As we discussed in the last post, the Drake Equation gives an estimate of the number of civilizations in our galaxy with which communication might be possible, N, as:

N = R * f_p * n_e * f_l * f_i * f_c * L

Today, we’ll talk about the first term, R = the average rate of star formation per year in our galaxy.

Caveat: I’m an amateur (hopefully in the best sense of the word), with a day job, a family, and a writing career. These estimates are chock-full of back-of-the-envelope-calculations (BOTEC) and rules of thumb. I’ll show my work, and professional astronomers are welcome to comment.

*Average rate of star formation*

More accurately, we’re interested in the rate of star formation a few billion years ago, on the assumption other intelligent life would only evolve billions of years after its home stellar system formed, just like us. I’ll assume the rate of star formation has been constant over that time frame. I’ll also assume the number of stars in our galaxy is in equilibrium–the number that form each year is equal to the number that die each year. (“leave the main sequence,” to be more technical).

One more assumption: I will ignore class L, T, and Y red and brown dwarf stars, on the assumption they have such low luminosity, any planets they might have would receive insufficient sunlight for life to arise.

From our equilibrium assumption, if we estimate the number of stars dying each year, we have an estimate for how many formed per year in the time frame of interest. Next question: approximately how many stars die each year?

Answer: approximately the number of stars of each spectral classification in the galaxy divided by the main sequence lifespan for that spectral type. (Number of stars from here * 100 billion stars in our galaxy, main sequence lifespan for a typical star of that classification estimated from here):

spectral type | Number in galaxy | max lifespan (yrs) | number at max lifespan |

O | 30000 | 5000000 | 0.006 |

B | 130000000 | 50000000 | 2.6 |

A | 600000000 | 1000000000 | 0.6 |

F | 3000000000 | 2000000000 | 1.5 |

G | 7600000000 | 10000000000 | 0.76 |

K | 12100000000 | 30000000000 | 0.4033333333 |

M | 76450000000 | 200000000000 | 0.3822 |

Summing up and rounding a bit, we get R = 6.25.

One down, six to go. Till next time.

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