Fraunhofer lines

solar spectrum

Lines in the solar spectrum. The range illustrated is from 3,900 to 6,900 angstroms, i.e. from violet through to red. (One angstrom equals one hundred millionth of a centimeter.) Each line can be identified; thus the D lines in the center of the spectrum are due to sodium, the H-alpha line to hydrogen.

Fraunhofer lines are absorption lines (dark lines) in the spectrum of the Sun, or of another star, first studied and named by Joseph von Fraunhofer in 1814. Altogether, Fraunhofer found some 700 lines in the solar spectrum. The nine most prominent he labeled with capital letters A to K, starting at the red end. The A and B bands are now known to be caused by absorption in Earth's atmosphere, while the rest are due to absorption in the Sun's photosphere. C and F are now better known as H-alpha and H-beta; the D lines are of sodium, the H and K lines of calcium, and the G band by neutral iron and the CH molecule. All these features occur generally in stars of spectral types F, G, and K.


A selection of Fraunhofer lines
lines due to wavelengths (Å)
A band O2 (molecular oxygen in Earth's atmosphere) 7594–7621
B band O2 (molecular oxygen in Earth's atmosphere) 6867–6884
C (H-alpha) H (hydrogen) 6563
a band O2 (molecular oxygen in Earth's atmosphere) 6276–6287
D1 & D2 Na (sodium) 5896 & 5890
E Fe (iron) 5270
b1, b2, b3, b4 Mg (magnesium) 5184, 5173, 5169, 5167
c Fe (iron) 4958
F (H-beta) H (hydrogen) 4861
d Fe (iron) 4668
e Fe (iron) 4384
f H (hydrogen) 4340
G Fe 4308
g Ca 4227
h (H-delta) H (hydrogen) 4102
H & K Ca (calcium) 3968 & 3934