The HST images of M100 for the Cepheid variable hunt

Although beautiful, the color image of M100 above is not what will be used for our Cepheid hunt. That image was taken simply to test the newly-corrected optics of the Hubble Space Telescope. To hunt for Cepheids, several exposures of the same region must be taken over the course of months to look for the brightness variations characteristic of Cepheid variables.

Although the color image of M100 was centered on the galactic nucleus at least partially for artistic reasons, that is not the best alignment for Cepheid hunting. The nucleus is so crowded and bright it would be difficult to detect Cepheids in that region. Including the bright nucleus would also increase the contrast in the images, making it more difficult to do the long exposures needed to examine faint regions. It is much wiser to minimize the amount of nucleus in the images. Dr. Mould and his colleagues chose to position WFPC2 as shown below.

Orientation of WFPC2 displayed on a ground-based image of M100 taken by Dr. R. Peletier at the prime focus of the INT telescope in La Palma (adapted from Figure 1 in Ferrarese et al., 1996). The four chips are labeled PC1 (the planetary camera), WF2, WF3 and WF4 (the three wide-field cameras). Note that the HST observers chose to place the high-resolution planetary camera on the faint outskirts of the galaxy, and that very little of the galactic nucleus is imaged. Click on PC1, WF2, WF3 or WF4 to view a large-format Hubble Space telescope image taken by that chip.

We'll have to zoom in to a scale where stars are easily distinguished in order to look for Cepheids. In this lab, we will only look for Cepheids in WF4.

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