Limits on Long-Term Solar Variability from Observations of Open Star Clusters

Increased carbon dioxide in the Earth's atmosphere has caused the mean global temperature to be about 1.4K above its pre-industrial value. However, there have been many instances on geological timescales when similar or larger temperature fluctuations have occurred. These were obviously not due to anthropogenic fossil fuel emissions. A possible explanation is that the luminosity of the Sun varies, with an amplitude of one to a few percent, on time scales of thousands to millions of years. There are a number of ways to investigate this possibility. In this paper, I discuss measuring the dispersion in brightness of stars similar to the Sun, so-called solar analogs. A particularly good sample is solar-type stars in an open star cluster with the approximate age of the Sun. I discuss the availability of data for this purpose, as well as some of the corrections which must be made for a meaningful result.