What do we mean by the frost line when we discuss the formation of planets in the solar nebula?
A) It is another way of stating the temperature at which water freezes into ice.
B) It marks the special distance from the Sun at which hydrogen compounds become abundant; closer to the Sun, there are no hydrogen compounds.
C) It is a circle at a particular distance from the Sun, beyond which the temperature was low enough for ices to condense.
D) It is the altitude in a planet's atmosphere at which snow can form.
Question 2
Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours.
The change in brightness alternates between a small change and a large change. The most likely explanation is that ________.
A) the star is a member of an eclipsing binary star system
B) the star is periodically ejecting gas into space, every 73 hours
C) the star is a Cepheid variable
D) the star is a white dwarf
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