Basically what the stars do is pulsate in and out. The shortest ones have a time period of one and a bit days, through to the very long ones which pulsate irregularly and have periods of up to a hundred and fifty days.

Someone else gets photometry, I get spectra, and we see just how these things vary over time and what sort of relationships you have between the very short-period ones and the very long-period ones. In theory, there’s a certain evolutionary development of the stars, and the older ones are much larger, much brighter, and pulsate with a lower frequency.