Henrietta Swan Leavitt

American astronomer and human calculator

Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921) was the American astronomer who discovered the relationship between the luminosity and the period of Cepheid variable stars. She made this discovery, among others, while employed at the Harvard College Observatory as a human computer examining photographic plates to measure and catalog the brightness of stars.

Henrietta Swan Leavitt
at about age 30


  • It is worthy of notice that in Table VI 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.
Fig.5 H 1255 "Ten Variable Stars of the Algol Type" Higher resolution time period for the horizontal axis, centering on minimum magnitude
Fig.6 H 1255 "Ten Variable Stars of the Algol Type" Lower resolution time period for the horizontal axis, centering on maximum magnitude
  • The discovery of variable stars, at this Observatory and elsewhere, has progressed so rapidly during the last five years, that the difficulty of keeping pace in observing and discussing them has become very great. In the study of distribution now in progress here, the actual time devoted to the search for new variables is small, but thorough observation requires much time, while the discussion of results may be prolonged almost indefinitely. When new lists of variables are published, therefore, it should be remembered that their discovery does not interfere materially with the study of individual objects. The number of these is so large that the publication of full results for all must be greatly delayed.
  • Apparently no sharp dividing line can be drawn between true Algol stars and those whose variations are continuous. Periods of nine variables in this region, which are of the Algol type or closely resemble it, have been determined and are here discussed.
    • "Ten Variable Stars of the Algol Type" (1908) Annals of the Astronomical Observatory of Harvard College Vol.60. No.5
  • The range of H 1255 is only four tenths of a magnitude, and on account of its brightness it is difficult to observe on all plates except those taken with the 1-inch Cooke lens. It seemed necessary, therefore, to take unusual precautions in order to secure accurate observations, and to give each one its full weight. Accordingly, one hundred and thirty six photographs were selected, including nearly all of those taken with the Cooke lens, and also those taken with the 8 inch Bache Telescope on which the variable was certainly faint. Four independent estimates of brightness were made on each plate, and means were taken, thus reducing the probable error one half. The phase was computed for each observation, thus covering all parts of the light curve. ...H 1255 and H 1303 differ from the other variables in a marked degree as in each case the duration of the phase of minimum is very long in proportion to the length of the period. This fact led to considerable difficulty in determining their periods as they were apparently at their minimum brightness for some time before and after the actual minima occurred. In H 1255, the change in brightness is obviously continuous throughout the period, although it is much more rapid near minimum than near maximum. This is clearly seen in Plate IV, Figs. 5 and 6.
with Edward C. Pickering, Harvard College Observatory Circular, vol. 173, pp.1-3
Fig.1 Separate graphs for maximum & minimum Cepheid magnitudes, with Period in days on horizontal axis & Apparent magnitude on vertical axis
Fig.2, "Periods of 25 Variable Stars in the Small Magellanic Cloud" same as Fig.1 but with logarithmically scaled Periods on the horizontal axis
  • A remarkable relation between the brightness of these [Cepheid] variables and the length of their periods will be noticed. In H.A. 60, No.4, attention was called to the fact that the brighter variables have the longer periods, but at that time it was felt that the number was too small the drawing of general conclusions. The periods of 8 additional variables which have been determined since that time, however, conform to the same law. The relation is shown graphically in Figure 1... The two resulting curves, one for the maxima and one for the minima, are surprisingly smooth, and of remarkable form. In Figure 2, the abscissas are equal to the logarithms of the periods, and the ordinates to the corresponding magnitudes, as in Figure 1. A straight line can readily be drawn among each of the two series of points corresponding to the maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods. The logarithm of the period increases by about 0.48 for each increase of one magnitude in brightness.
  • Since the [Cepheid] variables 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.
  • It is to be hoped, also, that the parallaxes of some variables of this type may be measured.
  • It is hoped that 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.

