# What is the largest (angular) dark area in the sky?

## Question

Within our current knowledge, what is the largest (circular, say) area in the sky which is empty/dark? I'm thinking in terms of angular size.

## Motivation

I was wondering what was the largest star as seen from the Earth, other than the Sun of course, in terms of milliarcseconds, say. A search quickly turned up R Doradus which is huge (57 mas as seen from the Earth). But I wondered if it was possible to rule out any (angularly) larger dark objects by finding stars whose light would be blocked if there was such an object.

Of course we should eventually be able to rule out each such area -- as Hubble Deep Field showed, if you look long enough there's a lot to find everywhere -- but relative to our current knowledge, what are the largest gaps?

I expect that the answer would be in a region which has been missed by a sky survey and which coincidentally doesn't have anything we can see.

• @CarlWitthoft The fundamental idea is that I'm looking for an area dark enough that we can't rule out the possibility of a large dark object at that position in the sky. A few scattered photons could potentially be from such an object, but if there are enough in a focused area we can call that a star and say that there is no obscuring object at that position. – Charles Nov 3 '16 at 14:51
• @JoeBlow Neither. I'm looking for the largest (from our perspective) angular patch of sky which has no stars we can detect. So like (B), except that I'm not requiring it to actually be empty, just empty as far as we know. This includes areas which seem dark because there's something in the way, but only if we can't tell if there's something in the way or not. (If we know something's in the way, then we know it's not empty.) – Charles Nov 4 '16 at 14:12
• I feel there are some confusing issues. Note that considering 'in our galaxy" ("stars") is incredibly different from considering "looking at the universe" (ie, "galaxies"). {Just as looking at stars is incredibly difference from looking at planets.} – Fattie Nov 4 '16 at 16:16
• @JoeBlow I'm talking about what can be seen from Earth, anywhere in the universe. Average angular separation between the smallest detectable stars is a lower bound on what I'm asking for (and would be a good starting place for an answer). If we can identify a region as having a blok globule, it's not empty for my purposes. – Charles Nov 4 '16 at 17:13
• @JoeBlow Yes, as I said in a comment on that answer, it's not what I was looking for. – Charles Nov 4 '16 at 17:14

Barnard 68 is the first thing that comes to mind for me, it is a little over 10' across and is opaque in the visible spectrum.

I'm sure there could be something much larger out there, but like I said, this was the first to come to mind.

• Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Nov 7 '16 at 14:55

What do you mean by, "relative to our ability to observe it"? If you have a detector sensitive enough you'll see some light no matter what direction you look, but the level of light will depend on the resolution of your detector. When you look at a patch on the sky, all of the light within one resolution element basically gets lumped in together. Think of the analog of looking at the Milky Way with your naked eyes versus using binoculars - with the eyes it looks like a smooth cloud, up the resolution and you can see the stars.

So, if you up the resolution enough to see past all of the foreground objects (galaxies and stars), you'll see the combination of the foreground milky way clouds and the cosmic microwave background. At a more realistic resolution you need to add in light produced by milky way stars and galaxies.

This is an area of study in astronomy known as the extragalactic background light, here's a recent review by Asantha Cooray.

So, the two short answers are, "Nowhere is dark," or "It depends on your detector's sensitivity and resolution."

• I think that what he really means is "where is the largest gap in our knowledge", i.e. what's the largest area of the sky that still appears empty on maps and such. Not that we can't see things there, necessarily, but that we just haven't looked. – Phiteros Nov 3 '16 at 4:18
• @Phiteros: Exactly. I'm not asking about what could have been detected, but what has actually been detected. – Charles Nov 3 '16 at 4:47
• @Charles now you're getting close to "known unknowns and unknown unknowns" :-) – Carl Witthoft Nov 3 '16 at 11:29
• I'm sure the OP simply means "with your naked eye in a fairly nonpolluted area" – Fattie Nov 3 '16 at 20:11
• @JoeBlow No, with the best telescopes or other equipment which we've used, in any wavelength. – Charles Nov 4 '16 at 4:58