The Struve–Sahade effect (S–S effect) occurs in a double-lined

spectroscopic binary A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

star system when the strength of the spectral lines of the components varies during the orbital motion. A spectroscopic binary is called double-lined when the absorption lines of both stars can be observed with a

spectroscope An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...

. As each member of the star system approaches the observer in turn, the

absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...

s of that star are shifted toward the blue end of the optical spectrum by the

Doppler effect The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...

. Likewise, as a star moves away, its lines are shifted toward the red end of the spectrum. Each of these absorption lines has a characteristic strength that depends on the physical properties of the

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

. The Struve–Sahade effect occurs when these lines become anomalously weaker as a star's spectrum is red-shifted, and stronger when it is blue-shifted, most noticeable in the secondary component. This effect is observed in the bright naked eye binary

Spica Spica is the brightest object in the constellation of Virgo and one of the 20 brightest stars in the night sky. It has the Bayer designation α Virginis, which is Latinised to Alpha Virginis and abbreviated Alpha Vir or α Vir. Analys ...

, which consists of two class B stars, and pairs of massive O class stars such as AO Cassiopeiae and HD 93403. The Struve–Sahade effect was first reported by

Otto Struve Otto Lyudvigovich Struve (; 12 August 1897 – 6 April 1963) was a Russian-American astronomer of Baltic German origin. Otto was the descendant of famous astronomers of the Struve family; he was the son of Ludwig Struve, grandson of Otto Wilhel ...

in 1937. It became important because the effect called into question the values of parameters such as mass and luminosity ratios in massive spectroscopic binary systems. In 1950, Struve attempted to explain the effect as the result of streams of gas trailing behind the secondary star, causing the star to be obscured when the star was moving away. In 1959, Jorge Sahade produced a model where a gaseous stream extended from the primary to the secondary member of the binary, and the opacity of this stream produced the weakening of the absorption lines. The effect then became known as the Struve–Sahade effect. In 1997, Gies and colleagues provided an alternative explanation, arguing that the collision between the stellar winds from the two stars results in a bow shock that is deflected by the

Coriolis force In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...

, placing it in an obscuring position along the line of sight to the secondary star. Other hypotheses have since been created to explain this effect, but models still do not fully reproduce the observed line strengths.

References

{{DEFAULTSORT:Struve-Sahade effect Emission spectroscopy - Struve family

See also

References