This week we have a look at the light curve of the star 2MASS20504836+4407398 in the field of the Pelican Nebula. We show the HOYS data in V, R, and I in the light curve, as well as an image from the Digitized Sky Survey of the field around the source – the star is positioned in the white square. According to Gaia the star is about 460pc away, and hence a foreground object and not part of the star forming region, which is at a distance of about 870pc. It looks like a perfectly boring main sequence foreground star.

Looking at the light curve, the question might be: What is so interesting about the object? Other than a bit of noise, there seems to be no variation. And this is what makes the star interesting. Indeed, it is the least variable star in the entire Pelican Nebula field, for which we have a light curve. We are using these non-variable stars to calibrate our photometry. Because we know they do not vary, we know how bright they should be in each image that is send to us.

If the stars do not have their nominal magnitude we know there are systematic issues with the brightness measurements. We can investigate how these off-sets vary with the brightness and colour of the non-variable stars in the field, and can correct for this. This requires us to identify all the non-variable stars over the entire range of magnitudes and with a wide range of colours. This procedure improves the accuracy of our brightness measurements and allows us to even use data that is taken under less than ideal observing conditions (thin cirrus) or with slightly non-standard filters.