NSES Content Standard D
Earth and Space Science: Origin and evolution of the universe Grades 9-12, page 190 Early in the history of the universe, matter, primarily the light atoms
hydrogen and helium, clumped together by gravitational attraction to form
countless trillions of stars. Billions of galaxies, each of which is a
gravitationally bound cluster of billions of stars, now form most of the
visible mass in the universe.
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Benchmark 4A The Physical Setting: The
Universe
Grades 9-12, page 65
On the basis of scientific evidence, the universe is estimated to be
over ten billion years old. The current theory is that its entire contents
expanded explosively from a hot, dense, chaotic mass. Stars condensed by
gravity out of clouds of molecules of the lightest elements until nuclear
fusion of the light elements into heavier ones began to occur. Fusion released
great amounts of energy over millions of years. Eventually, some stars
exploded, producing clouds of heavy elements from which other stars and
planets could later condense. The process of star formation and destruction
continues.