Image 1 Caption: An emission line spectrum showing the first 4 atomic spectral lines of the hydrogen Balmer series (AKA the Balmer lines). The Balmer lines are the atomic spectral lines of atomic hydrogen gas. The Balmer lines are part of the overall hydrogen spectral series. The 4 shown Balmer lines are the only Balmer lines in the fiducial visible band (fiducial range 0.4--0.7 μm). Image 1 looks like a simulation, NOT a real photograph.

Features:

1. Going in order of decreasing vacuum wavelength (i.e., going blueward), the shown Balmer lines are Hα (red), Hβ (aqua), Hγ (blue), and Hδ (violet). The Hα line is often noticeable in astronomical images: it gives that nebulous pink color in true-color images of some gaseous nebulae particularly H II regions.

2. Note the Balmer lines are for an atomic hydrogen gas (i.e., a gas of hydrogen atoms) and NOT molecular hydrogen gas (i.e., a gas of hydrogen molecules H_2). Molecular hydrogen is the normal state of hydrogen near the Earth's surface. However, it is easy to create atomic hydrogen gas discharge tube where heating by an electric current causes chemical dissociation of the molecules.

3. How do you get a emission line spectrum? If you have any relatively dilute hot gas, it will emit a emission line spectrum, NOT a continuous spectrum like that of white light. The lines of an emission line spectrum are nearly all you get when the light from the gas is dispersed through a diffraction grating or prism. The background will be black or nearly so as in Image 1.

4. Emission line spectra are used to identify atoms, molecules, and ions (which could be either atomic or molecular) in the technique known as spectroscopy.

   

5. Image 2 Caption: The emission line spectrum of hydrogen (H, Z=1) in the visible band (fiducial range 0.4--0.7 μm) (4 lines shown) and the nearest part of the near ultraviolet (fiducial range 0.3--0.4 nm) (2 lines shown, but they are a bit hard to see). Image 2 looks like a real photograph.

6. These spectral lines are from the aforesaid Balmer series. As aforesaid, there are only 4 Balmer lines in the fiducial visible band, but the Balmer series extends into the fiducial near ultraviolet and ideally there is an infinity of Balmer lines, but they become increasingly closely spaced as they approach the Balmer limit λ=3646 Å (AKA the Balmer ionization edge) which is the ionization wavelength of hydrogen from principal quantum number n = 2 energy level. In fact, perturbations always truncate the Balmer series as it extends toward the Balmer limit in a complicated and time-varying way.

7. Note that the human eye does actually see into the near part of the near ultraviolet and many Balmer lines in this band are detectable by the human eye of most people.

   The first 7 going blueward (which is the direction of decreasing strength of absorption lines in stars) and the Balmer limit are:

   1. Hα (6564.5377 Å, red line): This is the spectral line that gives the pink glow in many astronomical images. The glow comes mainly from H II regions which are ionized hydrogen recombining to give a recombination cascade of many emission lines of which the strongest is the Hα which gives the aforesaid pink glow.
   2. Hβ (4861.3615 Å, aqua line) .
   3. Hγ (4340.462 Å, blue line).
   4. Hδ (4101.74 Å, violet line).
   5. Hε (3970.072 Å, ultraviolet line).
   6. Hζ (3889.049 Å, ultraviolet line).
   7. Hη (3835.384 Å, ultraviolet line): NOT shown in Image 2.  

     ∞. Balmer limit (3646 Å, ultraviolet line): NOT shown in Image 2.

Images:

1. Credit/Permission: Merikanto, before or circa 2006 / Public domain.
   Image link: Wikipedia: File:Emission spectrum-H.png.
   
2. Credit/Permission: © Jan Homann, 2009 / CC BY-SA 3.0.
   Image link: Wikimedia Commons: File:Visible spectrum of hydrogen.jpg.
   

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