Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating the rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars is approximately 3 M ☉ per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 M ☉ of gas being assembled in molecular clouds in the Milky Way per year.