LECTURE

What followed should be familiar by now. Gravity drew the cloud together. As it became denser, it heated up—particularly in the center. When the center reached 10 million degrees Celsius, hydrogen fusion began and our Sun lit up. It acquired the typical structure of stars, with fusion reactions in the center, a middle layer containing reserves of hydrogen, and a surface from which energy radiates into space. We can see areas of star formation in our galaxy even today. Similar processes occur even at smaller scales. For We cannot determine the example, Jupiter is mainly gaseous age of the solar system by and is large enough for its center to dating Earth rocks because be extremely hot, but not quite hot the Earth is so geologically enough for fusion. It is almost a star, but not quite.

active that its original surface

is now unrecognizable. Now we shift away from the Sun to the debris surrounding it. Through a process known as accretion, the planets and other bodies of our solar system were created from the 0.1% of the solar nebula that was not incorporated in the Sun. As it contracted, the solar nebula spun faster, like an ice skater doing a pirouette. Centrifugal force attened the spinning nebula into a disk, so that the material not gobbled up by the Sun orbited the Sun in a single plane. Saturn’s rings consist of orbiting debris, so they may give us a good idea of the shape and structure of the solar nebula. In each orbit, particles of matter were drawn together by electrostatic forces or crushed together in the course of violent collisions. Gradually, larger objects appeared and began colliding with each other and sometimes merging with each other. They grew like large snowballs. Stray bodies in the solar system, such as comets, are thought to be remnants of these early stages of accretion. Within a few million years, the large bodies in each orbit formed “planetesimals.” The largest planetesimals drew in most of the remaining material through their gravitational pull until a single large body appeared in each orbit.