Are nebulae only visible from a distance?

Question

There's plenty of data about the density of nebulae, and obviously they're pretty thin. This is space, after all, and these things are really, really big.

But I'm wondering about the practical experience of that density. Are the colorful patterns of gas only visible when you're far away and looking through them, or if you were inside a nebula would you see colors all around you instead of the black of space? Are any dense enough to obscure the view between planets of the same solar system? Between a planet and its moon?

Answer 1

The colorful patterns of gas are caused by excitation of gas atoms, e.g. oxygen, from radiation of a nearby star. You would see the same colors as from a distance, as the light is emitted in almost random directions. But you wouldn't see the same region of nebula shining at the same time as seen from Earth, when excitation is caused by a flash of radiation from a central star, because the radiation travels concentrical away from the star, and the distances the light has to travel from the excited part of the nebula to the observer is dependent of the location of the observer.

Light may be scattered non-uniformly by dust-particles, so looking different from differnt positions, e.g. forming a halo around stars, depending of the composition and shape of the particles. The interplanetary dust in our solar system causes zodiacal light.

Our solar system is in an interstellar dust cloud right now, but it's not very dense.

Very dense clouds can form, but would become instable and collapse to a star or a protoplanetary disk, compare this site about dark nebula.

Answer 2

You might be interested in these two answers from the Physics.SE site:

How dense are nebulae?

Excerpt:

If you were within a nebula, it is hard to say what it would look like. But nebulae are so large that the optical depth of the cloud would actually probably be quite high, and I would guess that it would look like you were surrounded by glowing green and red gas in the far distance - instead of space looking black and dark, it would be colored all over. But this would only be an effect caused by the fact that you are looking through so much gas - even if your spaceship were a thousand kilometers away, it probably wouldn't look much different if you were inside a nebula versus outside of it.

If we were to travel through space, how close to the false-color images would the galaxies we see be?

Excerpt:

Unfortunately, even from a near distance, most of these objects won't look much better. They are, after all, faint, rarefied clouds of dust and gas. They are just not bright enough for the human eye to see color.

There is another question that I cannot find which asks what a nebula would look like from a nearby spaceship. It basically states that the nebula are too disperse and too large to see from up close. I'm mentioning this in an answer instead of a question with the hope of finding the relevant link and providing more details.

Answer 3

The nebula colours are mostly due to forbidden line emission from oxygen (green) and nitrogen (red) and from the balmer series of hydrogen and occasionally, ionised helium. The key point is that this is usually optically thin emission, meaning that what we see is proportional to the number of excited atoms/ions in the nebula in the line of sight.

This is important for your question, because in such cases, the surface brightness is independent of distance. The luminosity increases as one over distance squared, but the area occupied on the sky also increases by the same factor.

From inside the nebula the surface brightness would be even lower.