Question

1. The sun is much brighter than the star.
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Is the star in generic use? Or does the star refer to the specific star?
In my opinion, some stars are brighter than the sun, aren't they?

Answer 1

In that sentence, "the star" seems to be referring to a particular star, one that has already been named in an earlier sentence. It depends on context, remember.

Just after having looked at the sky at night over many decades, yes, I agree that some look brighter than others. I don't know if that's because the brighter ones are larger than others or closer to the earth than others or what.