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.
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(m 1 m 2 )/d 2 Most mass concentrates at the center as the nebula contracts.
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 !!!
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 9 (possibly 8) planets and an asteroid belt. Earth is one of those nine planets. Earth condensed approximately 4.6 billion years ago.
Evidence of Age of the System Oldest Earth rock: 3.98 Ga Acasta Gneiss Chemistry of the Sun and rate of fusion Age of oldest Moon Rocks: 3.3 - 4.2 Ga Age of Meteorites: 4.5 Ga