Answer to Question 1
More massive atoms, such as gold atoms, are made only by short-lived nuclear reactions that occur during a supernova explosion. This explains why lower-mass atoms such as carbon, nitrogen, and oxygen are more common and why more massive atoms-such as gold, silver, platinum, and uranium-are so rare and, often, valuable.Most of the matter in stars is hydrogen and helium. Other elements, including carbon, nitrogen, oxygen, and the rest of what astronomers call metals, were cooked up inside stars. When the galaxy first formed, there should have been no metals because stars had not yet manufactured any. Judging from the composition of stars in the oldest clusters, the gas from which the galaxy originally condensed must have contained about 90 percent hydrogen atoms and 10 percent helium atoms.
Answer to Question 2
The measured motion of stars shows that the disk does not rotate as a solid body. Each star follows its own orbit, and stars in some regions have shorter or longer orbital periods than the Sun. This is called differential rotation.If all of the mass in the galaxy were concentrated at its center, then orbital velocity would be high near the center and would decline away from the center. This kind of motion has been called Keplerian motion because it follows Kepler's third law, as in the case of our Solar System, where nearly all of the mass is in the Sun.
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