by gwolfchase scientist
Some of you may have noticed that most of the bubbles you've marked are distinguished by 'red' interiors and 'green' rims. Here is an example of a bubble where this is reversed! In the more typical bubbles, the red depicts emission from warm dust in a region where
most of the gas is ionized, so you don't have molecules. The 'green' rims are regions where complex molecules known as Polycyclic Aromatic Hydrocarbons (PAHs) can form and emit infrared light (at the wavelength that is colored 'green' in the images). These regions are typically fairly small rings -- too far away, and PAHs aren't sufficiently heated; too close, and these complex molecules are destroyed.
In the case of a bubble that appears 'reversed' (and please tag any such objects you find for the science team!!), the central young stars may not be hot enough to ionize the gas so you just get a PAH region that appears circular (more or less), with a dusty ('red') exterior. Alternately, some of these objects may be more evolved stars near the end of their lifetimes. We plan to explore this! In either case, it will give us a better picture of how stars affect their environments! -- Thanks, Grace Wolf-Chase