Which event marks the beginning of a supernova?
A) the expansion of a low-mass star into a red giant
B) the beginning of neon burning in an extremely massive star
C) the onset of helium burning after a helium flash in a star with mass comparable to that of the Sun
D) the sudden collapse of an iron core into a compact ball of neutrons
E) the sudden outpouring of X-rays from a newly formed accretion disk
Question 2
As an interstellar cloud of hydrogen gas shrinks in size, its temperature increases
A) because thermal energy is converted to radiative energy.
B) because kinetic energy is converted to radiative energy.
C) because gravitational potential energy is converted to thermal energy.
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