(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

When the Sun becomes a red giant star its brightness will increase by a factor of 5000 and its mass will decrease to half of its present mass. When this happens particles smaller than a critical size will be blown out (light force = gravity force) of the solar system by the pressure of sunlight. What is the size for a black spherical particles ( they absorb all light that falls on them) with density of 1000 kg/m^3.

2. Relevant equations

radiation force on a spherical particle=F=((flux)*π a^2 Q)/c

3. The attempt at a solution

So in the problem it says that the light force = grav force

((flux)*π a^2 Q)/c = GMm/r^2 = GM/r^2 * (4π a^3 ρ)/3

(from example in book)

But I don't know what a, Q or flux is. I would estimate r= 1AU.

This also wouldn't use the change in brightness.

Need some help on where to go with this problem because I think I'm setting this up all wrong :(

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# Homework Help: Astronomy - Light Force

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