The Sun far outweighs all other components of the solar system combined In fact the Sun contains more than 99 percent of the mass of the entire solar system Nevertheless the Sun is a fairly average-sized star From Earth it looks so much larger and brighter than other stars only because it is so much nearer to Earth than any other star If the Sun were much farther away it would look much like many other stars in the night sky But if this were so life as we know it could not exist on Earth The Sun provides nearly all the heat light and other forms of energy necessary for life on Earth In fact the Sun provides the great majority majority of the energy of the solar system Astronomers believe that the solar system formed as a by-product of the formation of the Sun itself some 4.6 billion years ago According to the prevailing theory the Sun and its many satellites satellites condensed out of the solar nebula a huge interstellar cloud of gas and dust The solar system began forming when the gravity of this interstellar interstellar cloud caused the cloud to start contracting and slowly spinning This could have been caused by random fluctuations in the density of the cloud or by an external disturbance such as the shock wave from an exploding star INTRODUCTION 6
The Sun and Moon during a solar eclipse As the interstellar cloud squeezed inward more and more matter became packed into the center which became the protosun the material that later developed into the Sun The contraction caused the cloud to spin faster and faster and to flatten into a disk Eventually the center of 7
The Sun and The Origins Of The Solar System the cloud collapsed so much that it became dense enough and hot enough for nuclear reactions to begin and the Sun was born Meanwhile away from the center the gas and dust in the spinning disk cooled Solid grains of silicates and other minerals the basis of rocks condensed out of the gaseous material in the disk Farther from the center where temperatures temperatures were lower ices of water methane ammonia and other gases began to form The spinning material in the disk collided and began to stick together forming larger and larger objects Ultimately some of the clumped together objects grew huge and developed into planets The inner planets formed mostly from chunks of silicate silicate rock and metal while the outer planets developed mainly from ices Smaller chunks of matter and debris that did not get incorporated into the planets became asteroids in the inner part of the solar nebula and comet nuclei in the outer part of the nebula At some point after the matter in the nebula had condensed and clumped into objects the intensity of the solar wind suddenly increased This blew much of the rest of the gas and dust off into space This general formation process is thought not to be unique to the solar system but rather to be how stars and planets throughout the universe develop Astronomers have detected disks of matter surrounding newly formed stars 8
Introduction The future of the solar system depends on the behavior of the Sun If current theories of stellar evolution are correct correct the Sun will have much the same size and temperature for about 5 billion more years By then all of the hydrogen in its core will have been used up Other nuclear reactions will begin in a shell around the core Then the Sun will grow much brighter and larger turning into a red giant and expanding beyond the orbit of Venus perhaps even engulfing Earth Much later when all its nuclear energy sources are exhausted the Sun will cool down evolving into a white dwarf star Around it will orbit the remaining planets They will have turned into frozen chunks orbiting their shrunken star 9
