overlapping stars, stellar training sets

On the phone with Schölkopf, Wang, Foreman-Mackey, and I tried to understand how it is that we can fit some insanely variable stars in the Kepler data using other stars, when the variability seems so specific to each star. In one case we investigated, it turned out that the crazy variability of one star (below) was perfectly matched by the variability of another, brighter star. What gives? It turns out that the two stars overlap on the detector, so their footprints actually share pixels! The shared variability is caused by the situation that they are being photometered through overlapping apertures. We also learned that some stars in Kepler have been assigned non-contiguous apertures.

Late in the day, Gail Zasowski (JHU) showed up. I explained in detail The Cannon—Ness, Rix, and my label-transfer code for stellar parameter estimation. She had many questions about our training set, both because it is too large (it contains some obviously wrong entries) and too small (it doesn't nearly cover all kinds of stars at all metallicities).