The Student Room Group

Stars and brightness and luminosities

Two distant stars are observed through a telescope. Star A is observed to be half as bright as star B. Star A is calculated to be twice as far away as star B.

Obviously this involves the equation:
F=L4πd2F=\frac{L}{4\pi d^2}
Where F is the received or perceived brightness (Flux).

So we have:
2Fa=FbFa=12Fb2F_a=F_b \therefore F_a=\frac{1}{2}F_b.
And so:
2Lada2=Lbdb2\frac{2L_a}{d^2_a}=\frac{L_b}{d^2_b}.
Is that it?
Doesn't the distance come into it? Why not? Is it because it is 'calculated' distance? So, presumably, that distance comes from the above observation?
The answer is Star A has twice the luminosity of Star B.
Original post by halpme
Two distant stars are observed through a telescope. Star A is observed to be half as bright as star B. Star A is calculated to be twice as far away as star B.

Obviously this involves the equation:
F=L4πd2F=\frac{L}{4\pi d^2}
Where F is the received or perceived brightness (Flux).

So we have:
2Fa=FbFa=12Fb2F_a=F_b \therefore F_a=\frac{1}{2}F_b.
And so:
2Lada2=Lbdb2\frac{2L_a}{d^2_a}=\frac{L_b}{d^2_b}.
Is that it?
Doesn't the distance come into it? Why not? Is it because it is 'calculated' distance? So, presumably, that distance comes from the above observation?
The answer is Star A has twice the luminosity of Star B.


distance could have been calculated by parallax - if an exam question doesn't tell you how something was calculated I wouldn't get stressed about it. it seems like you're supposed to just take the calculated value as 'true' and get on with it.

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