Composition of Stars
The following are spectra of five stars from the spectral atlas of Jacoby, Hunter and Christian, presented in visually intuitive way. This is what you would see if you projected the light of a star passed through a prism, or diffracted by a grating, and projected on a screen. (In fact, the data were originally taken with digital detectors and processed digitally to produce this form. In this process, we have adjusted the contrast to make the relevant features easier to spot, for example.)
Dark bands should be visible in these images.
The following are emission line spectra of five elements (hydrogen, helium, oxygen, calcium, and sodium from top to bottom). These are not actulal laboratory data, but constructed from atomic data and software at NIST.
All the spectral images cover the same wavelength range and have the same physical size on computer screen or when printed. This allows direct comparison of features among stars, among elements, and between a star and an element.
Some dark bands are common among several stars. Many of these dark bands appear at the same place as the bright lines in the spectra of the elements. These lines are characteristics of elements, which can appear in emission (bright lines) or in absorption (dark lines) depending on the situaiton. For the purpose of this exercise, you don't have to match all the lines between the star and the element. Just try to see if at least one line is common.
- Exercise for the readers: which elements can you see in which star?
The correspondence between emission lines of elements and absorption lines in stars allow chemical elements to be identified in stars, from may light years away.
Calcium at Different Temperatures
The following thre images are emission line spectra of calicium (calculated using NIST database/software), at three different temperatures (the middle one is the same as the one used above).
Although you can't tell from just three snapshots, individual lines do not move. If we make a movie, changing the temperature continuously, you'll see that the lines just brighten and fade.
Why have I included this comparison? Because, contrary to what students might think, all five stars used above have roughly the same chemical composition. The difference in spectral features are mostly the results of different temperatures.