How dense are giant gas clouds like the “Pillars of Creation”?

The biggest pillar in the classic Hubble Space Telescope picture of the Eagle Nebula (also known as the “Pillars of Creation”) measures about 3 light-years. The pillars are about half a light-year thick. The image leads us to believe that the contrast between the pillars and the space between them is large — the pillars look almost solid. However, the amount of material we are looking through, rather than the density of the gas, determines this contrast.

The terrestrial analog is looking into a fog bank. What we call “dense fog” is not actually denser; it is just a larger cloud of material — more water droplets suspended in the air along our line of sight obscure our view. And so it is with these types of nebulae. Because we’re looking a half light-year through the pillars, we can see sharp edges and apparently dense structures — even though the material is quite tenuous.

In the Eagle Nebula, the density of material within the columns is about 4,000 particles per cubic centimeter. Air density at sea level has more than 10¹⁹ particles per cubic centimeter — that’s a factor of 10 million billion times more dense than in the pillars. The 4,000 particles per cubic centimeter density is far lower than that of the best vacuum we can achieve on Earth. So, it is the vast scales of these nebulae that give them the appearance of solid structures. — Paul Scowen, Arizona State University, Tempe