Leavitt returned to the Harvard College Observatory from her travels in 1903.
 Because Leavitt had independent means, Pickering initially did not have to pay her.
Later, she received $0.30 an hour for her work, being paid only $10.50 per week.
She was reportedly "hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career".
 One of the women that Leavitt worked with in the Harvard Observatory was Annie Jump Cannon, who shared the experience of being deaf.

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.
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.

In another paper published in 1912, 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.
 This paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt".
Leavitt made a graph of magnitude versus logarithm of 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.

She then used the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately the same distance, so that their intrinsic brightness could be deduced from their apparent brightness as registered in the photographic plates, up to a scale factor since the distance to the Magellanic Clouds were as yet unknown.
She expressed the hope that parallaxes to some Cepheids would be measured, which soon happened, thereby allowing her period-luminosity scale to be calibrated.
 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).

Leavitt also developed, and continued to refine, the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness over 17 magnitudes.
She initially analyzed 299 plates from 13 telescopes to construct her scale, which was accepted by the International Committee of Photographic Magnitudes in 1913.

In 1913, Leavitt discovered T Pyxidis, a recurrent nova in Pyxis, and one of the most frequent recurrent novae in the sky with eruptions observed in 1890, 1902, 1920, 1944, 1967, and 2011.

Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers.
In 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry.
By the end of that year she had succumbed to cancer and was buried in the Leavitt family plot at Cambridge Cemetery in Cambridge, Massachusetts.
Leavitt returned to the Harvard College Observatory from her travels in 1903.  Because Leavitt had independent means, Pickering initially did not have to pay her. Later, she received $0.30 an hour for her work, being paid only $10.50 per week. She was reportedly "hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career".  One of the women that Leavitt worked with in the Harvard Observatory was Annie Jump Cannon, who shared the experience of being deaf.
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. 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.
In another paper published in 1912, 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.  This paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt". Leavitt made a graph of magnitude versus logarithm of 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.
She then used the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately the same distance, so that their intrinsic brightness could be deduced from their apparent brightness as registered in the photographic plates, up to a scale factor since the distance to the Magellanic Clouds were as yet unknown. She expressed the hope that parallaxes to some Cepheids would be measured, which soon happened, thereby allowing her period-luminosity scale to be calibrated.  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).
Leavitt also developed, and continued to refine, the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness over 17 magnitudes. She initially analyzed 299 plates from 13 telescopes to construct her scale, which was accepted by the International Committee of Photographic Magnitudes in 1913.
In 1913, Leavitt discovered T Pyxidis, a recurrent nova in Pyxis, and one of the most frequent recurrent novae in the sky with eruptions observed in 1890, 1902, 1920, 1944, 1967, and 2011.
Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. In 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry. By the end of that year she had succumbed to cancer and was buried in the Leavitt family plot at Cambridge Cemetery in Cambridge, Massachusetts.
Henrietta Swan Leavitt