From my understanding D4000 is a ratio between the 'flux densities between 4050 and 4250 Angstroms and that between 3750 and 3950 Angstrom', from Poggianti et al (1997). However I do not understand why this is a direct relationship with stellar age. Did someone just notice that D4000 has a strong correlation to stellar age, if so what is the scientific explanation of this relationship.

  • $\begingroup$ Are there strong Lithium lines in this wavelength range? $\endgroup$ – AtmosphericPrisonEscape Aug 9 '18 at 17:22
  • $\begingroup$ The Lithium abundance and age (up to very approx 600 Myr) is measured from the Li I 6708 angstrom line as it it by far the strongest. According to physics.nist.gov/PhysRefData/Handbook/Tables/lithiumtable2.htm there are basically no lines at all in the wavelength range of the question. Suspect the lines in this region are gravity-sensitive and so will give crude measure of age. Main sequence stars have log g ~4.5, decreasing as they age and evolve off the main sequence and go up the giant branch. $\endgroup$ – astrosnapper Aug 9 '18 at 19:09

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