# How much starlight do black holes absorb?

Can it be calculated how much light from other stars a black hole of certain sizes would absorb? And how long will it take for the number of active stars to decrease to the point where Hawking radiation would surpass it?

• Much ambiguity in that last sentence. What are we measuring the Hawking radiation up to? Average brightness of surrounding stars (in a region of average stellar density) as seen from the black hole? Are we measuring against the stars' emissions, as seen from their atmospheres? What is the mass of the black hole, when does the black hole occur? I don't think your question can be answered unless that's cleared up.
– BMF
Nov 16, 2019 at 23:22
• @BMF I specified in the question that I was talking about black holes "of certain sizes". I'm asking for a range of answers, not in regards to one specific case. Nov 16, 2019 at 23:34
• I think that most light will just continue to try to go in a striaght path and curve around it, with a minority touching the event horizon. But right, what's the size of your black hole? Anyway, the longest living stars (red dwarf) have a lifespan of about 10^12 years, whereas the largest blackholes will live for about 10^100 years before evaporating away. Nov 16, 2019 at 23:43
• Most of the photons in the universe are in the CMB, not starlight. It will take a long time before the CMB is cooler than the Hawking radiation temperature of even the smallest stellar mass black holes. See en.wikipedia.org/wiki/Timeline_of_the_far_future Nov 17, 2019 at 9:16
• @PM2Ring That is true on average, i.e. in intergalactic space. Inside a galaxy, however, the energy density in starlight will be comparable or bigger than the CMB energy density. Nov 18, 2019 at 10:29