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The Balmer jump (or Balmer discontinuity, Balmer edge, or Balmer break) is a spectral feature consisting of a steep downward slope within an object's SED (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.
The term Balmer break is typically used for this discontinuity in SEDs of high-redshift galaxies and quasars, the term Balmer edge similarly for accretion disks, and Balmer jump for stellar spectra.
Some spectra show an analogous Lyman jump at the Lyman limit, which is seen, for example, in the SED of some active galactic nuclei (AGNs).
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