Analysis / Space Clouds
In the real world, this does not actually happen. Simply put, the "cloud" would quickly disperse due to intermolecular collisions and thin out into the interstellar medium. Or if it did have sufficient mass or was being compressed by, say, gravity or the shockwave from a nearby super nova, it would collapse into a star. Now there are clouds of a sort in space (nebulae), but these are objects light-years across, with enough mass in them that gravity can keep them together, and their density is far below even industrial-grade vacuums. A typical interstellar cloud has a molecular density of between 100-1000 particles per cubic centimeter. The air you breathe just walking around on Earth is around 2.5x1019 particles per cubic centimeter, or about 250-million-billion times denser, and artificially created vacuums on Earth have an average density of ~40 per cubic centimeter, which is still too dense when compared to some nebulae. The nebula's density may be 1000 times the density of the interstellar medium, but it's still a near perfect vacuum. If you found yourself suspended in the midst of a nebula, you likely would never know. That is to say, if you flew your rocket ship up to, say, the Orion Nebula, it would pretty much look like nothing was there. Another example is how galaxies have a distinct "disc-like" shape when observed from far away by our telescopes, but when you're in one (such as the Solar System being in the Milky Way) you won't even notice you are in a disc and will just see vast areas of nothing. The reason that interstellar clouds appear opaque and dense is because they are very, very, very, very far away and very, very, very, very large, and this perspective makes the clouds appear compressed and thus solid. The Pillars of Creation are four to seven light-years tall; large enough to swallow not only our Solar System but also Alpha Centauri, Sirius and several other nearby systems. Note that despite lacking density, nebulae do contain some matter such as asteroids, but they travel with high speeds. When you fly through such with a spaceship, collisions will ensue which might, or depending on the speed of your spaceship, will wreak havoc to the hull. Just to give a sense of scale, a single fog-sized water drop which collides with a spaceship at half the speed of light has the impact force of about 200 kilograms of TNT. This is an example of force=mass*acceleration. Space Clouds may contain more than one drop of water. One point that is never taken into consideration is that the various astronomical nebulae that are known by appearance (Crab Nebula, Orion Nebula, Pillars of Creation, ect), are known solely by the appearance that they had at some time in the distant past. All the pictures we have now were made with light that took many thousands of years getting to us. The Crab Nebula is estimated, for example to be approximately 6300 light years from Earth. This means that the image of the nebula that we are most familiar with is of its appearance 6300 years ago. Astronomers are somewhat certain that if we were able to instantaneously observe the present form of these nebula, they would look nothing like the pictures we have of them and for all that is known, they may no longer exist. By the time you arrive, the whole thing could have collapsed into a new generation of stars or been blown to the far ends of the universe. To add to the confusion, some of the most famous pictures taken of outer-space phenomena are in false-color — to help illustrate some aspect of it, such as ultraviolet radiation. In other words, the colorfully magnificent Space Clouds you're looking at in astronomy books was, in essence, Photoshopped. This information is not commonly provided in images released to the public, and "true color"note pictures of these stellar phenomena are not as easy to find, which means artists are more likely to use the wrong images as a basis for what they look like to human eyes. A final note for those readers who may be interested on a telescope, hoping to see Space Clouds or something so coloured as photographs: not only most real nebulae are often so extended that their surface brightness is so low, being very difficult or impossible to see with a telescope, even with big ones and very dark skiesnote , and appear only in long-exposure photographs, but also at very low levels of light the eye cells that work in those conditions (rods) capture black and white, meaning you'll see no colorsnote .