hydrogen Balmer series
/ BAL-mer /
Hydrogen, the simplest and most abundant atom in the universe, has one tidy family of spectral lines that fall in visible light, where the human eye can see them. This family is the Balmer series, named for the Swiss schoolteacher Johann Balmer, who in 1885 found a simple formula for their wavelengths years before anyone understood why it worked.
Every Balmer line is made when an electron in hydrogen jumps between the second energy level and a higher one. Landing on (or leaving from) the second level releases (or requires) a photon of visible light. The first and strongest is H-alpha, a deep red line at 656 nm; then H-beta at 486 nm in the blue-green; H-gamma at 434 nm violet; and so on, the lines crowding closer together as they climb toward the blue limit at 365 nm.
Because hydrogen is everywhere, the Balmer lines are among the most useful in astronomy. H-alpha glows brilliant red in nebulae and reveals where gas is being lit up by young stars. In a star's spectrum the strength of the Balmer absorption lines is a sensitive thermometer — they peak in stars around 10,000 degrees, where conditions are just right to populate that second energy level, and fade in stars much hotter or much cooler.
H-alpha at 656 nm is the red glow that floods photographs of star-forming nebulae; in cooler stars the same line appears as a strong dark absorption notch instead.
Hydrogen's visible lines — born from jumps to the second level — and a built-in stellar thermometer.
Strong Balmer lines do not mean a star is mostly hydrogen versus another being mostly something else — almost every star is mostly hydrogen. The lines' strength tracks temperature, because temperature decides how many hydrogen atoms sit on the second level ready to absorb.