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Fine-Tuning the Ratio of Small to Large Stars

For decades, astronomers have assumed that the ratio of small-mass stars to high-mass stars observed in our Milky Way Galaxy (MWG) is a universal constant for all galaxies. A growing body of data, however, now challenges that assumption.

Astronomers refer to this ratio as the stellar initial mass function (IMF). The IMF is important for establishing whether a galaxy could possibly host life since it determines the galaxy’s chemical enrichment history. High-mass stars produce more elements that are heavier than helium and only high-mass stars produce elements heavier than iron. Life, especially advanced life, requires that the abundance of elements in the periodic table be available at specified levels.

Furthermore, for planetary systems, the abundance of elements heavier than helium is strongly correlated with the size and number of asteroid and comet belts. As I explained in a previous Today’s New Reason to Believe article, for advanced life to be possible on a planet, the number, sizes, and locations of asteroid and comet belts in its planetary system must be fine-tuned. Too many high-mass stars would mean that the planet on which advanced life could conceivably exist would be buffeted by destructive gravitational perturbations exerted by the high-mass stars.

The latest and most potent challenge to the IMF being a universal constant for all galaxies comes from an analysis by the Calar Alto Legacy Integral Field Array (CALIFA) survey team.1 The CALIFA survey is an integral field spectral survey of 600 nearby galaxies. The study is designed to investigate how galaxies form and evolve over the history of the universe.

The CALIFA team performed detailed observations of 24 early-type galaxies characterized by old stellar populations where only stars less massive than the Sun are still burning. Analysis of these observations showed that the IMF not only varies from galaxy to galaxy but also varies within each galaxy. The IMF depends strongly on distance from the center of each galaxy and on the age of the galaxy. These characteristics led the CALIFA team to conclude that the local abundance of elements heavier than helium is a major factor in determining the IMF.

The variations in the IMF noted by the CALIFA team yield more evidence for fine-tuning design in the position of a planet capable of sustaining advanced life. Such a planet must reside in an environment with the just-right IMF, which implies that it must reside in the just-right position within a galaxy that possesses the just-right mean IMF.

  1. Ignacio Martín-Navarro et al., “IMF–Metallicity: A Tight Local Relation Revealed by the CALIFA Survey,” Astrophysical Journal Letters 806 (June 2015): id. L31, doi:10.1088/2041-8205/806/2/L31.