Binary stars (7 marks)The following shows the spectrum of a binary stellar system, measured at fourdifferent instants (which are in order of increasing time). As a whole, the system is at afixed distance from us. It is found that the spectral lines shift back and fro. When thespectral lines of the two stars do not overlap, they are identified to belong to star Aand star B.Instant 0AB ABA BInstant 1Instant 2BA BAB AInstant 3> Wavelength (nm)400500600700(a) At which instant (0/1/2/3) is star A moving towards us?(b) What is the reason of the periodic shift of the lines?(c) Which star is more massive? Why?