Transcript: Main sequence fitting can be applied in principle to any cluster. However, the rare variable stars like RR Lyraes and Cepheid variables are particularly valuable because the physics of their variations allows their luminosities to be estimated, and their luminosities allow them to be seen to large distances. RR Lyraes are a hundred times more luminous than the Sun, so they can be seen ten times further away than a Sun-like star could. Cepheid variables are ten thousand times more luminous than the Sun and so can be seen at distances a hundred times that of a Sun-like star. But we really need a large cluster to be able to detect even a few versions of these very rare stars, and that is a practical limitation.