Which of the following is NOT an indication of a binary star system?
a-orbital motion
b- variation of luminosity
c-characteristic of spectral lines
The question probably belongs in astronomy, but the answer is c. Orbital motion is an indicator of a binary star system because the stars pulling on each other will make their orbits appear to wobble. Variation of luminosity will occur because at times one star will be behind the other, blocking its light. Spectral lines won’t matter, however, because they are due to the composition of the stars, which does not change.
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Hmm, checking the Wikipedia article on binary stars, Ken (next poster… uh, make that the former next poster – why’d he delete an informative message?) is right about how the spectral lines will produce a detectable Doppler shift. However, the same article has this to say about luminosity:
"Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse."
Seems like all three are possible indicators of a binary system — you sure there wasn’t a fourth choice? 
January 14th, 2010 at 10:01 am
The question probably belongs in astronomy, but the answer is c. Orbital motion is an indicator of a binary star system because the stars pulling on each other will make their orbits appear to wobble. Variation of luminosity will occur because at times one star will be behind the other, blocking its light. Spectral lines won’t matter, however, because they are due to the composition of the stars, which does not change.
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Hmm, checking the Wikipedia article on binary stars, Ken (next poster… uh, make that the former next poster – why’d he delete an informative message?) is right about how the spectral lines will produce a detectable Doppler shift. However, the same article has this to say about luminosity:
"Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse."
Seems like all three are possible indicators of a binary system — you sure there wasn’t a fourth choice?
References :
http://en.wikipedia.org/wiki/Binary_star