Atomic energy levels within atoms give rise to different spectra and each element has a unique atomic structure. Thus the spectral lines of hydrogen will occur at different wavelengths than the spectral lines of Carbon. As a result, each element in the periodic table of elements has a unique set of spectral features. To familiarize yourselves with this concept, refer to this interactive simulation:

Open up the Periodic Table of elements Applet


Let's examine the spectrum of Hydrogen (H). You will see that there are features in that spectrum (these are called spectral lines - they refer to specific atomic transitions that we have already discussed). Click on H in the periodic table to bring up the spectrum. If you click the left mouse button you can measure the wavelengths of these features. Go ahead and do so, you should find the following wavelengths (measured in units of angstroms; 1 angstrom = 10^-8 cm:

6563 angstroms

4861 angstroms

4340 angstroms

4101 angstroms

Now click on the element C (Carbon). You will notice more spectral lines than in the case of Hydrogen and although some of the wavelengths are close to the 4 wavelengths listed above, there is not an exact match. Therefore, each element has a unique set of wavelengths associated with their spectral lines. By precisely measuring the wavelengths of lines in stellar atmospheres, astronomers (and now you) can identify the chemical composition of the star.

Note that the spectra of the atomic elements displayed are those that are obtained under laboratory conditions on the Earth. While the wavelengths of the features are the same in stars as on the Earth for the same element, the relative strength of the features may be quite different in stars than in the laboratory on earth because the environments are much different. Therefore, it is not useful or practical or even relevant to expect that if a specific line is strong for an element under Earth lab conditions, that it would be similarly strong in a stellar atmosphere.

Now back in the latter 19th century, when the field of spectroscopy was first established, there were some spectral features in the yellow part of the spectrum (observed during an 1868 eclipse by the French astronomer Pierre Janssen) of the Sun that could not be readily associated with any known element on the Earth at that time. In 1870, this element was named as Helium (for Helios, the sun god) by Sir Joseph Lockyer. It wasn't until 1895 that Helium was actually discovered to also exist on the Earth.