In this activity, from the Royal Observatory Greenwich, students use real data, taken from a scientific paper, to plot the rotational curve of M31 (Andromeda), our neighbouring spiral galaxy. They will look at Kepler’s third law to predict the motion of stars around the centre of M31. They will then measure the wavelengths of hydrogen emission spectra taken at a range of radii. The Doppler equation will be used to determine whether these spectra come from the approaching or receding limb of the galaxy and the velocity of rotation at that point. A velocity vs radius graph is plotted and compared with their predicted result.
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