The Drake Equation: The R* Term R*: The number of suitable stars "born" per year This is probably the only factor in the Drake equation that is known with any certainty! The rate at which stars are forming in the Milky Way galaxy now is of order of 1 per year, but it is thought that in the past, say a few billion years ago, the rate was higher, perhaps between 3 and 5 stars per year. It is, of course, that larger number which would determine the number of civilisations arising now! (New estimates seem to be increasing these numbers somewhat). The key criteria However, we need the rate of formation of suitable stars, i.e., those which would allow the evolution of life on the surface of a planet in their solar systems. These must satisfy two criteria. Firstly, they must provide a stable heat output for a period of time long enough for an advanced civilisation to evolve. We only have one example on which to base this estimate (guess which one!), but evolution is a slow process and almost certainly a period of several billion years is required. This eliminates stars which are significantly more massive than the Sun as their lifetimes are too short. Secondly, the star must provide sufficient heat that a planet could be sufficiently warm. Stars which are progressively less massive than the Sun produce less heat and consequently a planet would need to be nearer to it to achieve the appropriate temperature (in the habitable zone). There comes a point when the planet would be so close that it becomes "tidally locked" (like the Moon is now), with one face permanently facing the star and the other facing away. Life is most unlikely to exist on such a planet. How many? It turns out that stars of spectral types F, G and K (the Sun is of type G) seem to provide the best prospects for the evolution of life within their solar systems. They comprise around 10% of all stars, so the value of R* is probably in the region of 0.3 to 0.5 per year.