coalesce (join together). Materials such as iron and nick- el and the elements of which the rock- forming minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and rocky clumps that orbited the Sun (Figure 1.23). Repeated collisions caused these masses to coalesce into larg- er asteroid-size bodies, called planetesimals , which in a few tens of mil- lions of years accreted into the four inner planets we call Mercury, Venus, Earth, and Mars. Not all of these clumps of matter were incorporated into the planetesimals. Those rocky and metallic pieces that re- mained in orbit are called meteorites when they survive an impact with Earth. As more and more material was swept up by these growing planetary bod- ies, the high-velocity impact of nebular debris caused their temperature to rise.
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