# How much starlight is there?

Hi All

Just wondering, but how much starlight from the Galaxy is there away from any star? How cold can a body in interstellar space get with a zero albedo? And I don't mean inside an opaque cloud, but in the relative emptiness between the stars?

Milan Cirkovic & Robert Bradbury have speculated that super-advanced Civilizations might move to the Galactic Halo to make best use of the cosmic heat-sink. Just how much energy would they get from the Galaxy's light?

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Hi All

Just wondering, but how much starlight from the Galaxy is there away from any star? How cold can a body in interstellar space get with a zero albedo? And I don't mean inside an opaque cloud, but in the relative emptiness between the stars?

Milan Cirkovic & Robert Bradbury have speculated that super-advanced Civilizations might move to the Galactic Halo to make best use of the cosmic heat-sink. Just how much energy would they get from the Galaxy's light?
You seem to be asking a number of questions here, the first of which makes no sense: the size and albedo of a body, and its distance from the star would be the issue for your first question. A zero albedo would be a function of the object's reflectivity, not what "starlight" falls on it. Then you ask about using some kind of cosmic heat sink, and for that I would just say that could be figured by asking: what is the total flux of all forms of energy in a given region of space, that your sink is capable of interacting with? I don't know that such a thing has ever been calculated, and I don't know why it would be. You also keep talking about light, ignoring all of the other EM radiation that would be useful, and particle collisions, friction with interstellar medium, and so on.

You seem to be asking a number of questions here, the first of which makes no sense: the size and albedo of a body, and its distance from the star would be the issue for your first question. A zero albedo would be a function of the object's reflectivity, not what "starlight" falls on it.
Did you read what I said before commenting? How cold can a zero albedo object get in Interstellar space - the big gap between the stars, in case you can't figure it out - is a perfectly sensible question and really just a rephrasing of the first question. When you think about it, that is.

Then you ask about using some kind of cosmic heat sink, and for that I would just say that could be figured by asking: what is the total flux of all forms of energy in a given region of space, that your sink is capable of interacting with?
Facing the CMB and not the stars is what I meant by "cosmic heat sink" or is that concept too novel?

I don't know that such a thing has ever been calculated, and I don't know why it would be. You also keep talking about light, ignoring all of the other EM radiation that would be useful, and particle collisions, friction with interstellar medium, and so on.
"Light" is an accepted synonym for EM radiation in most conversations amongst astrophysicists. "Visible light" is the stuff human eyes react to. "Friction with the ISM"??? Huh? You're talking about the particle flux? Insignificant in most cases.

Chronos
Gold Member
Olber's paradox suggests we are surrounded by a finite, and relatively modest number of photons in this universe.

Olber's paradox suggests we are surrounded by a finite, and relatively modest number of photons in this universe.
In the Universe at large but I meant the Galaxy's starlight. For example how much starlight is available per square metre to collect for power a few light years out from any star in particular?

Chronos
Gold Member
The total brightness [including airglow] of the sky on a clear, moonless night is about 2E-003 lux. Sirius, the brightest visible star, contributes about 1E-005 lux. The brightness of the night sky during a full moon at the equator is about 1 lux [one lumen per square meter].

You seem to be asking a number of questions here, the first of which makes no sense:
And this, my friends, epitomizes the smug Physics Forums poster attitude.

Regarding the question, I feel like an object with zero albedo would absorb heat and thus be warmer than a reflective planet, just as reflective snow cools mountains. But it seems that you are using "albedo" to characterize interstellar space, as in areas with no objects obviously reflect no light.

I feel like there must be matter located in the "void" you describe for it to act as a heat sink. While a perfect blackbody would absorb all incident electromagnetic energy, this is because of electrodynamics, ie, the interaction with matter and EM waves. Thus with no matter present in the interstellar void, no heat is absorbed or reflected, it just moves on.

I'm guessing the Bradbury reference is discussing an object placed in the space near an object with zero albedo, as the object would "sink" all electromagnetic energy. I feel like the temperature of the object would be dependent only on the energy that is incident upon it, and the zero albedo blackbody would only be an "umbrella" from the EM in one direction.

The lowest possible temperature would depend on angular size of the blackbody relative ot the object, location/size of nearby starts, but I'm guessing it could approach (but not reach) absolute zero.

And this, my friends, epitomizes the smug Physics Forums poster attitude.

