(Ha, Hα, H-α)
(hydrogen line for electrons settling from n = 3 to n = 2)
H-alpha (Ha, Hα, or H-α)
is the first line in the Balmer series
of hydrogen spectral lines.
It has a wavelength of 656.3 nm (in air; 656.5 nm in a vacuum)
and is red.
Being visible light, it is conveniently within
an atmospheric window.
It is bright in the sky but does not stand out visually
because it is so red as to be outside the most sensitive part
of the eye's light sense.
An H-alpha filter (or hydrogen-alpha filter)
is a narrow-bandwidth filter centered on the H-alpha wavelength.
They have many uses, but are a basic tool in the study of the Sun.
Deuterium's H-alpha wavelength is about 0.2 nm shorter.
With spectroscopy of sufficient spectral resolution,
it produces a discernible feature on the H-alpha spectral line,
enabling an estimation of the ratio between 1H and deuterium.
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Referenced by pages:
Balmer series (H)
Dickel-Wendker-Bieritz Catalog (DWB)
HII region (HII)
spectral line shape
MASH Catalog of Planetary Nebulae
Palomar Four-filter Survey (PC)
P Cygni profile
rare designator prefixes
star formation rate (SFR)
The Survey for Ionization in Neutral Gas Galaxies (SINGG)
spectral line designation
star formation (SF)
VLT Survey Telescope (VST)
wind-momentum luminosity relationship (WLR)