When we accept the evidence of our unaided eyes and describe the Sun as a yellow star,we have summed up the most important single fact about it-at this moment in time. It appears probable, however, that sunlight will be the color we know for only a negligibly(微不足道的）small part of the Sun's history.
Stars, like individuals, age and change. As we look out into space, we see around us stars at all stages of evolution. These are faint blooded dwarfs so cool that their surface temperature is a mere 4,000 degrees Fahrenheit; there are scaring ghosts blazing at 100,000 degrees Fahrenheit and almost too hot to be seen, for the great part of their radiation is in the invisible ultraviolet range. Obviously, the "daylight" produced by any star depends on its temperature; today (and for ages to come) our sun is at about 10,000 degrees Fahrenheit, and this means that most of the Sun's light is concentrated in the yellow band of the spectrum, falling:lowly in intensity toward both the longer and shorter light waves.
That yellow "hump" will shift as the Sun evolves, and the light of the day will change accordingly. It is natural to assume that as the Sun grows older, and uses up its hydrogen fuel-which it is now doing at the spanking rate of half a billion tons a second-it will become steadily colder and redder.
1.What is the passage mainly about?
[A]Faint dwarf stars
[B]The evolution cycle of the Sun
[C]The Sun's fuel problem
[D]The dangers of invisible radiation
2. What does the author say is especially important about the Sun at the present time?
[A] It appears yellow
[B]It always remains the same
[C] It had a short history
[D] It is too cold
3. Why are very hot stars referred to as "ghosts"?
[A] They are short-lived
[B]They are mysterious
[C]They are frightening
[D] They are nearly invisible
4. According to the passage, as the Sun continues to age, what color is it likely to become?[A]Yellow
5. In the last sentence of the third passage, to which of the following does "it" refer?
[D] Hydrogen fuel