A star's apparent brightness<span> is its </span>brightness<span> seen from Earth. A star's </span>absolute brightness<span> is the </span>brightness<span> the star would have if it were at a standard distance from Earth
</span>so if your riding in a car at night and your passing some one on the road in there point of view your apparent brightness is lets say 10 and if your are farther a way your estimated brightness or star's absolute brightness would be 10.<span />
"Sex Cells" or "haploids" are needed in order to add variation in DNA. If there was no "haploids" (which are just cells with 23 Chromosomes), then the female would not need the male for reproduction, basically meaning birth of an identical twin to the mother. The two haploids, the female's ova (Egg) and male's sperm meet, creating a zygote, or a fertilized egg.
This makes sure there is "immediate variation", and it isn't just ten generations of the same "Bob"!
Target cells that are known to be having specific receptor for an antigent or antibody and the main reason why all cells are not target cells for the hormones is that they act as the focus for contact by phagocyte.
A rain forest would have the most species diversity if you are talking about complex eukaryotic organisms (reptiles, mammals etc), though the polluted lake would have the most simple prokaryotic organisms ( bacteria, protists, fungi etc.)