For stellar spectra, the absorption lines represent how much light is removed at a particular wavelength, allowing astronomers to interpret the composition and temperature of a star. Energy transitions within a single ion determine the particular wavelengths at which that atom can absorb energy. For a given atomic element, a large number of atoms which are experiencing conditions that allow a certain energy transition to occur will combine to form a strong (thick and dark) absorption line. An absorption line which is strongest at one temperature will always be weaker at lower and higher temperatures, with some ion's lines getting stronger at a certain temperature and other ion's lines getting weaker. Thus, one star's spectrum can be compared to another's based on the position and strength of the absorption lines.
A handful of spectral lines determine the letter classification for a given star (see table 1). The hotter the star is, the more highly ionized the gas in its outer layers The degree of ionization governs the pattern of spectral lines that will form. For the O stars, ionized Helium, which requires a very high temperature for its formation, is the principal species that reveals the spectral type. Class A stars are noted for their distinct Balmer lines of Hydrogen, while G stars begin exhibiting strong lines due to ionized metals, which take less energy to ionized. The spectrum of the low surface temperature M stars are dominated by molecular features. Molecules absorb energy very effectively, but they are easily broken apart by high temperature. Thus molecular absorption lines are only present in the coolest stars.
In this lab, you should look for a specific spectral features that characterize each spectral type. Are lines of neutral or ionized helium present when the hydrogen Balmer lines are weak or absent? Then the spectrum is an O or B type, if you identified the helium correctly. Are the hydrogen lines the dominant feature? Then the spectrum is probably of a type A. How about the metals? Molecular bands? When the star is cooler, probably of a type F, G, K, or M, depending on what is present and how strong it is. In this way you will make a rough classification.
| Spectral Class |
Surface Temp |
Prominent Absorption Lines |
| O | 30,000 K | faint Hydrogen; b ionized Helium; multiply ionized heavy elements |
| B | 20,000 K | moderate strength Hydrogen and neutral Helium; singly ionized heavy elements |
| A | 10,000 K | strong Hydrogen; very faint ionized Helium; singly ionized heavy elements |
| F | 7,000 K | moderate strength Hydrogen; neutral and singly ionized heavy elements |
| G | 6,000 K | faint Hydrogen; neutral and singly ionized heavy metals at moderate strength |
| K | 4,000 K | faint Hydrogen; strong lines from neutral metals; moderate strength lines from singly ionized heavy metals. |
| M | 3,000 K | very faint Hydrogen; strong neutral heavy elements; moderate strength molecules |
To match the spectra more exactly, you will need to look at more subtle characteristics. For example, in stars of types A to G, you can compare the strengths of the hydrogen Balmer lines at 434 nm and the ionized calcium line at 393 nm. The hotter the star, the weaker the calcium line will be in comparison to the hydrogen line. At spectral type A7, they are about equal in strength, whereas in F and G stars, the line of neutral calcium is stronger. In a similar way, you can compare the strength of the neutral calcium line at 423 nm, and of the group of iron lines at 427 nm, with the hydrogen line at 434 nm. The iron and calcium lines will become more and more prominent as the star gets cooler.
Start off by accepting the invitation to practice.
You should see spectra of stars with well determined spectral classification (O4, B0, B2, B5, A0, A5, F0, F5, G0, G5, K0, K5, M0, M2, and M4) above and below a spectrum of unknown classification. The goal of this exercise is to determine the best fit for the spectral classification of each unknown spectra. Click on the buttons above the spectrum of the unknown star to display a known spectrum with a similar spectral features. Then click on the buttons below the spectrum of the unknown star to display the known spectrum with the next higher or lower classification.
Be careful to look at the trends for the strength of every absorption feature. Repeat this process until you have selected one comparison spectra above which is very similar to the unknown and selected below another comparison spectrum which is one step different below the unknown. The spectrum of the unknown star could, but in most cases will not, be identical to either comparison. If the pattern of the unknown spectrum clearly lies about halfway between two known spectra, the spectral type that is about halfway between the two known spectra. For instance, if your unknown spectrum lies halfway between A0and A7, then you could write down either A3 or A4). You are expected to be within 2 or 3 spectral sub-classes of the accepted type.
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