The Struve-Sahade Effect is one seen in massive binary star systems where the spectral emission lines from the secondary star of the system appear to be enhanced when the star is approaching the observer, and diminished when the star is receding.

(Note: This is not simply a Doppler effect, although it sounds similar to a first order approximation. The emission lines are not shifted in frequency in any way; rather, their intensity waxes and wanes in ways that are not seen in any other sort of system.)

There are a couple of theories which have been put forth to explain this effect, but researchers have yet to find any compelling evidence to either support or disprove them. The first, referred to as the Stream Model, proposes that mass is flowing from the primary star toward the secondary, and that one is actually looking through the flowing ‘stream’ of mass when observing the star as it recedes. As a result, it appears to be obscured and one sees diminished spectral emissions. The second theory, known as the Wind Model, says that the motion of the stars creates a shock boundary between them which heats up the surface of the secondary star. The increased temperature makes the star appear to have stronger emissions than usual when it is approaching; one sees the star just as it normally is when it is receding, and notes apparently weaker lines.

Recent studies of the effect showed it to be less prevalent than had been supposed, introducing even more questions about its real nature, and whether or not it might have some dependence on phase and epoch.

(gleaned from a PhD dissertation by
R. L. Riddle of Iowa State University,
as presented at the January 2001 meeting
of the American Astronomical Society)

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