Quotes about Leavitt

  • A determination of the visual magnitudes of the stars had a very large place in the work of the Observatory during the first half of Professor Pickering's directorate. As soon as the need of photographic magnitudes became urgent, the importance of a standard sequence, from which the photographic magnitudes could be derived for stars anywhere in the sky, became evident. A sequence of stars of varying magnitudes had been early selected near the North Pole, and the determination of the magnitudes of the stars involved was assigned to Miss Leavitt. This work was carried out with unusual originality, skill, and patience.
    • Solon I. Bailey, "Henrietta Swan Leavitt" Popular Astronomy (1922) Vol. 30, No. 4, Whole No. 294
  • About 1906 a Durchmustering of variable stars was proposed at the Harvard Observatory. Somewhat later this was undertaken on plates included in the Map of the Sky... A comparison of the photographs of a number of these regions by Miss Leavitt led to the discovery of several hundred variables and other special objects. Among them were a number of stars of the Algol type.
    • Solon I. Bailey, "Henrietta Swan Leavitt" Popular Astronomy (1922) Vol. 30, No. 4, Whole No. 294
  • One of the most striking accomplishments of Miss Leavitt was the discovery of 1,777 variable stars in the Magellanic Clouds. These results were [made] possible by photographs of long exposure made at Arequipa with the 24-inch Bruce refractor and forwarded to Cambridge. Some of these plates had exposures of from two to four hours and showed very faint stars, among which nearly all the variables are found. ...from a study of 25 of them, the important law was derived, that the length of period bears a definite relation to the absolute magnitude.
    • Solon I. Bailey, "Henrietta Swan Leavitt" Popular Astronomy (1922) Vol. 30, No. 4, Whole No. 294
  • In addition to these larger labors, Miss Leavitt took part in various minor investigations. She gave considerable time to the discovery of new celestial objects. Altogether, she found 4 new stars, 2400 variable stars, or about one half of the known variables, and various asteroids and other objects.
    • Solon I. Bailey, "Henrietta Swan Leavitt" Popular Astronomy (1922) Vol. 30, No. 4, Whole No. 294
  • Miss Leavitt was of an especially quiet and retiring nature, and absorbed in her work to an unusual degree. She had the highest esteem of all her associates at the Harvard Observatory, where her loss is keenly felt.
    • Solon I. Bailey, "Henrietta Swan Leavitt" Popular Astronomy (1922) Vol. 30, No. 4, Whole No. 294
  • Apart from her few and very important scientific papers, Leavitt left behind almost no traces of her life. She was born on the Fourth of July 1868 in Lancaster, Mass., and died of cancer on Dec. 12, 1921. Her will tells nearly all. She left an estate worth $314.91, mostly in Liberty Bonds, with a few items such as a desk valued at $5. She never married and had few living relatives. She also left behind a legacy of a great astronomical discovery.
  • Hubble tackled two of the most fundamental questions of the universe: how old is it, and how big? To answer both it is necessary to know two things—how far away certain galaxies are and how fast they are flying away from us. The red shift gives the speed at which galaxies are retiring, but doesn't tell us how far away they are to begin with. For that you need what are known as "standard candles"—stars whose brightness can be reliably calculated and used as benchmarks... Hubble's luck was to come along soon after an ingenious woman named Henrietta Swan Leavitt had figured out a way to do so.
    • Bill Bryson, A Short History of Nearly Everything (2003)
  • As a senior in 1892 Leavitt was introduced to astronomy. She was fascinated by it, and after graduation she enrolled in a course to study the subject full time. Tragically Henrietta Leavitt was suddenly struck down by a serous illness, and she was forced to spend over two years at home recovering. Her illness left her profoundly deaf. ...when she felt fit enough she put forward her name in 1895 as a volunteer worker at Harvard College Observatory.
    • David H. Clark & Matthew D. H. Clark, Measuring the Cosmos: How Scientist Discovered the Dimensions of the Universe (2004)
  • The photographic plates from Peru that Leavitt was studying in Harvard covered two clouds of stars, known as the Large and Small Magellanic Clouds... During the course of her painstaking work, Leavitt noticed that the Cepheids in the Small Magellanic Cloud (SMC) showed an overall pattern of behaviour in which the brighter Cepheids... went through their cycle more slowly. The initial discovery was reported in 1908, and by 1912 Leavitt had enough data to pin down this period-luminosity relationship in a mathematical formula, established from her study of twenty-five Cepheids in the SMC. ...Leavitt found a clear mathematical relationship between the apparent brightness of a Cepheid in the SMC and its period... This could only mean that the absolute magnitudes of Cepheids are related to one another in the same way, since the distance effect is essentially the same for all of the Cepheids in the SMC. All that was needed now was to find the distance to just one or two Cepheids in our neighborhood... so that distances... could be worked out from the period-luminosity law that Leavitt had discovered.
    • John Gribbin, The Scientists: A History of Scient told through the Lives of its Greatest Inventors (2002)
  • She and others realized that one needed only to calculate the distance to these [Magellanic] Cepheids, which almost certainly were roughly the same distance to the earth, to have a useful yardstick for measuring other distances.
    • Jacob Darwin Hamblin, Science in the Early Twentieth Century: An Encyclopedia (2005)
  • She deserved the Nobel Prize for her work.