Regarding the question, I feel like an object with zero albedo would absorb heat and thus be warmer than a reflective planet, just as reflective snow cools mountains. But it seems that you are using "albedo" to characterize interstellar space, as in areas with no objects obviously reflect no light.
Well no. What I'm asking is the equilibrium temperature of an object in interstellar space - what's the coolest it could get? Plus just how much energy would impinge upon a surface or volume per second, in interstellar space. Illumination at all frequencies via the collective glow of the Galaxy's stars. I know that the Galaxy's collective luminosity is about ~30 billion Suns worth, which seems surprisingly low and illustrates that most stars are significantly dimmer than the Sun. But within the spiral arms just how much starlight, collectively, would a surface receive from an arbitary direction? Intuitively it seems a straight-forward question, but I guess I'm not explaining what I mean to get a specific enough answer.

Ich
Chronos' answer gives you a flux of the same order as the CMB in the visible light. Now double that figure to account for invisible radiation, and you get some 3.5 K equilibrium temperature. Just a guess, but an educated one.

And this, my friends, epitomizes the smug Physics Forums poster attitude.

Regarding the question, I feel like an object with zero albedo would absorb heat and thus be warmer than a reflective planet, just as reflective snow cools mountains. But it seems that you are using "albedo" to characterize interstellar space, as in areas with no objects obviously reflect no light.

I feel like there must be matter located in the "void" you describe for it to act as a heat sink. While a perfect blackbody would absorb all incident electromagnetic energy, this is because of electrodynamics, ie, the interaction with matter and EM waves. Thus with no matter present in the interstellar void, no heat is absorbed or reflected, it just moves on.

I'm guessing the Bradbury reference is discussing an object placed in the space near an object with zero albedo, as the object would "sink" all electromagnetic energy. I feel like the temperature of the object would be dependent only on the energy that is incident upon it, and the zero albedo blackbody would only be an "umbrella" from the EM in one direction.

The lowest possible temperature would depend on angular size of the blackbody relative ot the object, location/size of nearby starts, but I'm guessing it could approach (but not reach) absolute zero.

Well, that was completely uncalled for, but thanks for joining me in the "swing and a miss" column.

Well, that was completely uncalled for, but thanks for joining me in the "swing and a miss" column.

Hi nismaratwork

I was wondering because I read of the idea of collecting starlight for power in a piece by Richard P. Terra on colonizing the Oort Cloud comets. Ben R. Finney & Eric M. Jones were the first to suggest it in a serious paper, according to Terra. They discussed it in Interstellar Migration and the Human Experience, a now classic book on interstellar colonization from 1985, plus an earlier paper "Interstellar Nomads" (1983), so it's quite a venerable idea.

Hi nismaratwork

I was wondering because I read of the idea of collecting starlight for power in a piece by Richard P. Terra on colonizing the Oort Cloud comets. Ben R. Finney & Eric M. Jones were the first to suggest it in a serious paper, according to Terra. They discussed it in Interstellar Migration and the Human Experience, a now classic book on interstellar colonization from 1985, plus an earlier paper "Interstellar Nomads" (1983), so it's quite a venerable idea.
Interesting, it seems like a very exotic notion compared to the usual ramscoop notion.

Interesting, it seems like a very exotic notion compared to the usual ramscoop notion.
It's not for propulsion, just power. The proposal is for colonizing Oort cometoids and starlight seems viable as a power source, given sufficiently large collectors - about 3000 km across per megawatt, which should be feasible with the very low tidal forces so far from any star. Fusion fuels would be saved for propulsion.

It's not for propulsion, just power. The proposal is for colonizing Oort cometoids and starlight seems viable as a power source, given sufficiently large collectors - about 3000 km across per megawatt, which should be feasible with the very low tidal forces so far from any star. Fusion fuels would be saved for propulsion.
That seems like a lot of work for a megawatt; what kind of material could be used in such large quantities as an effective medium for absorbing and converting ambient energy? Do you have a link to this paper? My curiosity is piqued, as this sounds like a kind of inverted Dyson Sphere, and I always found that notion very romantic.

Chronos
Gold Member
Pardon me for quantifying the answer. Liking it is optional.

That seems like a lot of work for a megawatt; what kind of material could be used in such large quantities as an effective medium for absorbing and converting ambient energy? Do you have a link to this paper? My curiosity is piqued, as this sounds like a kind of inverted Dyson Sphere, and I always found that notion very romantic.
The conference book chapter is available through Google Books...