Main Sequence FittingOpen up the Cluster Fitting Explorer. Note that the main sequence data for nearbystars whose distances are known are plotted by absolute magnitude in red on the HRDiagram. In the Cluster Selection Panel,choose the Pleiades cluster. The Pleiades dataare then added in apparent magnitude in blue.Note that the two y-axes are aligned, but thetwo main sequences don’t overlap due to thedistance of the Pleiades (since it is not 10parsecs away).If you move the cursor into the HRdiagram, the cursor will change to a handle,and you can shift the apparent magnitude scaleby clicking and dragging. Grab the clusterdata and drag it until the two main sequencesare best overlapped as shown to the right.We can now relate the two y-axes.Check show horizontal bar which willautomate the process of determining the offsetbetween the m and M axes. Note that itdoesn’t matter where you compare the m andM values, at all points they will give the properdistance modulus. One set of values gives m –M = 1.6 – (-4.0) = 5.6 which corresponds to a distance of 132 pc.Question 8: Note that there are several stars that are above the main sequence in theupper left. Can you explain why these stars are not on the main sequence?Question 9: Note that there are several stars below the main sequence especially neartemperatures of about 5000K. Can you explain why these stars are not on the mainsequence?
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