  • He acknowledges the use and calibration of her period-luminosity relation first by Hertzsprung and later by Shapley and ends the “Period- Luminosity Relations to Cepheids” section in his book without ever mentioning that he, Hubble, had used Shapley’s technique. ...Hubble’s underwhelming acknowledgment of Henrietta Leavitt is an example of the ongoing denial and lack of the professional and public recognition that Henrietta Leavitt suffers from, despite her landmark discovery. With the exception of naming a moon crater after her, the profession of astronomy has not done much to celebrate her work. No astronomy prize is named after her and the period-luminosity relation has not been renamed as the H. Leavitt law.
  • I should be willing to pay thirty cents an hour in view of the quality of your work, although our usual price, in such cases, is twenty five cents an hour.
    • Edward Charles Pickering, Letter to H.S. Leavitt following her 4 year absence from volunteering at the Harvard College Observatory (~1900) as quoted by George Johnson, Miss Leavitt's Stars (2005)
  • The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt.
    • Edward Charles Pickering, "Periods of 25 Variable Stars in the Small Magellanic Cloud" (1912) Introductory sentence to article written by Leavitt and published under both names.
  • By the death of Miss Leavitt on December 12, 1921, the Observatory lost an investigator of the highest value. She had obtained a comprehensive experience in photographic photometry, and had developed a clear appreciation of the difficulties involved in the theory and practice of this important research. Her work on standard magnitude sequences was nearly concluded at the time of her death, but she had hardly begun work on her extensive program of photographic measures of variable stars. In the foregoing summary no mention has been made of Miss Leavitt's work on standard photometry...
Inverse Square Law
  • How far are the spiral nebulae? How large is the universe? We cannot begin to answer these questions unless we measure the distance of heavenly objects. The breakthrough was made by Henrietta Leavitt, who was interested in a rather special class of stars, the Cepheids. The intensity of light coming from Cepheids rises and falls regularly with time... Concentrating on one of the Magellanic Clouds, she found that there was a very close relationship... The brighter the Cepheid was, the longer its period. The distance of the Magellanic Cloud is so great that the stars there can be regarded as all being effectively the same distance from the Earth. If you are in Los Angeles, everybody in Carnegie Hall is about the same distance from you. ...Suppose that a Cepheid in the cloud has a certain brightness and a period of one week. Now look at another Cepheid in some more distant galaxy. If it has the same period, we can assume it has the same intrinsic brightness, and yet it is dimmer than it should be. ...we can work out the relative distance from Earth. A star of the same intrinsic brightness that is twice as far away will be four times dimmer. ...It is slightly complicated by the effects on brightness of interstellar dust clouds, but it was a huge step forward.
    • Brian L. Silver, The Ascent of Science (1998)
Henrietta Swan Leavitt
acrylic portrait, 2013
  • Miss Henrietta Swan Leavitt, for more than twenty years a member of the staff of the Harvard Astronomical Observatory, died at her home in Cambridge on Dec. 12. She was a graduate of Radcliffe College, and had studied astronomy as a graduate student. She joined the staff of the Harvard Observatory in 1895, and finally was in charge of the department of photographic stellar photometry. She determined the brightness of a series of stars near the north pole ranging from the fourth to the twentieth magnitude; discovered four new stars and 2,400 variables of about half of all the known variable stars; formulated a law establishing a definite relation between the brightness and the length of period of such variables; and made other noteworthy achievements in astronomy. The scientific results of her work form parts of volumes 60, 71, 84, and 85 in the "Annals of the Harvard Observatory."
  • To the Editor of the Bulletin: In Professor Hart's most interesting and illuminating article printed in the Alumni Bulletin he remarks that barring certain exceptions "petticoats are considered to have no place in Harvard or a Harvard Catalogue." Unfortunately this statement is only too true, and I believe the time is ripe to take serious account of the important and indispensable services that women are rendering to the University in technical and administrative positions in her offices and her institutions. We have recently read in the papers of the death of Miss Henrietta S. Leavitt of the Astronomical Observatory, whose work in photographic photometry gave her an international reputation... in fact, the services that the women have rendered at the Observatory are too well known in the scientific world to need further comment. ...Harvard should follow the lead already taken by the other large universities of the country, including California, Chicago, Columbia, Princeton, and Yale, in recognizing high grade service afforded by women on its staff, and this recognition should be not merely the inclusion of their names in the Catalogue... but should carry with it privileges of retirement and pension funds and of leave of absence at stated periods in order to afford opportunity for study and research. Several of the universities named are already ahead of Harvard in this respect, and in some of them women occupying high grade technical positions take rank with instructors and assistant professors when their acquirements and the nature of their work make them worthy of it. ...my heading "Petticoats in Harvard" is not an attempt to bring up the question is... only a plea for fitting recognition of scholarly work efficiently and faithfully performed in our midst by an unrecognized body of experts.
    • T. Franklin Currier, "Petticoats at Harvard" Harvard Alumni Bulletin (1921) Vol. 24, Part 1 No.14 Thursday Jan.5, 1922
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