The strength of gravity, when measured against the strength of electromagnetism, seems fine-tuned for life (Rees 2000: ch. 3; Uzan 2011: sect. 4; Lewis & Barnes 2016: ch. 4). If gravity had been absent or substantially weaker, galaxies, stars and planets would not have formed in the first place. Had it been only slightly weaker (and/or electromagnetism slightly stronger), main sequence stars such as the sun would have been significantly colder and would not explode in supernovae, which are the main source of many heavier elements (Carr & Rees 1979). If, in contrast, gravity had been slightly stronger, stars would have formed from smaller amounts of material, which would have meant that, inasmuch as still stable, they would have been much smaller and more short-lived (Adams 2008; Barnes 2012: sect. 4.7.1).