Astronomy
Stellar Metallicity from Fluctuations in Hydrogen Recombination Lines
Quick fact
In stars of similar temperature, a stronger hydrogen Balmer line (higher equivalent width) actually indicates LOWER metallicity, because metals cool the outer atmosphere and suppress the hydrogen lines.
Why this is interesting
You might think that the brightness of hydrogen's glow tells you how much hydrogen is present—but sometimes, the opposite is true. How can a star with strong hydrogen lines actually be metal-poor, while one with faint lines is metal-rich?