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In physics and astronomy, H-alpha, also written Hα, is a specific emission line created by hydrogen.

According to the Bohr model of the atom, electrons exist in quantized energy levels surrounding the atom's 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 transition between these states.

The set of transitions from n ≥ 3 to n = 2 are called the Balmer series and are named sequentially by Greek letters:

• n = 3 to n = 2 is called Balmer-alpha or H-alpha,
• n = 4 to n = 2 is called H-beta,
• n = 5 to n = 2 is called H-gamma, etc.

For the Lyman series the naming convention is:

• n = 2 to n = 1 is called Lyman-alpha,
• n = 3 to n = 1 is called Lyman-beta, etc.

H-alpha has a wavelength of 656.281 nanometers, 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 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.

The H-alpha line saturates (self-absorbs) relatively easily due to the fact that 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 methyl cyanide are typically used to determine the mass of a cloud.

The Sun observed through a telescope with an H-alpha filter

A hydrogen-alpha filter is an optical filter designed to transmit a narrow 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.^[1] These filters are manufactured by multiple (~50) layers of vacuum-deposited layers. These layers are selected to produce interference effects that filter out any wavelengths except at the requiste band.^[2]

• Bohr model
• Hydrogen spectral series
• Rydberg formula
• Balmer series
• Lyman series
• Paschen series
• Brackett series
• Pfund series

1. ^ Filters (English). Astro-Tom.com. Retrieved on 2006-12-09.
2. ^ D. B. Murphy, K. R. Spring, M. J. Parry-Hill, I. D. Johnson, M. W. Davidson. Interference Filters. Olympus. Retrieved on 2006-12-09.