Forum for Amateur AstroSpectroscopy
Characteristic Lines by Spectral Class

Below is a basic list containing a few elemental lines in stellar spectra. Some of the prominent, or characteristic, lines are shown that might be used to broadly identify the types. Observing all of them will depend upon equipment, the seeing conditions as well as your eye acuteness. Wavelengths given are in Angstroms.
  • Type O: Hottest blue stars (>55,000 °K to 30,000 °K). Ionized Helium (4541 and 4687) predominates. Occasionally the Hydrogen Balmer lines are seen weakly (see Balmer Series below under Type A) as well as very weak neutral Helium (3888, 4472 and 5877). Other very faint ionized Oxygen, Nitrogen, Silicon and Carbon lines may be present.
  • Type B: Hot blue stars (30,000 °K to 10,000 °K) .Neutral Helium (3888, 4121, 4472 and 5877) lines dominate and max at B2. Hydrogen Balmer lines become progressively stronger through this type. Other very faint ionized Magnesium, Silicon and Carbon lines may be present.
  • Type A: Blue stars (10,000 °K to 7,400 °K). Hydrogen Balmer lines dominate and max at A0. The H and K lines of ionized Calcium (3968, and 3934) become strong as neutral metals may appear weak. The Hydrogen Balmer lines: Ha - 6563; Hb - 4861; Hg - 4340; Hd - 4102; He - 3770.
  • Type F: White stars (7,400 °K to 6,100 °K). Hydrogen lines are weakening. H and K lines of ionized Calcium strengthen as well as many fine lines due to other metals.
  • Type G: Yellow stars (6,100 °K to 5,300 °K). Many fine lines appear due to many neutral metal elements such as Iron, Manganese and Calcium. The broad molecular CH G-band (4314) appear.
  • Type K: Orange-red stars (5,300 °K to 3,800 °K). Hydrogen lines are gone. Broad TiO bands (4661, and 4955) begin to appear. The ionized Calcium lines are strong. CH band is very strong.
  • Type M: Red stars (3,800 °K to 2,200 °K). Broad TiO bands (4661, and 4955) dominate the spectrum. Neutral metal lines throughout the spectrum.
Notes and Suggestions
  1. Everyone’s eyes respond differently to light. To "Calibrate" your eye response to the grating or prism work sheet spectrum, observe a bright A0 star, such as Sirius or Vega, which contains the very strong Balmer absorption lines of Hydrogen. Compare the positions of these Hydrogen lines with respect to where you perceive the location of the color boundary lines (i.e. between violet and blue, blue and green, etc.). Determine if they match the standard dashed color boundary lines shown on the blank work spectrum as illustrated below.


    If they do not, you may wish to make more personalized boundaries on your work sheet copies for better accuracy when recording the more complicated lines of the cooler (F, G, K and M) stars. You might also like to estimate the UV and IR cutoff sensitivity of your eye.
  2. When observing spectra for line details, just like observing visual objects such as planets, galaxies, etc., you should take your time on each subject. The middle and cooler stars usually have a few strong lines with many fine ones that may take some practice to detect visually.
  3. If you are taking photographs, the use of black and white film (ISO 400 or greater) is recommended over color. The photosensitive dye layers of the color films will yield poor line definition, and uneven response between the blue, green, and red color layers. Large gaps in the spectrum will also result due to the low stellar exposure levels, and the steep spectral responses of the dye layers.