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3d.  The Cepheid period-apparent magnitude relationship

By 1908, Leavitt had discovered nearly two thousand variable stars in the Magellanic Clouds and published her results in a paper which is now considered an astronomical classic (Leavitt, 1908).  Leavitt reported 1777 variables in the Magellanic Clouds found on plates taken with the Bruce Telescope using exposures of 2 to 5 hours.  She noted the periods of many variables were short, and plates taken within 2 to 3 days of each other were sufficient for demonstrating these variables.  Near the end of the paper in her discussion of variable stars in the SMC, Leavitt states “It is worthy of notice that in Table VI [containing data for 16 SMC variables] the brighter variables have the longer periods.  It is also noticeable that those having the longest periods appear to be as regular in their variations as those which pass through their changes in a day or two” (Leavitt, 1908).  This subdued statement is the first pronouncement of the now acclaimed Cepheid period-luminosity relationship.  It should be noted that Leavitt never used the term “Cepheid period-luminosity” relationship and never used the term “Cepheid variable” star in her papers.  These terms were applied by others, but Leavitt makes it clear that the period of these special variable stars in the SMC are intrinsically related to their luminosities.  The relationship was first referred to as the period-apparent magnitude relationship, but today it is commonly called the Cepheid period-luminosity relationship. 

Most textbooks cite Leavitt’s 1912 paper “Periods of 25 Variable Stars in the Small Magellanic Cloud” as the first announcement of her discovery, but that paper merely reiterates and elaborates on her 1908 publication.  However, in the 1912 publication she more explicitly delineates the period-luminosity relationship she had previously discovered.  “A remarkable relation between the brightness of these variables and the length of their periods will be noticed.  In H.A. 60, No.4 [her 1908 publication], attention was called to the fact that the brighter variables have the longer periods, but at that time the number was too small to warrant the drawing of general conclusions.  The periods of 8 additional variables which have been determined since that time, however, conform to the same law...It should be noticed that the average range, for bright and faint variables alike, is about 1.2 magnitudes.  Since the variable stars are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light as determined by their mass, density, and surface brightness” (Leavitt, 1912).  Figure 5 show the two graphs used by Leavitt to demonstrate this period-luminosity relationship.

  

Figure 5.  Original graphs from Leavitt’s 1912 paper describing the period-luminosity relationship for a select group of SMC variable stars.  The graphs show the stars’ apparent magnitudes for their maxima and minima versus their periods in days (fig.1) and the logs of their periods (fig.2).  

While Leavitt’s 1912 publication received little initial fanfare and her 1908 publication had been overlooked, the importance of her discovery was quickly recognized by others in the astronomical community.  Leavitt’s variable stars had periods of 1-270 days, but she and Pickering provided no calibration for the stars they had noted.  Almost immediately, Ejnar Hertzsprung (1873-1967) worked on calibrating these stars shortly after he had visited Harvard.  In 1913 he published his work and established the distance to the Small Magellanic Cloud as thousands of light years.  His value is not close to the modern value, but it was the first use of the Cepheid period-luminosity relationship, and it was the first serious attempt to accurately measure distances far beyond those available by parallax methods.  This was also the first attempt to establish distances beyond the Milky Way (Belkora, 2003). 

Unfortunately, Leavitt fell ill shortly after her 1908 publication and was briefly hospitalized in December 1908.  She then went to Beloit to stay with her parents and her two unmarried brothers.  She lingered in Beloit for months due to an unstated illness and corresponded off and on with Pickering.  At this time, Pickering was especially interested in the North Polar Sequence, which was a large project to measure more accurately than before the magnitude of 96 stars near Polaris.  Pickering arranged for her to work on this project from Beloit even sending her plates from the newly commissioned 60-inch telescope at Mt. Wilson (Jones, 2005).

Leavitt finally returned to Cambridge in May 1910, but her visit was short, because she returned to Beloit in March 1911 after the death of her father.  She finally returned to her uncle’s house in Cambridge by the fall of 1911 to resume her work on variable stars.  While in Beloit, she had worked intermittently on the North Polar Sequence.  Her work at this time also led to the publication of her now famous 1912 paper which was published under the name of Pickering but stated “The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt” (Leavitt, 1912; Jones, 2005). 

For the next four years Leavitt plugged away at her work and did not concern herself about the mechanism involved in the periodicity of variable stars.  She worked on the North Polar Sequence and was periodically ill.  In 1913 she was absent 3 months due to stomach surgery (Jones, 2005). Finally, in 1917 she published the completed North Polar Sequence, a large voluminous work [reference #26 in Table 1].  As far as can be determined from her writings and minimal notes, Leavitt was very proud of this publication, and she may have considered it more important than her work with Cepheid variable stars.  Jones (2005) notes that PhD’s have been awarded for less. 

