How many times fainter (or brighter) will 100 stars of the same luminosity, distant at a distance of 5r, shine compared to one such star at a distance of r?
The brightness of a group of stars is proportional to their number and inversely proportional to the square of the distance E2 / E1 = (n2 / n1) * (r1 / r2) ^ 2, (4 points) in our case we get E2 / E1 = 100/5 ^ 2 = 4 -group of stars will be 4 times brighter.
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