Bubbles bubbles everywhere. As seen by Spitzer, blue: 4.5 µm, green: 8 µm, red: 24 µm.

As the data from the Milky Way Project are starting to come in, and Rob is making progress with the data reduction of  many clicks and drawings, I’ve been giving a lot of thought to these gorgeous bubbles we’re seeing. How were they created, why do they appear the way they do, and what do they tell s about the process of star birth on the scale of an entire galaxy? We understand pretty well these days how stars are formed, how they live and how they meet their ends. But when it comes to forming a picture of the lifecycle of material on large scales, lots of questions remain.

From our kiddie play with washing up liquid, we know that we can make bubbles by blowing air into some soapy liquid. Intuitively, that’s how we interpret bubble shapes: something has inflated them from the inside. The interstellar bubbles seen all throughout the disk of the Galaxy look just like that too. At 8 μm, we can see that something is illuminating the dense cloud material in the rim from the inside. At 24 μm, we can see heated dust glowing in the centres of many of them. At radio wavelengths we can see that something, somewhere in the bubble, is producing enough ultraviolet radiation not only to break up hydrogen molecules, but to knock the electrons off the atoms.

All this evidence points towards the bubbles being inflated by young hot stars that are blowing away the dense cloud of gas and dust they were born in, pushing the material back with their hot winds and heating and ionizing it with their UV radiation. But new simulation results reported in a Monthly Notices paper, posted to astro-ph this week, call this scenario into question.

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