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S Eclipsing binary. (a) The binary system is shown above; the light curve below.The fainter secondary passes alternately in front of and behind the primary. Most of the time, as at position C,we see light from both stars.When the secondary eclipses the primary, part of the primary light is blocked and there is a dip in the intensity, as at point A. When the secondary passes behind the primary, as at B, its light is lost. Since the secondary is not as bright as the primary, the loss of brightness is not as great as at A. (b) Part of the light curve for the eclipsing binary, NN Ser, around the eclipse part of the orbit. [(b) ESO]

A Doppler shift is a change in the wavelength (and frequency) of a wave, resulting from the motion of the source and/or the observer. It is most easily visualized for a sound wave or a water wave,

[jQglQBQI Astrometric binary.Two stars orbit about a common center of mass, which in turn moves across the sky.The fainter star is too faint to see, so we only see the brighter star, moving back and forth across the path of the center of mass.

where the waves are moving through a particular elastic medium (Fig 5.3).

5.2.1 Moving sources and observers

We first look at the case of the moving observer. If the observer is moving toward the source, then the waves will be encountered more frequently than if the observer were standing still. This means that the observed frequency of the wave increases. If the frequency increases, then the wavelength decreases. If the observer is moving away from the source then the situation is reversed. Waves will be encountered less frequently; the frequency decreases; the wavelength therefore increases. It should be noted that if the observer moves perpendicular to the line joining the source and observer, no shift will be observed.

We now look at the case of the moving source. Each wavefront is now emitted in a different place. If the source is moving toward the observer, the waves will be emitted closer together than if the source were standing still. This means the wavelength decreases. The decreased wavelength results in an increased frequency. If the source is moving away from the observer, the waves will be emitted farther apart than if the source were standing still. The wavelength increases and the frequency decreases. Again, if the source is moving perpendicular to the line joining the source and observer, no shift results.

S Eclipsing binary. (a) The binary system is shown above; the light curve below.The fainter secondary passes alternately in front of and behind the primary. Most of the time, as at position C,we see light from both stars.When the secondary eclipses the primary, part of the primary light is blocked and there is a dip in the intensity, as at point A. When the secondary passes behind the primary, as at B, its light is lost. Since the secondary is not as bright as the primary, the loss of brightness is not as great as at A. (b) Part of the light curve for the eclipsing binary, NN Ser, around the eclipse part of the orbit. [(b) ESO]

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