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ULP Cepheids


 

Astronomers use an array of tools to determine the distance to an object. Parallax works well for objects that are relatively close but extragalactic objects are another matter. One “standard candle” that astronomers use is the Cepheid variable. Henrietta Leavitt discovered these stars about 100 years ago. She was one of Dr. Pickering’s “computers” at the Harvard Observatory. She noticed that the luminosity of these stars was related to the time between peaks in the star’s variable nature. Ejnar Hertzsprung (yes, the one celebrated for the Hertzsprung-Russell diagram) was able to use parallax to get distances to a few Cepheid variables and thus Leavitt’s relationship could now be calibrated. Since astronomers could now compute the difference between a star’s absolute magnitude and its apparent magnitude, distances to these stars could be computed. This turned out to be critical in determining that “spiral nebulae” like the Andromeda Galaxy are located well outside of the Milky Way.

Now astronomers have a new tool, ultra long period Cepheid variables. These ULP Cepheids allow for more accurate determinations of distances to far out objects. This in turn can be used to improve the accuracy of another standard candle, Type 1A Supernovae. It was these supernovae that led to the discovery by Alex Filippenko (and team) that something, now called dark energy, was accelerating the expansion rate of the universe. But a key question remained, was this acceleration due to a change in the universe or was dark energy at work from the very beginning? This is the kind of question that might be answered with more accurate distance measurements.

One example of the improved accuracy is seen in the more accurate number of the Hubble Constant. This is a number that tells us the current rate of the universe’s expansion. Previously, the uncertainty in this number was 10%. The new number, determined by Adam Riess and the SHOES team using ULP Cepheids and other tools, is 74.2 +/- 3.6, an uncertainty of 5%.

ULP Cepheids are so useful because they can be used over longer distances than normal Cepheids. In general, Cepheids can be used to measure distances up to 100 million light-years. The ULP Cepheids can be used for distances up to 100 million parsecs (1 parsec = 3.26 light-years).

 


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