Measuring the star formation rate (SFR), defined as the mass of gas converted to stars per year, is taking the pulse of a galaxy. The SFR is a fundamental parameter in theoretical models of galaxy evolution and governs the observed properties of galaxies. The most widely-used empirical measurements of the SFR in both the Milky Way and external galaxies rely on indirect observational tracers sensitive to only the most massive 1% of stars. Hence state-of-the-art astrophysical methods for diagnosing galactic star-formation activity are analogous to using only the tax returns and investment portfolios of the richest 1% of earners to diagnose U.S. economic activity.
In the Milky Way, it is possible to directly resolve very young stars in many of the most massive star-forming regions---the kinds of high-luminosity regions that dominate observations of distant galaxies. In this talk, I will describe new work by my group to model the 3.6 µm through 10 mm spectral energy distributions (SEDs) for a sample of 30 Galactic massive star-forming regions. Our SEDs were constructed from archival imaging data obtained using an armada of space-based observatories: Spitzer, MSX, IRAS, Herschel, and Planck. Because the massive stellar content in the majority of these regions has been spectroscopically cataloged, we can balance the "energy budgets" in each region by comparing the luminosity input by stars to the luminosity output in the SEDs. We find that typically ~50% of stellar bolometric luminosity is absorbed and reprocessed by dust in these infrared-bright H II regions and surrounding photodissociation regions. We calibrate infrared and radio diagnostics of obscured SFRs to the known ionizing stellar populations. Widely-employed extragalactic SFR calibrations based on 24 µm luminosity agree with our calibrations to within 30%, while analogous extragalactic calibrations based on 70 µm luminosity must be corrected for the smaller physical size of individual Galactic regions.
To conclude, I will discuss how we can apply our new SFR calibrations based on SED modeling to the upcoming catalog of ~4000 Galactic star-forming regions produced by citizen scientists for the Milky Way Project to map the SFR distribution in our home Galaxy.