From dust to stars

By Daniel Holz | April 26, 2010 9:37 pm

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.

Planck's view of OrionThe 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!

  • V H Satheeshkumar

    Good to know that Planck has finished imaging of the entire sky. When will the full data/map be out? How better is it compared to WMAP? Can we know the time variation of Dark Energy with this data? Is it any better when it comes to the signatures of Inflation?

  • Tom

    Thanks for the post. I have some questions about the actual astronomy, not the cosmology and physics, of what those images are showing.

    First, what exactly is “dust” ? Is it just clouds of atoms / gas, such as hydrogen? (I assume not, atomic and molecular hydrogen should be too small to scatter anything in the visible or longer). Is it larger, particulate matter? If so, what is it composed of and what is the origin? (Supernovae, I would presume).

    Second, why is the cold gas emitting at higher energies (300 um) compared to the hot electrons emitting way out in the 1 cm band?

  • Brian137


    Here is a Wikipedia article on cosmic dust:

  • daniel

    @Tom (#2). Yes, the numbers appear counterintuitive. The trick is that we are not only talking about thermal emission. The hot gas is ionized, and we are looking at emission from transitions; this occurs at a range of frequencies, including some in the green band. The hot electrons are emitting synchrotron, which has a distinct (non thermal) spectrum. So cold gas can, in some cases, be detected at higher frequency (and hence energy) than hot gas.

  • Andy Fleming

    The image of the Orion Nebula with the red ring of dust from the ancient exploded star is amazing. Until recently, I never realised that only 10% of the normal, visible matter in the universe is comprised of stars… the rest is actually intergalactic gas and dust!


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