We're all waiting for the Planck map of the cosmic microwave background (CMB), which should become the definitive map of the early Universe for the foreseeable future. While we're on tenterhooks, the Planck team has been feeding us tidbits to keep us occupied. The first was a gorgeous map of the dust. Now they've released some images of a stellar daycare. Planck's key science goals have to do with mapping the CMB, which is an image from the far edge of the Universe. All the foreground stuff in between (like our galaxy, and all its dust and stars) is a nuisance, and need to be removed. Most of the Planck team would be just as happy if no stars existed at all. In that case the images of the CMB would be pristine and spectacular, and the whole mission would be a lot easier. Of course, it'd be pretty cold and lonely Universe, since there'd be no Sun, and no Earth, and no Planck team, and (shudder to think) no blogs. For better or worse, there are dusty regions in our galaxy, filled with newly-born stars. Planck has been specifically designed to map out these annoying foregrounds, so as to be able to remove them from its images. The trick is that stars generally form in these regions, because it is precisely this dust which collapses to form stars. But this same dust obscures our view of what's happening, at least at optical wavelengths. At microwave wavelengths, one can image the dust directly, and Planck observes at multiple frequencies precisely to do this. It makes detailed maps of stars and dust, just to subtract them off. But in the process, we get these lovely pictures.
The image is of Orion. The right panel is a composite image, while the left shows the three individual color bands: red corresponds to synchrotron emission from hot electrons in our galaxy's magnetic field, green corresponds to hot gas (presumably heated by the stars), and blue corresponds to cold gas (this is the stuff that collapses into stars). The giant red circle in the image is from a star which exploded roughly 2 million years ago, and blew out its surrounding dust (inhibiting further star formation in that region). We're seeing the aftermath of the birth (and death) of a star! The details of how stars are born, live, and die are pressing astrophysical questions, and these images show us the process as it unfolds. Whatever. Enough with the distractions. Planck has now imaged the entire sky in at least three frequency bands, and it looks like the data is good. Hopefully the full-sky CMB maps aren't too far behind!