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| [[Image:Bohr-atom-PAR.svg|thumb|right|210px|'''H-alpha Emission''': In the simplified '''Rutherford Bohr model''' of the [[hydrogen atom]], the Balmer lines result from an electron jump between the second energy level closest to the nucleus, and those levels more distant. The <math>\scriptstyle 3 \rightarrow 2</math> transition depicted here produces an H-alpha photon, and the first line of the [[Balmer series]]. For hydrogen (<math>Z = 1</math>) this transition results in a photon of [[wavelength]] 656 [[nanometre|nm]] (red).]]
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| '''H-alpha''' ('''Hα''') is a specific red visible [[spectral line]] in the [[Balmer series]] created by [[hydrogen]] with a wavelength of 656.28 [[nanometre|nm]], which occurs when a [[hydrogen]] electron falls from its third to second lowest energy level. It is difficult for humans to see H-alpha at night, but due to the abundance of [[hydrogen]] in space, H-alpha is often the brightest wavelength of visible light in [[stellar astronomy]].
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| ==Balmer series==
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| According to the [[Bohr model]] of the [[atom]], [[electrons]] exist in [[Quantum|quantized]] energy levels surrounding the atom's [[atomic nucleus|nucleus]]. These energy levels are described by the [[principal quantum number]] ''n'' = 1, 2, 3, ... . Electrons may only exist in these states, and may only transit between these states.
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| The set of transitions from ''n'' ≥ 3 to ''n'' = 2 is called the [[Balmer series]] and its members are named sequentially by Greek letters: | |
| *''n'' = 3 to ''n'' = 2 is called Balmer-alpha or H-alpha,
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| *''n'' = 4 to ''n'' = 2 is called Balmer-beta or H-beta,
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| *''n'' = 5 to ''n'' = 2 is called Balmer-gamma or H-gamma, etc.
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| For the [[Lyman series]] the naming convention is:
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| *''n'' = 2 to ''n'' = 1 is called Lyman-alpha,
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| *''n'' = 3 to ''n'' = 1 is called Lyman-beta, etc.
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| '''H-alpha''' has a [[wavelength]] of 656.281 [[nanometre|nm]],<ref name="cox2000">{{cite book
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| | author=A. N. Cox, editor
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| | title=Allen's Astrophysical Quantities
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| | year=2000
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| | publisher=Springer-Verlag
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| | location=New York
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| | isbn=0-387-98746-0}}</ref> is visible in the red part of the [[electromagnetic spectrum]], and is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Since it takes nearly as much [[Rydberg_constant#Rydberg_constant_for_hydrogen|energy to excite the hydrogen]] atom's electron from ''n'' = 1 to ''n'' = 3 as it does to ionize the hydrogen atom, the probability of the electron being excited to ''n'' = 3 without being removed from the atom is very small. Instead, after being ionized, 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), emitting [[photons]] with each transition. Approximately half the time, this cascade will include the ''n'' = 3 to ''n'' = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line occurs where hydrogen is being ionized.
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| The H-alpha line saturates (self-absorbs) relatively easily because hydrogen is the primary component of [[nebulae]], 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 as [[carbon dioxide]], [[carbon monoxide]], [[formaldehyde]], [[ammonia]], or [[acetonitrile]] are typically used to determine the mass of a cloud.
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| [[File:Emission spectrum-H.svg|800px|thumb|center|The four visible hydrogen emission spectrum lines in the Balmer series. The red line at far-right is H-alpha]]
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| {{clr}}
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| ==Filter==
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| [[Image:HI6563 fulldisk.jpg|thumb|right|The Sun observed through a telescope with an H-alpha filter]]
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| [[File:WHAM survey.png|thumb|right|Milky Way view by Wisconsin H-Alpha Mapper survey]]
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| [[File:NGC6888 Ha JeffJohnson.jpg|thumb|right|Amateur image of NGC 6888 using H-alpha (3nm) filter.]]
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| A hydrogen-alpha filter is an [[optical filter]] designed to transmit a narrow [[Bandwidth (signal processing)|bandwidth]] of light generally centered on the H-alpha wavelength. They are characterized by a bandpass width that measures the width of the wavelength band that is transmitted.<ref>{{cite web
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| | url = http://www.astro-tom.com/technical_data/filters.htm | title = Filters | publisher = Astro-Tom.com | accessdate = 2006-12-09 }}</ref> These filters are manufactured by multiple (~50) [[Vacuum deposition|vacuum-deposited]] layers. These layers are selected to produce [[interference (wave propagation)|interference]] effects that filter out any wavelengths except at the requisite band.<ref>{{cite web | author = D. B. Murphy, K. R. Spring, M. J. Parry-Hill, I. D. Johnson, M. W. Davidson | url = http://www.olympusmicro.com/primer/java/filters/interference/index.html | title = Interference Filters | publisher = Olympus | accessdate = 2006-12-09 }}</ref>
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| Alternatively, an [[etalon]] may be used as the narrow band filter (in conjunction with a "blocking filter" or energy rejection filter) to pass only a narrow (<0.1 [[nanometer|nm]]) range of wavelengths of light centered around 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 (an interference filter relies on the interference of internal reflections). 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) to cope with the associated [[Doppler effect]].
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| ==See also==
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| * [[Hydrogen spectral series]]
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| * [[Rydberg formula]]
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| ==References==
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| <references />
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| [[Category:Atomic physics]]
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| [[Category:Astronomical spectroscopy]]
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| [[Category:Hydrogen physics]]
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