Hbeta

Hbeta, denoted Hβ, is a spectral line of neutral hydrogen that belongs to the Balmer series. It arises when an electron in a hydrogen atom transitions from the fourth energy level (n = 4) to the second level (n = 2). In vacuum its rest wavelength is 4861.33 angstroms (486.133 nm). Hβ can appear in both emission and absorption in astronomical spectra and is one of the principal Balmer lines observed in the optical range.

Physically, the Balmer lines result from electron transitions to n = 2, with Hβ representing the n = 4 → n = 2 transition. The line strength and profile are influenced by the temperature and density of the emitting or absorbing gas. In stellar atmospheres, Hβ is strong in A-type stars, where Balmer lines reach peak strength, and its wings are broadened by the Stark effect. In nebulae and other ionized gas regions, Hβ is often seen in emission and can be combined with other Balmer lines to diagnose physical conditions.

Astronomical usage: Hβ is a key diagnostic line in optical spectroscopy. It is used to estimate interstellar reddening via the Balmer decrement by comparing the observed Hβ to Hα (and other Balmer lines) ratios with theoretical values to infer dust extinction. After correcting for extinction, Hβ luminosity contributes to estimating star formation rates in galaxies. In emission-line diagnostics, Hβ features alongside lines such as [O III] 5007 Å in diagnostic diagrams like the BPT diagram to classify ionization sources. In stellar spectroscopy, the equivalent width and shape of Hβ help determine stellar temperature and luminosity class.

Hbeta thus serves as a versatile tracer of physical conditions and processes across stars, nebulae, and galaxies.