Astrophysics (Index)About

Balmer jump

(BJ, Balmer discontinuity, Balmer edge)
(abrupt change in stellar spectrum at the Balmer series wavelength limit)

The Balmer jump (or Balmer discontinuity or Balmer edge) is a spectral feature consisting of a steep downward slope in the intensity-per-wavelength (sloping downward as you look at shorter and shorter wavelengths) at the Balmer limit, the "infinity end" of the Balmer series spectral lines, which is 364.6 nm. The jump occurs in a star sufficiently hot that hydrogen atoms in or near its photosphere with electrons at electron shell 2 are generally being ionized rather than moving to a higher level of atomic excitation. Photons of below the above-mentioned wavelength can be absorbed, resulting in lowering of the continuum below the above-mentioned wavelength, and the sufficiently-hot star has enough such excited hydrogen atoms and such high-energy photons to show a significant such "jump". The shape of the jump is dependent on the absorbing material's density and temperature and its analysis can help determine those. The jump is strongest in early stars, e.g., A-type stars.

A spectrum can also show an analogous Lyman jump (aka Lyman edge) at the Lyman limit, which is seen, for example, in some active galactic nuclei, and used for analysis regarding their accretion disks. I believe the ISM hides its rest wavelength, making it more interesting for redshifted extra-galactic objects.

(stars,spectrum,Balmer series,absorption)
Further reading:
364.6nm822THz3.5eVBalmer jump

Referenced by pages:
Balmer-break galaxy (BBG)
blue horizontal branch (BHB)
continuous spectrum
flux-weighted gravity-luminosity relationship (FGLR)
galaxy age determination
spectral feature