Imagine that you know the mass of a nearby star and you know that there is a planet orbiting around the star with a mass much smaller than the mass of the star. Explain with a sentence or two how, using the Doppler effect technique, you can measure the semi-major axes of a planet orbiting the star.]

By measuring the time taken for the stars line of sight velocity to cycle from peak to Peak, and by calculation using newtons version of Kepler's third law

Explanation:

The motion of orbiting planets using planet-hunting techniques can rely on doppler effect. The light from the stars they orbit, as seen from Earth. As the star moves back and forth, the Doppler shift causes a slight change in its apparent colour which can be detected using spectroscopy. The blue shift and the red shift.

The Doppler technique of blue shift and the red shift can be used to estimate the semi major axis of the planets orbit by

Measuring the time it takes for the stars line of sight velocity to cycle from peak to Peak, and using newtons version of Kepler's third law