Luminosity

zebra1707

1. The problem statement, all variables and given/known data

Question 1 - If two stars A of luminosity 9000 and B of luminosity 90 (both relative to the Sun) appear equally bright from the Earth, how much further away is A from B?

Question 2 - Star A is twice as bright as Star B, but B is twice as far away as A. Which star has the greater luminosity, and by what ratio?

Question 3 - If two stars, A and B, are the same distance away, but B is 20 times more luminous than A, how much brighter will B appear than A?

I could use some assistance as to how I should set these out in accordance with

brightness (proportional to) Luminosity / R^2


2. Relevant equations

See above

3. The attempt at a solution


Question 1 - Worked out to - by moving Star A 10 times further away we have diminished the apparent brightness of Star A by a factor 10 x10 = 100 so that it now matches the brightness of Star B.

Question 2 - Worked out to - Luminosity of A
Luminosity of B = (½)2 x 2 = ½

This means that Star A really only has half the luminosity of Star B, but appears brighter because it is closer.
Ratio 1:2

Question 3 - Worked out to 1/20th .

Just need assistance to set these out correctly - and also make sure that they are correct.

zebra1707

Any thoughts?

Redbelly98

Looks good so far.

So are you saying that B appears to be 20 times dimmer than A? I will suggest rethinking this one. Hint: distance is not a factor here, since it is the same for both stars.

arunma

Zebra, you seem to be really close on the last one. When distance is constant, the apparent brightness is just proportional to the luminosity.

zebra1707

Many thanks for the replies.

Yes, I think I understand what you are saying about question 3.

Again, many thanks for the replies.

Cheers