Henrietta Swan Leavitt was born in Lancaster, Massachussets. Her father was the Rev. George Roswell Leavitt, who was descended from early settlers in Hingham, Massachussets. Her mother was Henrietta Kendrick, who also came from colonial stock.
She attended Oberlin College and in 1892, was graduated from Radcliffe College, then called the Society for the Collegiate Instruction of Women.
After graduation Leavitt spent several years in travel and teaching and as an advanced student and volunteer research assistant in the Harvard College Observatory, becoming a permanent member of the staff of the Observatory in 1902. At that time Edward Charles Pickering was directing the activities of the Observatory toward the determination of the photographic magnitudes of the stars. Assigned, as an assistant to the study of the brightness of variable stars on the large number of photographic plates already accumulating, she soon became, by reason of her originality and intelligent industry, the head of the department of photographic stellar photometry.
Her principal achievements in this field were her determination of the magnitudes of the stars in sequences near the North Pole and in other regions; her discoveries of variable stars; and her studies of variable stars in the Magellanic Clouds which led to her discovery of the "period-luminosity" law. The photographic study of variable stars and any attempts at a systematic photometry of the stars with photographic plates called imperatively for standards of reference. Owing to the different colors of different stars, visual magnitudes, no matter how good, would not suffice. A sequence of stars, ranging from the second magnitude to the faintest star easily photographed, was charted near the North Pole. Methods had to be developed for the determination of the photographic magnitudes of these stars. Miss Leavitt was assigned to this work. The results were published in the Harvard College Observatory Circular 170, February 21, 1912, and in the Annals of the Astronomical Observatory of Harvard College, vol. LXXI (1917). They have since been in constant use by many astronomers.
She followed this work with similar measurements of sequences of stars in the forty-eight "Standard Regions" (Annals, Vol. LXXI, 1917) and of sequences (Annals, Vol. LXXXV, 1919) for use in connection with the international campaign of the Astrographic Catalogue.
Much of her time in her last years was devoted to the determination of standards in the Kapteyn "Selected Areas. " Her study of the light-curves of ten variables of the Algol type appeared in the Annals (Vol. LX, 1908). The most powerful method of measuring distance developed from her discovery that the brightest of the "cluster" variables in the Magellanic Clouds had the longest periods of variation. With the calibration of this relation between period and luminosity it has become possible to estimate distance from period and apparent brightness wherever variables of this type are found.
