The presence of elements heavier than helium in stars is very important because
A) they provide the raw materials for the formation of planets and life
B) without them they could not form
C) they help stabilize their interiors
D) they are responsible for the fusion reactions at their centers
A
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An explosion of genetic diversity similar to that which occurred during the Cambrian Period hasn't happened recently, most likely because
A) evolution of life on Earth is no longer occurring B) such an explosion can occur only for simple organisms and not complex ones C) there are no more ecological niches available to allow this to happen D) of the widespread presence of efficient predators that make it much more difficult for entirely new organisms to appear
Why are collisions between galaxies more likely than collisions between stars within a galaxy?
A) Galaxies have higher redshifts than stars. B) Galaxies travel through space much faster than stars. C) Galaxies are much larger than stars. D) Relative to their sizes, galaxies are closer together than stars.
If an object is placed at the center of curvature of concave mirror, the image formed by the mirror is located
A) out beyond the center of curvature. B) at the center of curvature. C) between the center of curvature and the focal point. D) at the focal point. E) at infinity.
Three forces, 5.0 N, 15.0 N, and 20.0 N, are acting on a 9.81 kg object. Which of the following forces could also be acting on the object if it is moving with constant velocity?
a. 1.0 N c. 39.0 N b. 19.0 N d. any of the above