Plot from Leavitt's 1912 paper. The horizontal axis is the logarithm of the period of the corresponding Cepheid, and the vertical axis is its
apparent magnitude. The lines drawn correspond to the stars' minimum and maximum brightness.[11][12]
Leavitt, a graduate of
Radcliffe College, worked at the
Harvard College Observatory as a "
computer", tasked with examining
photographic plates in order to measure and catalog the brightness of stars. Observatory Director
Edward Charles Pickering assigned Leavitt to the study of variable stars of the
Small and
Large Magellanic Clouds, as recorded on photographic plates taken with the Bruce Astrograph of the
Boyden Station of the Harvard Observatory in
Arequipa,
Peru. She identified 1777 variable stars, of which she classified 47 as Cepheids. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period.[13] Building on this work, Leavitt looked carefully at the relation between the periods and the brightness of a sample of 25 of the Cepheids variables in the Small Magellanic Cloud, published in 1912.[11] This paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt".
In the 1912 paper, Leavitt graphed the
stellar magnitude versus the logarithm of the period and determined that, in her own words,
A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods.[11]
Using the simplifying assumption that all of the
Cepheids within the Small Magellanic Cloud were at approximately the same distance, the
apparent magnitude of each star is equivalent to its
absolute magnitude offset by a fixed quantity depending on that distance. This reasoning allowed Leavitt to establish that the
logarithm of the
period is linearly related to the logarithm of the star's average intrinsic optical
luminosity (which is the amount of power radiated by the star in the
visible spectrum).[14]
At the time, there was an unknown scale factor in this brightness since the distances to the Magellanic Clouds were unknown. Leavitt expressed the hope that parallaxes to some Cepheids would be measured; one year after she reported her results,
Ejnar Hertzsprung determined the distances of several Cepheids in the
Milky Way and that, with this calibration, the distance to any Cepheid could then be determined.[14]
The relation was used by
Harlow Shapley in 1918 to investigate the distances of
globular clusters and the
absolute magnitudes of the
cluster variables found in them. It was hardly noted at the time that there was a discrepancy in the relations found for several types of pulsating variable all known generally as Cepheids. This discrepancy was confirmed by
Edwin Hubble's 1931 study of the globular clusters around the
Andromeda Galaxy. The solution was not found until the 1950s, when it was shown that
population II Cepheids were systematically fainter than
population I Cepheids. The cluster variables (
RR Lyrae variables) were fainter still.[15]
The Classical Cepheid period-luminosity relation has been calibrated by many astronomers throughout the twentieth century, beginning with
Hertzsprung.[17] Calibrating the period-luminosity relation has been problematic; however, a firm Galactic calibration was established by Benedict et al. 2007 using precise HST parallaxes for 10 nearby classical Cepheids.[18] Also, in 2008,
ESO astronomers estimated with a precision within 1% the distance to the Cepheid
RS Puppis, using
light echos from a nebula in which it is embedded.[19] However, that latter finding has been actively debated in the literature.[20]
with P measured in days.
[21][18] The following relations can also be used to calculate the distance to
classical Cepheids.
Impact
Phase lightcurve of variable star Delta Cephei.
Classical Cepheids (also known as Population I Cepheids, type I Cepheids, or Delta Cepheid variables) undergo pulsations with very regular periods on the order of days to months. Cepheid variables were discovered in 1784 by
Edward Pigott, first with the variability of
Eta Aquilae,[22] and a few months later by
John Goodricke with the variability of
Delta Cephei, the eponymous star for classical Cepheids.[23] Most of the Cepheids were identified by the distinctive light curve shape with a rapid increase in brightness and a sharp turnover.
Classical Cepheids are 4–20 times more massive than the Sun[24] and up to 100,000 times more luminous.[25] These Cepheids are yellow
bright giants and
supergiants of
spectral class F6 – K2 and their radii change by of the order of 10% during a pulsation cycle.[26]
Leavitt's work on Cepheids in the Magellanic Clouds led her to discover the relation between the
luminosity and the period of
Cepheid variables.
Her discovery provided astronomers with the first "
standard candle" with which to measure the distance to faraway
galaxies. Cepheids were soon detected in other galaxies, such as
Andromeda (notably by
Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are independent galaxies located far outside of the
Milky Way. Leavitt's discovery provided the basis for a fundamental shift in cosmology, as it prompted
Harlow Shapley to move the Sun from the center of the galaxy in the "
Great Debate" and Hubble to move the Milky Way galaxy from the center of the universe. With the period-luminosity relation providing a way to accurately measure distances on an inter-galactic scale, a new era in modern astronomy unfolded with an understanding of the structure and scale of the universe.[27] The discovery of the expanding universe by
Georges Lemaitre and Hubble were made possible by Leavitt's groundbreaking research. Hubble often said that Leavitt deserved the Nobel Prize for her work,[28] and indeed she was nominated by a member of the
Swedish Academy of Sciences in 1924, although as she had died of cancer three years earlier she was not eligible.[29][30] (The Nobel Prize is not awarded posthumously.)
References
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^Udalski, A.; Soszynski, I.; Szymanski, M.; Kubiak, M.; Pietrzynski, G.; Wozniak, P.; Zebrun, K. (1999). "The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud". Acta Astronomica. 49: 223–317.
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