COLOR  INDICES
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Image Captured by Blue Sensitive Detector
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Magnitudes:  bc>bb>bd>ba

 

 Image Seen
by the Eye
(same sky area)
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Magnitudes:
vb>va=vd>vc

 
        
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(bc - vc) > (bb - vb) > (bd - vd) > (ba - va)
So Star "a" is hottest, then Star "d," then Star "b."  Star "c" is coolest.
(Note that all of the stars are too faint for the eye to perceive color.  The stars are sensed with retinal rods.)