The core of a star emits a continuous spectrum of light. As this light passes through the cooler outer atmosphere of the star, the atoms there absorb some of it. This light is absorbed by electrons which are excited to higher energy levels in the atoms. Since the energy levels are quantized, only photons with the corresponding amount of energy will be absorbed. Thus, the light is absorbed only at certain wavelengths. These appear as dark lines on top of a continuous spectrum of light emitted by the star and is called the absorption spectrum.
The image in the file, REDSHIFT.JPG shows a simulation of the absorption spectrum of the sun and of BAS 11. (Actual data is not taken with color photography.) The dark lines in the red and in the blue is from absorption by hydrogen atoms. The dark line in the yellow is from sodium atoms. The dark lines in the green are from magnesium and iron atoms. The dark lines in the violet are from hydrogen, iron, calcium, and potassium atoms.
The shift in the spectrum of BAS 11 from the spectrum of the sun is clearly seen. The lines from BAS 11 are all shifted towards the red end of the spectrum. This shift (called the "red shift") is toward longer wavelengths (smaller frequencies) and is caused by the Doppler effect. The shift in frequency is about 7%. This means that BAS 11 is traveling away from us at about 7% of the speed of light.
