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Message: <blockquote data-quote="imhotep" data-source="post: 27073419" data-attributes="member: 562115">Cepheid Variables - Henrietta Swan Leavitt - The unnoticed US woman who gave the world the "Universal Cosmic Yardstick" to measure the Universe.Henrietta Swan Leavitt was one of many women "computers" who worked at Harvard University, cataloging stars around the turn of the last century. Women could be paid less than men, and were generally seen as detail-oriented and suited for the often boring and rote work of data analysis. They were also barred from operating Harvard’s telescopes, limiting their other astronomical options. Leavitt’s particular assignment was Cepheid variable stars. It was one of the Harvard Observatory's great projects, begun by Edward C. Pickering, of determining the brightnesses of all measurable stars.A Cepheid variable star is a type that pulsates and vary regularly in brightness in periods ranging from a few days to several months. A Cepheid progresses through a complete cycle from maximum brightness to minimum and then back to maximum again. Outer layer pulsations alternately increase and decrease the star's size and temperature, both of which affect the star's luminosity.The "Polaris" (or the "Pole Star"/"North Star") is a Cepheid variable.What Leavitt discovered in 1908 was that the fundamental brightness of a Cepheid is directly related to its pulse rate. This means that if two Cepheid stars have the same pulse rate but one is dimmer than the other, then we can tell that the dimmer one is farther from us. Since there is a precise relationship between distance and brightness – like gravity it is an inverse-square law – precise distances to all Cepheid stars were now within the grasp of astronomers.Leavitt published her work and pointed out that someone merely needed to work out the "Parallax" (a way of calculating distance that works only on very nearby stars) of a tiny handful of her variable stars to calibrate the system, and turn her rough picture of “near or far” into an accurate map complete with marked distances.  Within a year, a Danish astronomer named Ejnar Hertzsprung did exactly this. Sadly, as with many other female scientists of her time, Leavitt's contributions to her field went largely unacknowledged by the scientific peers. Her work went unnoticed for nearly  a decade. She never received any fame for her work. She was made head of stellar photometry at the Harvard College Observatory in 1921, but did not live long enough to enjoy her new role. She died of stomach cancer on the 12 December 1921 aged 53 years.It was Edwin Hubble who used her work and showed how large the Universe is.Today there are efforts to acknowledge her work and offer due recognition, as one article mentions - "As she had lived quietly, unnoticed, so her death left barely a ripple among her peers: to the extent that when, in 1925, the Swedish mathematician Gösta Mittag-Leffler wrote her a letter: 'Honoured Miss Leavitt, your admirable discovery ... has impressed me so deeply that I feel seriously inclined to nominate you to the Nobel Prize in Physics for 1926,' he had to be informed that she had in fact been dead for four years. As the Nobel Prize is not awarded posthumously, Leavitt never received her nomination."Using Leavitt's Law - or  the Period-Luminosity relationship that she discovered, others later calculated the distances to Cepheid variables in galaxies outside our own Milky Way. In doing so, they discovered that our Universe is expanding, starting from a single point more than 14 billion years ago at the Big Bang - a discovery that would have never been possible without the Leavitt Law.The most dramatic application was Hubble’s use in 1924 of a Cepheid variable to determine the distance to the great nebula in Andromeda, which was the first distance measurement for a galaxy outside the Milky Way.[ATTACH=full]150501[/ATTACH]</blockquote>

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