Transcript: In the early part of the twentieth century, astronomers calculated the distances to stars by assuming that interstellar space was perfectly transparent. But eventually comparisons of distance to clusters in different directions in the sky yielded inconsistent results, and in 1930 Robert Trumpler showed that interstellar extinction or obscuration dims the light from all stars, groups, and clusters, that are larger than a distance of a few dozen parsecs. What this means is that...

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...

Transcript: Cepheid variables are luminous stars with variations in a range of periods of one to fifty days. The physics of their pulsation is well understood, and empirically for stars with well measured distance by parallax, there’s a well determined relationship between the period of the pulsation and the luminosity of the star. More luminous Cepheids have longer periods. Astronomers therefore isolate Cepheids in a distant cluster by taking images over a period of several months to...

Transcript: Stars of a particular photospheric temperature can have vastly different sizes and luminosities in an HR diagram. For example at a temperature of about three thousand Kelvin there is Proxima Centauri, a low mass main sequence star only three percent the size of the Sun, Aldebaran, that’s twenty times larger than the Sun, and Betelgeuse, a red supergiant a thousand times the Sun’s size. Yet spectroscopy alone can distinguish between these situations. This is because spectral...