什么是Emission Nebula？
An emission nebula is a cloud of interstellar gas that glows in its own right, because hot young stars embedded in it are stripping the electrons off the hydrogen around them. It is the same material as a dark molecular cloud, a few million years later, once the first stars inside have switched on.
The colour is a fingerprint, not a choice. When a hydrogen nucleus recaptures an electron, that electron cascades down through the atom's energy levels and emits at fixed wavelengths on the way. The brightest of them in visible light is H-alpha at 656 nanometres, a deep red, which is why these nebulae are overwhelmingly red or pink in long-exposure images. Closer to the hottest stars, where the excitation is higher, doubly ionized oxygen emits in the green-teal at 500.7 nanometres — a line so unfamiliar when it was first measured that astronomers attributed it to a new element they called nebulium. It is not a new element. It is ordinary oxygen doing something it cannot do anywhere that has enough atoms around to bump into.
These objects are dismantling themselves. The same ultraviolet flood that makes the gas glow also boils it away, and the stellar winds off the massive stars punch a growing cavity through the middle of the cloud. Only the densest lumps of gas hold out, shielding the material directly behind them and leaving columns that point back at the stars eroding them. That is what the Pillars of Creation are: not structures that were built, but the last parts of a cloud that has not been destroyed yet.
The model runs that erosion rather than drawing its result. The density threshold in the volume rises with how strongly each point is being irradiated by the embedded cluster, so the cavity opens up and the pillars form on their own, aimed correctly, out of noise that contains no pillars. The ionization colour follows the same field: teal where the excitation is highest, red where the gas has recombined further out.