IC 1396 H-Alpha Close-Up. Image Credit: Nick Wright (University College London), IPHAS Collaboration
H alpha, also written as H-alpha and abbreviated as H?, is a red visible spectral line that is created by hydrogen with a wavelength of 6562.81 angstrom.
According to the Bohr atomic model, electrons exist in quantized energy levels surrounding the nucleus. These energy levels are described by the principle quantum number n=1,2, or 3. Electrons may only exist in these states, and may only transfer between these states. The transitions from n?3 to n=2 is called the Balmer series. In that series n=3 to n=2 is the H alpha transition.
H-alpha 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) and emits 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, so the H-alpha occurs where hydrogen is being ionized.
A hydrogen-alpha filter is designed to transmit a narrow bandwidth of light centered on the H alpha wavelength. They have a bandpass width that measures the width of the wavelength band that is transmitted. These filters are manufactured to produce interference effects that filter out any wavelengths except at the requisite band. Also, an etalon may be used in conjunction with a energy rejection filter to pass only a narrow 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, but the implementation is different. Due to the high velocities sometimes associated with features visible in h-alpha light, solar h-alpha etalons are often able to be tuned (by tilting or changing the temperature) to cope with the associated Doppler effect. How it is observed is not nearly as important as all of the information that has been gained from understanding the H alpha spectral line.
We have written many articles about H Alpha for Universe Today. Here’s an article about astronomical telescopes, and here’s an article about the H Alpha Solar Telescope.
If you’d like more info on H Alpha, check out an article about H Alpha by Wikipedia, and here’s a link to an article about Hydrogen Imagery of the Sun.
We’ve also recorded an entire episode of Astronomy Cast all about Optical Astronomy. Listen here, Episode 133: Optical Astronomy.
Source: Wikipedia
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