In many of our papers with HOYS data you find introductory statements like this one: “Historically, young stars were first discovered based on their irregular and large-amplitude optical variability (Joy 1945).” – copied from our first HOYS paper. What these statements imply is that young stars are variable, while older stars are not (so much). Now, can we somehow test this for the HOYS clusters? Yes we can!

First we can find out how many stars belong to each cluster. Using data from the Gaia satellite we can count all stars that have the correct distance and proper motion.

Second we check what fraction of these stars has a colour between 2.1micron and 4.6micron that indicates it has a disk (K-W2>0.5mag). A warm inner disk usually creates a large excess colour for these sources compared to more evolved young stars that have lost their disk. The disk fraction in a cluster is hence correlated with the age of the cluster.

Third we extract all the HOYS light curves for the cluster stars and determine the fraction of them that is variable. In this specific case we select stars as variable if their changes in brightness is on average larger than the photometric uncertainties.

This is what we have plotted in the above figure. The size of the symbols scales with the number of stars in each cluster. The numbers next to each circle are HOYS cluster numbers. A few clusters are missing due to as yet insufficient data.

The plot shows a very nice correlation of the fraction of stars with disks and the fraction of stars that vary. Indeed, with very few outliers, the correlation is much clearer than one would expect, given that observing the variability of stars depends on a number of things as viewing angles, cadence, etc..