In 1912 Harlow Shapley (1885-1972) completed his doctoral work at Princeton under the supervision of Henry Norris Russell (1877-1957), the leading theoretical astronomer in America in the early part of the 20th century.  Shapley had grown up on a Missouri farm, but his brilliance was recognized early, and it lead to a doctoral fellowship at Princeton.  Shapley’s doctoral thesis concerned eclipsing binary stars, and he and Russell realized about this time that Cepheid variable stars were probably not binary systems, and their variations reflected intrinsic physical properties of these stars.  During a visit to Harvard in 1914, Shapley met with Solon Bailey who advised Shapley to study variable stars in globular clusters.  Shapley later credited Bailey with leading him to an important area of research that helped establish Shapley's scientific reputation, and it influenced him for the rest of his career (Belkora, 2003). 

Figure 6. Harlow Shapley.  From: http://www.phys-astro.sonoma.edu/BruceMedalists/Shapley/index.html

After completing his doctorate at Princeton, Shapely started his professional career at Mt. Wilson Observatory.  While there, he used observations of variable stars in globular clusters and the observed distribution of globular clusters in the sky to establish the basic size of the Milky Way.  He also proved to the satisfaction of the astronomical community that the Solar System was not in the center of the Galaxy, a revolutionary concept not too dissimilar to Copernicus’s idea that the Earth was not the center of the Solar System but revolved around the Sun.          

Shapley was quite familiar with Leavitt’s work and her ability to find and classify variable stars.  He believed the variable stars in globular clusters, the so-called cluster variables, were the same type of stars described by Leavitt in her 1908 and 1912 papers.  Hertzsprung and others pointed out the stars classified by Leavitt had periods of days or longer, while most of the cluster variables had periods under a day and seemed dimmer.  We now know these latter stars are RR Lyrae stars, which are important variable stars, but they have different characteristics than Cepheid variable stars and are intrinsically fainter.  Most of Shapley’s work with the Milky Way globular clusters was done with RR Lyrae stars which are very common in globular clusters and far more numerous than Cepheid variables.  Shapley had also noted faint variable stars in the Magellanic Clouds, and in 1917and 1918 through correspondence with Pickering, he asked Leavitt if these fainter stars obeyed the same period-luminosity relationship she had established for the brighter variables in the Magellanic Clouds.  The answer was never forthcoming as Leavitt had not gotten around to analyzing these stars, and she suffered from a variety of ailments in these last years of her life.  Moreover, Pickering died somewhat suddenly of pneumonia at the age of 72 in early 1919 (Jones, 2005).   

 

3e. Pickering’s death and Leavitt’s final years

After Pickering’s unexpected death, Solon Bailey became the acting director of the Harvard College Observatory.  When Henry Norris Russell turned down the directorship for the observatory and recommended Shapely for the position, it was offered to Shapley, and he assumed this position in early 1921.  Henrietta and her widowed mother had by then moved into an apartment near the observatory, and in 1920 while Shapley was being considered for the directorship, she had written him asking about what projects she should pursue.  He still wanted her to work on the faint variables stars in the Magellanic Clouds (Jones, 2005).

By the time Shapley assumed the directorship of the Harvard College Observatory, Leavitt was the head of stellar photometry, and Cannon was the curator of the photographic plate collection and chief overseer of the Henry Draper Catalog.  Unfortunately, by this time Leavitt was nearing the end of her life.  She had been deaf for years and had long periods of absences for a variety of illnesses that are poorly documented.  She never recovered the pace of her work that had transpired up to her seminal publication in 1912.  After her publication of the North Polar Sequence, she did little further work, though she was encouraged to do so by Pickering and Shapley.  She left so few personal papers and notes that it is hard to reconstruct what, if anything, she thought about her discovery.  Sometime in 1921, Leavitt became terminally ill with stomach cancer.  Shapley called on her a few days before her death, and she passed away December 12, 1921.  She left her estate of appraised value of $314.91 to her mother (Jones, 2005).

Leavitt also left behind a photographic survey of the southern hemisphere and a study of novae which were partially completed.  This work was collated and later completed by Shapley and others [references #30-35 in Table 1].  Leavitt’s desk was given to Cecilia Payne [-Gaposchkin] (1900-1979).  This was a particularly appropriate passing of the baton, because Cecilia Payne went on to a remarkable career at Harvard becoming a full professor and chair of the astronomy department.  Leavitt, Cannon, Fleming, and Maury had been denied academic titles despite the importance of their work.  Payne’s doctoral thesis on stellar atmospheres was considered by Otto Struve (1897-1963) as “the most brilliant PhD thesis ever written in astronomy” (Moore, 2002).  In this work, Payne demonstrated for the first time that hydrogen and helium were the most common elements in the Universe, and a star’s spectral type was related to its temperature.  In 1934, Cecilia Payne married Sergei Illiaronovich Gaposchkin (1898-1984) an émigré Ukrainian astronomer who joined Harvard’s staff.  Payne never met Leavitt but felt she had been done a great injustice (Jones, 2005).  In the years following her death, Leavitt gained more recognition, including having a crater named for her on the far side of the Moon (figure 7). 

 

Figure 7.  The crater Leavitt and surrounding craters.  From Gillis 2004.

 
 

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