Unit II: The Solar Nebula Hypothesis

Read Seeds: Section 5.1

Figure 1: 93KB Text Credit: Hui Yang (University of Illinois) and NASA Figure 2: GIF Text Credit: Robert P. Kirshner/ Harvard-Smithsonian Center for Astrophysics, NASA Figure 3: 124 KB Text Credit: Credit: Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O'Dell (Rice University), and NASA Figure 4: 80KB Text Credit: Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O'Dell (Rice University), and NASA Figure 5: 760KB Credit: NASA

Lecture: The solar nebula hypothesis is in one form or another the most widely accepted theory of how our solar system formed approximately 4.6 billion years ago. Although details may vary, the general picture is widely accepted. Originally a large cloud of dust and gas (75% H and 24% He) become unstable. The most dense part of the cloud started to collapse under the force of gravity (figure 1). The causes of the instability can vary. One possibility may have been a supernova explosion (def.) (figure 2). As the size (radius) of the cloud decreased,, the collapsing cloud increased its rate of rotation in order to conserve angular momentum (def.) The effect is similar to that of an ice skater who must pull in his or her arms in order to increase his or her rate of spin. Just as the oblateness (def.) of a planet depends on its rotational velocity, so too, as the rotational velocity of the cloud increases, it becomes more oblate forming a disk called a solar nebula (def.) (figure 3) Most of the matter in the collapsing cloud ended up in a central bulge As the cloud collapse, gravitational energy (def.) is released heating the central portion of the nebula where a protosun (def.) forms. (figure 4) Meanwhile, condensation(def.) was occurring within the disk surrounding the protosun. Because temperatures within the disk varied with distance from the center of the nebula, different materials condensed at different locations within the disk. Closer to the center, where temperatures were high, high temperature condensates such as iron and silicates formed. Farther from the center, where temperatures were low, hydrogen, water and other low temperature condensates formed. Collisions between the newly condensed particles cause larger bodies called planetesimals to accrete. This accretion (def.) process continued eventually forming the planets and moons. These violent and cataclysmic process of planetary formation is today evidenced by the cratered surfaces of Mercury and our Moon (figure 5). The recent collision between comet Shoemaker-Levy and Jupiter (Mpeg Animation - 636KB) also gave us a glimpse at what probably happened on a much grander scale 4.6 billion year ago. The heat generated by these impacts and by radioactive decay (def.) of elements resulted in molten planets which subsequently became differentiated (def.). The evolving star at the center of the solar nebula becomes a T-Tauri star at which point it releases burst of energy. These bursts sweep light elements such as hydrogen out of the outer solar system and into the solar system where it is swept up by the distant outer planets. The young protosun gets hot enough to ignite the hydrogen its core. Thermonuclear reactions in the core is what distinguishes a "sun" from a protosun. The terrestrial planets evolve their secondary atmospheres. Complete these Self-check Questions: How do the stages described in the solar nebula hypothesis (above) help to explain the general. Refer to this multimedia presentation. This presentation has several screens to it... you have to be patient because the download is self-timed (meaning you cannot control it.)   Homework Questions: (To submit your answers to the homework questions, first copy the questions from this page and paste them into the homework form. Insert your answers below the questions. Fill in the remaining form elements and submit. Your homework will be e-mailed to me. I will return your graded homework to you in the private e-mail of the comm center). Describe what a T-Tauri star is and its significance in the evolution of solar systems. (1 point) View These Optional Related Web Sites: Visiting these sites are optional, not required. The Origin of the Solar System A Star is Born- Take a virtual tour though the life of a young star Giant Starbirth Region In Neighboring Galaxy Lee Carkner's World of T Tauri Stars