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Hydrogen-alpha, typically shortened toH-alpha orHα, is a deep-red visiblespectral line of thehydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in theBalmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level. H-alpha has applications inastronomy where its emission can be observed fromemission nebulae and from features in theSun'satmosphere, includingsolar prominences and thechromosphere.
According to theBohr model of theatom,electrons exist inquantized energy levels surrounding the atom'snucleus. These energy levels are described by theprincipal quantum numbern = 1, 2, 3, ... . Electrons may only exist in these states, and may only transit between these states.
The set of transitions fromn ≥ 3 ton = 2 is called theBalmer series and its members are named sequentially by Greek letters:
For theLyman series the naming convention is:
H-alpha has awavelength of 656.281 nm,[1] is visible in the red part of theelectromagnetic spectrum, and is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Since it takes nearly as muchenergy to excite the hydrogen atom's electron fromn = 1 ton = 3 (12.1 eV, via theRydberg formula) as it does to ionize the hydrogen atom (13.6 eV), ionization is far more probable than excitation to then = 3 level. After ionization, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emittingphotons with each transition. Approximately half the time, this cascade will include then = 3 ton = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line occurs where hydrogen is being ionized.
The H-alpha line saturates (self-absorbs) relatively easily because hydrogen is the primary component ofnebulae, so while it can indicate the shape and extent of the cloud, it cannot be used to accurately determine the cloud's mass. Instead, molecules such ascarbon dioxide,carbon monoxide,formaldehyde,ammonia, oracetonitrile are typically used to determine the mass of a cloud.
AnH-alpha filter is anoptical filter designed to transmit a narrowbandwidth of light generally centred on the H-alpha wavelength.[2] These filters can bedichroic filters manufactured by multiple (~50) vacuum-deposited layers. These layers are selected to produceinterference effects that filter out any wavelengths except at the requisite band.[3]
Taken in isolation, H-alpha dichroic filters are useful inastrophotography and for reducing the effects oflight pollution. They do not have narrow enough bandwidth for observing the Sun's atmosphere.
For observing the Sun, a much narrower band filter can be made from three parts: an "energy rejection filter" which is usually a piece of red glass that absorbs most of the unwanted wavelengths, aFabry–Pérot etalon which transmits several wavelengths including one centred on the H-alpha emission line, and a "blocking filter" -a dichroic filter which transmits the H-alpha line while stopping those other wavelengths that passed through the etalon. This combination will pass only a narrow (<0.1 nm) range of wavelengths of light centred on the H-alpha emission line.
The physics of the etalon and the dichroic interference filters are essentially the same (relying on constructive/destructive interference of light reflecting between surfaces), but the implementation is different (a dichroic interference filter relies on the interference of internal reflections while the etalon has a relatively large air gap). Due to the high velocities sometimes associated with features visible in H-alpha light (such as fast moving prominences and ejections), solar H-alpha etalons can often be tuned (by tilting or changing the temperature or air density) to cope with the associatedDoppler effect.
Commercially available H-alpha filters for amateur solar observing usually state bandwidths in Angstrom units and are typically 0.7Å (0.07 nm). By using a second etalon, this can be reduced to 0.5Å leading to improved contrast in details observed on the Sun's disc.
An even more narrow band filter can be made using aLyot filter.