Stellar    Spectra



Composition
How can we tell the composition of our Sun or of another star out side of our solar system?  Astronomers apply the basic concepts about spectroscopy in more sophisticated ways.  Measuring the spectrum of a star and examining it are the first two steps in unraveling clues as to its physical and chemical make-up.  Astronomers study the  Balmer series of a star to determine its temperature.  The three series depend on how readily an atom is ionized; that is, how easily it is willing to give up or take an electron.  Since hot gases tend to produce more collisions, it is likely that their atoms are often ionizing.1  This will produce heavy lines in the Balmer series.1  When a stars spectrum reveals weak Balmer lines, it is a clue that the gas is not ionizing.  This clue will allow an astronomer to identify the type of gas absorbing at those wavelengths. 


Temperature
Astronomers can detect whether or not a star is relatively  "hot" or "cool" by the wavelength of maximum intensity.  For instance, shorter wavelengths exist in the bluer regions which require a lot of emitted energy.  This leads to the conclusion that the star should be relatively hot because a great deal of energy is radiating from it.  Similarly, red stars should be cooler because the energy they are emtting is weaker, found in the longer wavelength region.  In order to find a more precise measurement of how hot a star is, astronomers use the Balmer series to detect ionization.  If a scientist knows the amount of energy required to ionize a particular atom of calcium, s/he can estimate how hot the surface of the star must be.  If a Balmer series shows weak lines for calcium, it is a clue that the star is not at a high enough temperature to ionize this material.1 

In the spectrum of a star, one might find helium, hydrogen, calcium, and titanium oxide.  Viewing the Balmer series will offer two important pieces of evidence for astronomers.  Astronomers can hint at the types of chemicals that make up the star as well as predict a more precise temperature.



Classification
Since temperature plays a large role indetermining the chemical composition of a star, astronomers classify these solar bodies based on their temperature.  There exist seven major classes ranging in temperature from around 3,000 K to 40,000 K.1  All stars in a classification will have roughly the same composition and will yeild similar Balmer lines, both strong and weak depending on their chemical make-up.  Figure 1 illustrates the attributes of the seven spectral classes of stars.

Class Approximate Temperature (K)
 Hydrogen Balmer Lines
 Other Balmer Lines
 Example
O
 40,000
 weak
 ionized helium
 Meissa
B
 20,000
 medium
 helium
 Achernar
A
 10,000
 strong
 weak ionized calcium
 Sirius
F
 7,500
 medium
 weak ionized calcium
 Canopus
G
 5,500
 weak
 medium ionized calcium
 Sun
K
 4,500
 very weak
strong ionized calcium
 Arcturus
M
 3,000
 very weak
 strong TiO
 Betelgeuse
Figure 1.  Modified spectral class diagram from Seeds text1




Dissection of a Star
Our sun is made up of mainly hydrogen and helium.  Its spectra can be seen in the ultraviolet, x-ray, and visible light regions.  It has strong but few Balmer lines produced from calcium and weaker lines from hydrogen.   Its moderate temperature of approximately 5,500 K places it slightly on the cool side.  A more detailed view of the Sun's solar spectra can be found here.  Frauenhofer's original dark lines can be seen as well as photographs of the Sun at x-ray and ultraviolet wavelengths.
Will Calcium or Helium produce stronger Balmer lines?


Will a star having a temperature of approximately 9,500 K have relatively weak or strong hydrogen balmer lines?





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References:

1.  Seeds, M.A. Foundations of Astronomy; Thomson Brooks/Cole: Canberra, 2007; pp 148-150.