How dark are dark nebula?

Recently I was asked* 'how opaque is a dark nebula?' I went with it as a topic since, as a dark nebula is just a nebula with no stars to illuminate it, it gives me a perfect excuse to look at lots of stunning space images.

However.... we get those images from huge telescopes,  which are designed to collect as much light and detail as possible. That means they aren't a very good indicator of what a human eye would see. So I went and did some reading.

And it turns out that 'nebula' are a bit more complicated than I realised**....

She's complicated, true. But someone should try to understand her. Ahem, sorry, Marvel movies geek here.

Before we start, we need a reference point: The air you're breathing (I assume you breathe air on planet Earth, no offence if you've logged on from... elsewhere) has roughly 10,000,000,000,000,000,000 molecules per cubic centimetre.

So let's start at the beginning: Go out on a clear night, somewhere far from any city lights. You'll be able to see countless objects that are immense distances away, including nebula - they don't look like they do in the pictures, but if you know what to look for they're visible. So, pretty obviously, space is empty and transparent, and the nebula are clouds of glowing (or dark) dust and gas, over there somewhere....

....and then, just before the Sun comes up, you see this: 

Above: Zodiacal light, appearing above the European Southern Observatory

That glow (called zodiacal light) is sunlight being reflected by the cloud of dust and gas that fills our solar system - yep, we are actually inside a disk shaped nebula right now, and it's barely visible to us - and that's a big hint. But our nebula is thin  only 100 atoms per centimetre (and varying amounts of space dust) at most, so it's surprising we can see it at all.

Above: A particle of space dust, as seen under an electron microscope.

In fact we're in a nebula inside another nebula - it's called the interstellar medium (ISM), a thin soup of dust and gas that varies in thickness from place to place, and it surrounds our solar system.

Beyond our solar system's boundaries the ISM makes a fantastically complicated (and mostly invisible) 'ecosystem', busily becoming stars and planets and then turning back into dust and gas when they die. And, while it's tenuous, it's huge - it makes up roughly 5% of the mass of our Milky Way galaxy. ESA has mapped it, and their results should give you get an idea of how complex it is:

Above: A map of the cool portion of the interstellar medium, from ESA.

That's the ISM, the 'soup' between the solid things in our galaxy, made bright enough to see.
There's no real distinction between this soup and a nebula: There's no edge you can cross and say 'now I'm inside a nebula' - the soup just gets steadily denser, up to maybe 10,000 atoms per cubic centimetre. But, ok, you can just head towards something like the Carinae nebula.....

Above: The Carinae nebula, as seen byt the European Southern observatory. Awww... pretty! Invisible to human eyes, but pretty.

.... and stop when your spaceship's sensors tell you you're in the densest bit. What do you see?

Weirdly... not much, just like with the cloud surrounding our solar system. Maybe a faint glow, or slight dimming of the background stars, but from inside it would be almost completely invisible to a human eye.  
True, the damn thing looks almost solid in the picture. But so do ordinary terrestrial mists at a distance, and once you get inside those you can usually see fine:


The view from inside a mist bank, courtesy of StevesDigicam.com. That mist bank is far thicker than any nebula, but it's not visible at all until you look at something in the far distance.

Factor in that most nebula are many, many, times thinner than the thinnest terrestrial mists... and the lack of a show inside one starts to make sense.  

The reason why there appear so vivid in our telescopes is because a telescope concentrates the light from a big collecting mirror into one small image. A telescope will also spend hours staring at one spot, slowly collecting enough light to bring out normally invisible details. But get up close and the light (or shadows) from the nebula would spread out all around you, too thin to trigger your eye.

So... is that the answer: There's only one nebula, and it's actually see through? Not quite. Let's go back to our picture of the Carinae nebula:

Here it is again.

Do you see those little clumpy bits? Those are called Bok globules: Regions where gravity has made the ISM 'soup' collapse in on itself. The globules still don't have a defined edge, and through most of their structure they only have maybe 1000,000 atoms per cubic centimetre - denser than the usual soup, but still far thinner than air, mist, or smoke. But they are big, light years thick, which means they can still block out the stars behind them, and appear dark. 

Above: Bok globules blocking out light. For reference, the big one in the top right is about five light years long. Courtesy of NASA/JPL

In the centre of these globules solar systems are congealing out of the gas and dust: Gravity is crushing the soup into things as dense and solid as rock and ice****. The density in the middle eventually gets up to something like a planetary atmosphere, and then even higher than that as the central protostar forms. 

Here, in the heart of the densest part of a nebula within a nebula (within a nebula), we've found a spot where the human eye would see something - or, more accurately, nothing: Surrounded by light years of globule, and with the star growing in the middle still to young to shine, it would be totally, utterly, black. 

Which is a pity, because it's also full of newly minted comet cores, and you might want to spot those coming...

Above: A comet nucleus, one of the first objects to form from the Bok Globule that gave birth to our solar system. Courtesy of NASA.

To answer the original question: There's only one nebula really, and how opaque it is varies a lot. But generally it's much more transparent than terrestrial air - you have to really go looking to find a very opaque spot.

But that's not the most... interesting... thing I discovered researching this.

Next post: Why the ISM space soup could destroy civilisation (no joke).

* Yes, you can just ask me stuff and (if I feel like it) I'll research an answer for you. The contact form should be in the border of the post.

**In fact they are really, really complicated, so I apologise to all nebula boffins*** for all the oversimplifications I've committed.

***Yes that is an actual job - I even applied to be one once. 

**** It's a lot more complicated than that. But you start with the ISM, apply gravity, and end up with rocks - that's the gist of it, and it's almost my lunch time******.

****** Plus the number of *'s I'm needing is getting ridiculous and I've never figured out how to make Blogger do superscript.