A planetary nebula is
a. the expelled outer envelope of a medium mass star.
b. produced by a supernova explosion.
c. produced by a nova explosion.
d. a nebula within which planets are forming.
e. a cloud of hot gas surrounding a planet.
A
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A crate is at rest on a horizontal floor, with nobody pushing. The friction force acting on the crate is
A) zero. B) equal to the weight of the crate. C) between zero and the weight of the crate. D) none of the above
We are among the products of nuclear fusion and gravitational contraction
a. True b. False Indicate whether the statement is true or false
Statistical evidence
a. can be used to draw conclusions about a specific cases. b. is applicable to classes of objects where only a few objects from the class have been studied. c. allows scientists to determine the mechanism that produces the dust ring in Centaurus A. d. allows scientists to deduce general characteristics about a class of objects with similar properties. e. provides insight into the behavior of an individual object based on careful observations of it.
All of the following statements about the Sun's corona are true. Which one explains why it is a source of X-rays?
A) The temperature of the corona's gas is some 1 to 2 million Kelvin. B) The corona lies above the visible surface of the Sun. C) The corona's gas consists mostly of hydrogen and helium. D) The corona's structure is largely shaped by magnetic fields.