The splitting of single spectral lines of an emission or absorption spectrum of a substance into three or more components when the substance is placed in a magnetic field. The effect occurs when several electron orbitals in the same shell, which normally have the same energy level, have different energies due to their different orientations in the magnetic field. A normal Zeeman effect is observed when a spectral line of an atom splits into three lines under a magnetic field. An anomalous Zeeman effect is observed if the spectral line splits into more than three lines. Astronomers can use the Zeeman effect to measure magnetic fields of stars. Compare Stark effect.