The Shapley-Curtis debate provides a unique opportunity for astronomical instruction. It combines for the students examples public speaking and the ability to explain what they have learned as well as the importance of historical context for scientific discovery. Have the students reenact the debate, perhaps having different actors play Shapley and Curtis for different points in the argument. Much of the information needed to research this debate, including lecture notes for undergraduate-level students, can be found at http://anctwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html. Another useful resource is a paper by Virginia Trimble called "The 1920 Shapley-Curtis Discussion: Background, Issues, and Aftermath." 27
It was Hubble who put an end to this debate in 1923 by finding a special type of star called a Cepheid variable star in the Andromeda galaxy. This special type of star changes brightness on very regular intervals and this interval is related to its luminosity. So by measuring the period (time difference) between the brightest moments of variability we can find its luminosity and calculate its distance just like the galaxies with known Luminosities. For more information and activities on finding distances using Cepheid Variable stars see http://www.itpa.lt/~astro/aol/market/experiments/middle/skills207.html and http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html. The distance that Hubble deduced in this manner placed the Andromeda galaxy far outside the confines of our own galaxy and determined it to be its own independent stellar system.
Hubble did make some mistakes however; here is some of his data:
Hubble's Data 28
With this data he calculated that the distances to galaxies, including the nearest galaxy, Andromeda, were too small. There are actually two types of Cepheid stars each of which have a different relationship between their period and luminosity. Hubble chose the wrong type of star to do his calculations and therefore miscalculated his distances. This led to another miscalculation of the age of the universe. With Hubble's data the age of the universe was calculated to be 1,840 million years. Not too long after these calculations were made, the geologists discovered that with radioactive dating, the rocks on Earth were at least 3.6 billion years old. These discoveries were in conflict and needed to be reconciled since the Earth could not be older than the universe itself. Another astronomer named Baade found the misuse of Cepheid stars and began the correction of Hubble's data. He also calculated a much larger age and size for the universe. Even today we are still unsure of the actual amount of expansion, but are sure that the universe is expanding. Scientists must have many ways to check their answers, as the astronomers could check with the geologists, because they are never sure they have done everything right.
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