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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).
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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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