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How did the Earth form? • • • • What processes were involved? When did it happen? How long did it take? How do we know? How do stars form? The Nebular Hypothesis Observations • Stars can be seen in various stages of formation. • Stars seem to have been forming continuously since the formation of the Universe. • Star formation continues today. • Observations synthesized into the Nebular Hypothesis. Our Sun: an example of stellar evolution • Our Sun began as a nebula, approximately 5 billion years ago. • A nebula is an enormous cloud of gasses (mainly Hydrogen) and dust • Nebula may become disturbed by shock waves, for example from a nearby supernova. Example of a nebula Nebula begins to contract • As the molecules of gas and dust move closer together, they experience stronger gravitational attraction. • Newton’s Law of Universal Gravitation • Fg = g(m1m2)/d2 • Most mass concentrates at the center as the nebula contracts. Nebula begins to take a more definite shape. Shape becomes spherical with equatorial disk Will a star form? • If mass is sufficient, gravity at the center of the sphere may be great enough to “squash” atoms together. • 4 H atoms are fused to form 1 He atom in a nuclear reaction (not chemical). • Nuclear reaction is nuclear fusion, which releases tremendous energy. • A star is born! Products of fusion • H is fused to form He • He is fused to form C and other, heavier chemical elements. • Heavier elements are recycled into new nebulae, and/or new stars and planets. • Implication? Heavy Elements • Since all elements heavier than H are produced by fusion in stars, • We are made of Stardust !!! Summary: Nebular Hypothesis So what happened to the disk? • The disk that surrounds the central star may • 1) be swallowed as the star initially expands. • 2) remain as a disk or a series of rings • 3) may form planets that orbit the central star. Our Solar System • Sun began to radiate energy about 5 billion years ago. • Surrounding disk condensed into 8 planets, an asteroid belt and other objects. • Earth is one of those eight planets. • Earth condensed approximately 4.6 billion years ago. Evidence of Age of the System • • • • • Oldest Earth rock: 3.98 Ga Acasta Gneiss Oldest Earth minerals: 4.4 Ga Chemistry of the Sun and rate of fusion Age of oldest Moon Rocks: 3.3 - 4.2 Ga Age of Meteorites: 4.5 Ga