The period of a Cepheid variable depends on its luminosity, but generally ranges from a few days to a few weeks. Specifically, Cepheid variables with longer periods tend to be more luminous than those with shorter periods.
For example, a typical Cepheid variable with a period of around 5 days might have a luminosity that is roughly 1,000 times that of the Sun. Cepheids with longer periods, such as those with periods of around 50 days, can have luminosities up to 100,000 times that of the Sun.
American astronomer Henrietta Swan Leavitt
Leavitt worked as a “computer” at the Harvard College Observatory in the early 20th century, analyzing photographic plates of stars to measure their brightness and position. In the course of her work, she discovered a relationship between the period and luminosity of a type of variable star known as a Cepheid. Specifically, she found that the brighter Cepheids had longer periods.
Leavitt’s discovery was a major breakthrough in the field of astronomy, as it provided a way to measure distances to far-off galaxies by observing the period and brightness of their Cepheid variables. This relationship, known as the period-luminosity relationship, is still used today to determine the distance to astronomical objects, including galaxies and supernovae.
So while Cepheid variables themselves were first observed and named by the British astronomer Edward Pigott and his sister Caroline Herschel in the late 18th century, it was Henrietta Leavitt who first developed the mathematical description of their period-luminosity relationship.
Edwin Hubble
A contemporary of Leavitt, Hubble recognized the importance of her work and used it to determine the distances to remote galaxies.
Hubble observed Cepheid variable stars in these galaxies, using Leavitt’s period-luminosity relationship to determine their intrinsic brightness. By comparing their intrinsic brightness to their observed brightness, he was able to calculate the distance to each galaxy. This allowed him to determine that these galaxies were much farther away than previously thought, and that the universe was expanding.
Hubble’s use of Leavitt’s period-luminosity relationship was a critical step in the development of the field of extragalactic astronomy, and it paved the way for our modern understanding of the structure and evolution of the universe.
Edwin Hubble spent much of his career working at the Mount Wilson Observatory, which is located in the San Gabriel Mountains near Pasadena, California. He joined the staff of the observatory in 1919 and spent many years there making groundbreaking astronomical observations that helped to revolutionize our understanding of the universe.
While at Mount Wilson, Hubble made a number of important discoveries, including the observation of Cepheid variable stars in other galaxies, which allowed him to measure the distances to these galaxies and determine that the universe is expanding. He also discovered that there are many more galaxies in the universe than previously thought, and he played a key role in developing the concept of the Big Bang, which is now the dominant scientific theory of the origin and evolution of the universe.
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