H-alpha

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. It is within the visible-light atmospheric window, thus observable from the ground and is the Fraunhofer C line. 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.

H-alpha is one of the lines used for intensity mapping at various redshifts for cosmology research.