Following Hartmann's identification of interstellar calcium absorption, interstellar sodium was detected by Heger (1919) through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and Beta Scorpii.
Despois da identificación de Hartmann da absorción do calcio interestelar, Heger (1919) detectou o sodio interestelar coa observactón da absorción estacionaria das liñas "D" de átomos a 589,0 e 589,6 nanómetros en dirección a Delta Orionis e Beta Scorpii.