
The rate at which new stars are born in a given volume of space.Star formation rate (SFR) is a measure of how quickly a galaxy, nebula, or other region of space converts gas into new stars. It is typically expressed in solar masses per year — the total mass of stars born each year in a given volume. This quantity is fundamental to understanding how galaxies evolve over cosmic time, from the early Universe's furious star birth to the quiet, nearly dormant state of modern elliptical galaxies.
The concept of a measurable star formation rate emerged in the mid-20th century with the work of Beatrice Tinsley and others, who connected galaxy colors and luminosities to their star formation histories. The term became standard in the 1970s–1980s as extragalactic surveys grew.
Astronomers estimate SFR by observing tracers that are directly tied to young, massive stars. These stars are short-lived and emit strongly in ultraviolet light, which ionizes surrounding hydrogen gas, producing characteristic emission lines (like H-alpha). Infrared emission from heated dust, radio synchrotron radiation from supernova remnants, and X-rays from high-mass X-ray binaries are also used. By calibrating these signals with stellar population models, researchers convert observed luminosities into a mass-per-year rate.
SFR is used to classify galaxies (starburst vs. quiescent), to test models of galaxy formation and feedback from supernovae and black holes, and to trace the cosmic star formation history from the early Universe to today.
The Milky Way's current star formation rate is about 1–2 solar masses per year, but during the peak of cosmic star formation about 10 billion years ago, the average galaxy formed stars 10–100 times faster.
| N | variable | Number of communicative civilizations — number of civilizations |
| R_* | variable | Average rate of star formation in the galaxy — star formation rate per year |
| f_p | variable | Fraction of stars with planetary systems — fraction with planets |
| n_e | variable | Average number of potentially habitable planets per star with planets — habitable planets per star |
| f_l | variable | Fraction of habitable planets where life actually develops — fraction with life |
| f_i | variable | Fraction of life-bearing planets where intelligent life evolves — fraction with intelligence |
| f_c | variable | Fraction of intelligent civilizations that develop detectable communication — fraction communicative |
| L | variable | Average lifetime of a communicative civilization — civilization lifetime year |













