Halpha

H alpha refers to the spectral emission line of neutral hydrogen at about 656.28 nanometers, arising from the transition of an electron from the third energy level (n=3) to the second (n=2) in the Balmer series. The line lies in the red portion of the visible spectrum and is one of the strongest features in many astronomical spectra.

In ionized hydrogen gas, recombination and cascades produce H-alpha emission, especially in H II regions, planetary nebulae, and active galaxies. In stars, the line may appear in absorption in cooler photospheres or in emission in extended atmospheres or circumstellar material. Because it is relatively strong and isolated, H-alpha is widely used to trace hydrogen-rich gas.

Observationally, H-alpha is imaged and analyzed with narrowband filters centered on the line, often with passbands of a fraction of an angstrom to several angstroms for various applications. This allows astronomers to map ionized gas, measure velocities via Doppler shifts, and study star formation, supernova remnants, and the structure of the interstellar medium. Spectroscopy of H-alpha also provides kinematic information and redshift measurements.

For solar astronomy, H-alpha observations reveal chromospheric features such as filaments and prominences and are used to monitor solar activity. The line forms higher in the solar atmosphere than the photosphere and responds to magnetic activity.

Equipment and technique commonly employ narrowband H-alpha filters or interferometric etalons to isolate the line. In solar work, passbands are typically around 0.5–1.0 Å; broader filters are used for other studies.

See also Balmer series, spectral line, narrowband imaging.