Text: In physics and astronomy, H-alpha, also written H?, is a particular 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 656.3 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 ionize the hydrogen atom as it does to directly excite the electron from n = 1 to n = 3, 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.

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