The first term in the Drake Equation is R*, the number of newly formed stars in the galaxy per year. The estimate given in 1961 was ten stars per year. Over the past fifty years, new instruments and methods have allowed us to better understand how stars begin their lives and how efficiently gas can create new ones.
Powerful instruments specifically adapted to the study of star formation include both space facilities – the Galaxy Evolution Explorer (GALEX), the Spitzer Space Telescope (SST), the Herschel Space Observatory (HSO), and the Hubble Space Telescope (HST) – and a host of ground-based optical, infrared, submillimeter, and radio telescopes. These instruments have described in unprecedented detail the key phases and physical processes that lead to the formation of individual stars.
In-depth case studies of individual star-forming regions have yielded an understanding of the central physical processes that determine how molecular clouds contract and fragment into clumps and cores and, finally, clusters and individual stars. The determination of the global star formation rate (SFR) for the Milky Way is rigorously based on measurements of the global parameters of several local star-forming regions. In general, any total flux measure that is related to the SFR of a galaxy (including the Milky Way) is completely dominated by high-mass stars, since these are responsible for virtually all of the luminosity of a galaxy.
The detailed picture of how gas is transformed into stars requires not only knowledge of the SFR but also the distribution of mass of stars at their birth, a function called initial mass function (IMF). Theoretical simulations have explored how large molecular clouds fragment into stars under very different physical conditions. These works have permitted us to identify the most important physical parameters and have led to analytical formulations of the SFR and the IMF. 39In particular, they give estimates of a factor that is particularly important for the Drake Equation: the fraction of stars that are binary.
Most estimates of the SFR of the Milky Way have relied on global observables. Such studies generally rely on indirect tracers of massive (O- and early-B-type) stars to determine a massive SFR. This value is then extrapolated to lower masses to derive a global SFR for our galaxy. For example, an analysis from the late 1970s led to a value of five solar masses per year by making use of the fact that the integrated flux density from an HII region is a direct measure of the number of ionizing photons required to maintain that HII region, and is therefore an indirect measure of the number of O- and early B-type stars. In 2006, an estimate of four solar masses per year was derived from observations using the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) mission, which measured the gamma rays emitted by radioactive aluminum as a proxy for the massive star population of the Milky Way. Another study from 2006 gave a value of 2.7 solar masses per year, by using the total 100-micron flux of our galaxy. Along the line of these examples, this chapter will review in detail the evolution of estimates of R*, which is now closer to five solar masses per year than the ten assumed in 1961.