Now you have a good sense of how astronomers use Cepheid variables to determine the extragalactic distance scale, the expansion rate of the universe, and the age of the universe. You've also seen how they can glean some information about the ultimate fate of our universe. Many other galaxies were studied as part of the HST Key Project on the Extragalactic Distance Scale.

Although Cepheids are primary distance indicators, the most fundamental means of determining distances is trigonometric parallax. It is difficult to make accurate parallax measurements for all but the nearest stars, but good parallax measurements supersede any other method of distance determination. Recent parallax measurements of galactic Cepheids made with the Hipparcos satellite require us to adjust the Cepheid distance scale.

The Hipparcos results

After the data used in this lab were published, results were released for a precise parallax survey completed with the European Space Agency's Hipparcos satellite. The new findings revealed that the current Cepheid distance scale must be increased by 10%. In other words, the distance to M100 you derived in this lab should be multiplied by approximately 1.1. Applying this revised distance to subsequent calculations, your estimate of the Hubble constant will decrease by 10%, and your estimate of the age of the universe will thus increase by 10%. How does your revised age of the universe compare with the presumed ages of the oldest globular clusters?

You can see that science is a process, not a collection of facts. Just as Edwin Hubble's discoveries required the astronomers of his day to adapt to new information, astronomers today must also continually reassess and adjust their models to fit new data. The process will continue as long as people survive. The role of an astronomer is to constantly push back the barriers of our understanding of the universe.

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