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)