Leavitt

NAME: Leavitt, Henrietta Swan

ACHIEVEMENT(S):  Helped reveal the size of the universe.

NICKNAME/ALIAS: None

DATE/PLACE OF BIRTH: 4 July 1868, at Lancaster, MA.

DATE/PLACE/AGE AT DEATH: 12 December 1921, in Cambridge, MA.  Aged 53

DISCIPLINE: Astronomer

FIELD: Astronomy

LANGUAGE: English

PARENTS: George Roswell Leavitt and Henrietta Swan Kendrick

SIBLINGS: Six siblings

SPOUSE: None

CHILDREN: None

EDUCATION: Public school in Cambridge; Oberlin College, Cleveland, 1885-88;

Society for the Collegiate Instruction of Women (later Radcliffe College),

1888-92

MAJOR WORK: "1,777 Variables in the Magellanic Clouds", 1908

LIFE AND TIMES:

In 1893, Leavitt left Harvard and spent two years travelling before returning to the Harvard College Observatory in 1895, where she worked as a volunteer research assistant in the programme on the measurement of stellar magnitudes.

Apart from periods of absence due to ill health, Leavitt spent the rest of her life at Harvard, where she was ultimately  appointed head of the department of photographic stellar photometry (the measurement, using photography,  of the brightness of stars and other celestial objects).

During this period, visual photometry (which had been made far more accurate by the photometers constructed first by <work related to> Steinheil* in 1842, and then by <work related to> Zollner* in 1861) was gradually being superseded by photographic photometry, because the photographic plate is more sensitive to light of certain wavelengths than the human eye. 

Leavitt's first research at Harvard concerned the accurate measurement of the brightness - and so the magnitude - of circumpolar stars. In 1904 she began work on a "north polar sequence" of star brightness to serve as a standard for the entire sky, needed because of the complexity of photographic process in astronomy, the brightness of star images on film not being proportional to their actual brightness, and each telescope giving different results for different-colored stars. 

This became a lifelong project, involving the study of 299 photographic plates from thirteen telescopes, and comparing stars ranging from the fourth to the twenty-first magnitude in brightness (each increasing unit of magnitude corresponding to a reduction in brightness by a factor of 2.512 on a logarithmic scale). 

Leavitt produced a standard by which brightness could be estimated by comparing one star with another, rather than referring to photographic images. In 1913 her system of the north polar sequence was adopted by the International Committee on Photographic Magnitudes for its projected astrographic map of the sky. Her results were published in the 'Annals of Harvard College Observatory' (1917).

In the course of her investigations, Leavitt developed a method by which she superimposed a negative of a photograph,  taken on one date,  on a positive made

from the same region of the sky and taken on another date, and then found places where the white and black images of a star did not precisely coincide. 

These stars were suspected of variability and were compared to images of the same star on other plates. During 1904-05, Leavitt found some 1,054 variable stars in the Magellanic Clouds of the southern hemisphere, and in 1908 - despite ill health - she published a list of the 1,777 variable stars she had found in these regions in the 'Annals of the Astronomical Observatory of Harvard College' (60, no. 2).

Between 1908 and 1912, Leavitt began concentrating her studies on a kind of variable star she detected in the Magellanic Cloud pictures called the Cepheid variable, (named after the first type to be discovered, Delta Cephei), the pulsating change in brightness of which is extremely regular.

As early as 1908, when she published a preliminary report on her findings ('Annals', 60, no. 4), Leavitt had discovered that the brighter the Cepheid was overall, the longer it took to change its magnitude.

This led her to suggest that since the Cepheids were nearly all the same distance from Earth, the apparent differences in the photographic magnitudes of the stars probably corresponded to real differences in their emission of light, so their periods (the length of time for a star to go from maximum luminosity to minimum and back to maximum) might be related to their light output - i.e. the longer the period of pulsation, the brighter the star.

In 1912, in the 'Harvard College Observatory Circular', Leavitt published a table of the length of the periods, which varied from 1,253 to 127 days, with an average of around 5 days, plotting the apparent magnitudes at both maximum and minimum brightness of 25 Cepheids against the logarithms of their periods. From these data, she proved that there was a direct,  linearly increasing relationship - i.e. the longer the period, the greater the brightness at maximum - between the average apparent magnitude and the logarithm of the period of these variable stars.

Before the development of this period-luminosity had been established, cosmic distances could only be determined up to about 100 light-years. A Danish astronomer at Harvard, Ejnar Hertzsprung, soon realized that, as a result of Leavitt's work, if the actual brightness of a Cepheid of a given period were known, then its distance from Earth could be estimated from the apparent brightness and that huge distances could be

calculated.

All of the Cepheids were too far from the Earth for their distances to be calculated using the standard parallax method, pioneered by <work related to> Friedrich Bessel*, but Hertzprung and astronomer Harlow Shapley at the Mount Wilson Observatory  were able to convert Leavitt's period-luminosity curve so that it could be read in terms of absolute rather than apparent magnitude.

Then, once they had determined the period of any Cepheid, they could return to the curve and read off its absolute magnitude, and when the Cepheid's apparent magnitude was compared with its absolute magnitude, the distance of the star from the Earth could be worked out.

To the astonishment of the astronomical community, Hertzprung and Shapley found that the Small Magellanic Cloud was in the range of 100,000 light-years from Earth - a distance almost beyond the comprehension of their colleagues, placing it far beyond our own galaxy.  They argued that nebular regions of the sky like the Magellanic Clouds were in fact distant "island universes", outside the Milky Way. 

In 1918 Shapley revised the figures, finding the smaller Magellanic Cloud to be 94,000 light-years away. Although these conclusions were hotly debated at the time, both the existence of galaxies not our own and the use of the period-luminosity relationship to find stellar distances would soon become widely accepted by astronomers.

Leavitt’s discovery made possible <influenced> Hubble*’s epochal work

ASSESSMENT: Her discovery was used by subsequent astronomers for calculating the distances from the earth of similar stars within our own galaxy and in distant galaxies.

EXTRA CONNECTIONS: None

ONELINER: Henrietta Leavitt,  discoverer of  Cepheid variable stars

BIBLIOGRAPHY:

DSB, EB, ADNB

Hoffleit, Dorrit, Women in the History of Variable Star Astronomy,  1993