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 transit between these states. 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, •
n = 4 to
n = 2 is called Balmer-beta or H-beta, •
n = 5 to
n = 2 is called Balmer-gamma or 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
nm, 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 (12.1 eV, via the
Rydberg formula) as it does to ionize the hydrogen atom (13.6 eV), ionization is far more probable than excitation to the
n = 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), 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 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. ==Filter==