Image:M3 colour magnitude diagram.jpg

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Colour-magnitude diagram of M3.


This is a color-magnitude diagram for the Messier 3 globular cluster (after A.R. Sandage 1953). The colored points on the chart represent the plotted positions of stars in the cluster. The horizontal scale is based on the B—V colour index of the measured stars.

From the main sequence stars at magnitude 19 and fainter, the plot forms a knee that bends toward the upper right. Based on the position of the knee, the distance modulus for this cluster is determined to be 15.7. The stars along the upper right branch have left the main sequence and entered the red giant stage.

From the red giants branch, a horizontal branch then runs to the left. Note the gap in this branch at visual magnitude 15–16 and spectral type A, where there is a population of variable stars.


The image was drawn using Paint Shop Pro, and is based after a diagram from:

  • Albrecht Unsöld, The New Cosmos, p. 264, Springer-Verlag, New York Inc., 1969.

which in turn credits:

  • W. Baade, Stars and Galaxies, p. 91, Harvard University Press, 1963.

The work by Unsöld is also the reference for the above description.




R. J. Hall


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