Celebrating 100 Years of Knowledge of Cepheid Variables

March 3, 1912 - March 3, 2012

Thanks to Henrietta Leavitt (and as some might include Edwin Hubble), we know what we know today about much of the universe; and it was achieved by hard work, as is anything life-changing. As we remember these astronomers who changed history, remember not only their discoveries, but their impact on the modern world of astronomy.

Henrietta Swan Leavitt

Born on July 4, 1868 in Lancaster, Massachusetts, Henrietta Swan Leavitt soon moved to Ohio, where she would attend college and start her work as an astronomer, an occupation which was predominately dominated by men at the time. As some might suppose her interests of astronomy was kindled during her youth, it wasn't actually until her senior year of college where she took a course in astronomy, and in 1892 (when she was twenty-four) she graduated from the Society for the Collegiate Instruction for Women, known today as Radcliffe College, previously attending Oberlin college.  

The Woman Astronomer records, "Three years after graduation, she became a volunteer research assistant at Harvard College Observatory. Seven years later, in 1902, [Edward Charles] Pickering hired her on the permanent staff at $.30 per hour." This was the beginning of her career, which she had no idea could change the world.Leavitt continued her education as an assistant at Harvard College; herself capable of anything, although being given very small portions of theoretical work. Because Pickering, Leavitt's employer, did not allow Leavitt to work telescopes, he assigned her a position (at the small pay, as noted above) to measure and catalog the brightness of stars in the photographic plate collection of the observatory, as well as being a "computer", or someone who calculates sums, etc. 

But, Leavitt did not let her small pay and tedious work get in the way of her passion, and she fiercely took on her job, eventually discovering thousands of variable stars in the Small Magellanic Cloud, a nearby satellite-galaxy. Today, these variable stars are called Cepheid variables, as they were the stars responsible in providing a foundation for a paradigm shift, or changing, in modern astronomy. A paper was published of her findings (which can be found here), although she literally was given no recognition for her efforts during the period of her life. Below are excerpts of this influential paper, Periods of 25 Variable Stars in the Small Magellanic Cloud, describing what was to become one of the greatest findings in astronomical history.

"The measurement and discussion of these objects [1777 variable stars in the two Magellanic Clouds] present problems of unusual difficulty, on account of the large area covered by the two regions, the extremely crowded distribution of the stars contained in them, the faintness of the variables, and the shortness of their periods, as many of them never become brighter than the fifteenth magnitude. ... With the adoption of an absolute scale of magnitudes for stars in the North Polar Sequence, however, the way is open for such a determination. 

"Fifty-nine of the variables in the Small Magellanic Cloud were measured in 1904, ... and the periods of seventeen of them were published ... They resemble the variables found in globular clusters, diminishing slowly in brightness, remaining near minimum for the greater part of the time, and increasing very rapidly to a brief maximum. 

A remarkable relation between the brightness of these variables and the length of their periods will be noticed. [find chart here, Table One]

"The facts known with regard to these 25 variables suggest many other questions with regard to distribution, relations to star clusters and nebulae, differences in the forms of the light curves, and the extreme range or the length of the periods. It is hoped that a systematic study of the light changes of all the variables, nearly two thousand in number, in the two Magellanic Clouds may soon be undertaken at this observatory."

Another paper (1777 variables in the Magellanic Clouds) was published earlier, and had the same general idea. The Woman Astronomer concludes, "Her study led to the period-luminosity relationship of these variables, which in turn led to the ability to determine distances of stars from a mere one hundred light years to ten million light years. Ejnar Hertzsprung used her discovery to plot the distance of stars; Harlow Shapley used it to measure the size of the Milky Way; and Edwin Hubble used her work to ascertain the age of the Universe." It is certain Leavitt's work was utmost influential, yet it is hard to think that none of her work was heralded during her lifetime; it took Edwin Hubble to bring Leavitt's work into the light, of which he pivoted most of his studies on.

Leavitt found a relationship between luminosity in variable stars, the Observatories of the Carnegie Institution for Science, relates the steps to achieve her relationship for confirmation, "a) Measure the period of the star. b) Use Leavitt’s graph to determine how bright it really is. c) Measure how bright it appears and determine its distance." And everything works perfect today!

Today, two astronomical objects/features bear her name: asteroid 5383 Leavitt and the crater Leavitt on the Moon, are both named in her honor. After her death, Leavitt was also nominated for the Nobel Peace Prize, but it is so that such an award could not be bestowed to individuals posthumously.