Harvard University

Astronomy Lab and Clay Telescope


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H-R Diagram of Pleiades Cluster

We will observe a bright and nearby open cluster, Pleiades or Seven Sisters, in the constellation Taurus. By measuring the B and V band magnitudes of stars in a few of the fields around the center of this cluster, you can measure the cluster distance, approximate age, and density of its stars (how many stars per cubic volume of space).

We will do this by constructing an H-R Diagram. You will find that many stars in the images we take will not be part of the cluster but instead are either low luminosity stars (which would those be?) in the foreground or more luminous stars (are these less numerous?) in the background. Remember that a CCD image of a region of the sky is just a 2-Dimensional projection (onto your computer screen) of what is really a 3-Dimensional space. The image alone has NO depth information: you can't tell the difference between a nearby low luminous main sequence M dwarf (the most common kind out there!) vs. a more distant and thus luminous (because it has comparable apparent magnitude) M giant! Here is where the H-R diagram comes to your rescue: order appears within an otherwise scatter diagram of brightness vs. colors: the stars at a common distance (relatively speaking) “know” to organize themselves in accordance with the principles of how stars work and are built, as nuclear burning (cores) which radiate their energy from a blackbody surface at a given luminosity predicted by their mass and a temperature predicted by their radius.



We will now determine the B and V magnitudes of many of the stars in our fields. Your TF will provide you with the B and V magnitude as a calibration star in each field. We will now do the photometry, again in MaximDL, but in a slightly different way than we did for the Betelgeuse/Rigel lab. The most important thing moving forward is to LABEL your stars when doing photometry!


The following steps and the anlaysis handout (pdf or docx) will guide you through solving for the distance to M45, the density of stars in the cluster and the approximate age of the cluster.