Henrietta Leavitt

Henrietta Swan Leavitt

Born: July 4, 1868, Lancaster, Massachusetts
Died: December 12, 1921, Cambridge, Massachusetts

Have you ever looked up at the stars at night and wondered how far away they are. Well it was this woman who answered that question for the most distant stars and galaxies

Henrietta Leavitt was the astronomer whose findings greatly enlarged the size of the known universe. It was she who discovered that that Cepheid variable stars have rate of change of brightness that is related to the star’s intrinsic brightness, how bright it looks close to it. In this way a measurement of the Cephid’s apparent brightness, what we see from Earth, and its variation rate could be used to estimate its distance from Earth since its apparent brightness diminishes at the square of its distance away from us, Leavitt’s Law. This work started a revolution in our understanding of the size and age of the universe that continues today.

Leavitt was born into religious family and was the eldest of seven children, two of whom died in infancy. She herself suffered from ill-health throughout her life. The family moved frequently, following her father’s moves for the church. When in Cleveland, Ohio, at the age of 17, Henrietta enrolled in Oberlin College in their preparatory course for a year. She studied undergraduate courses for the next two years.

When the family moved to Cambridge, Massachusetts, Leavitt found that Harvard did not admit women. She was able to enroll in the Harvard Annex, now Radcliffe College. She graduated aged 23 in 1892 with the equivalent of a Harvard BA that included the studies of mathematics and astronomy. Financially supported by her parents, she spent the next two years as a volunteer research assistant at the Harvard observatory. Women were not allowed to operate the telescope, but she did have access to the large collection of photographic plates taken with it.

In 1893 Leavitt had earned enough credits for a graduate degree in astronomy but, because of her ongoing illness, she never completed it. It was after this illness that she discovered that she was losing her hearing, which got progressively worse during her life. In 1898 she became a member of the Harvard staff, but it was not until 1903 that she returned to the observatory. The director,  Edward Charles Pickering, did not initially pay her since she was financially independent. Later she received payment of 30c an hour, $10.50 a week.

Pickering assigned her to study variable stars on photographic plates from their telescope in Peru, cataloging their apparent brightness and period of variation. In 1908 she published the results of her studies, noting that the longer period variable stars were brighter. In 1912 she published her most influential paper where she took 25 variable stars in the  Small Magellanic Cloud, a small galaxy just outside the Milky Way, and she plotted their apparent brightness against their period. Making the assumption that the stars of the  Small Magellanic Cloud were all about the same distance away, her result showed that the period of variability of the Cephids could be used as a method of measuring their distance away. All that was needed was calibration by measuring the period of a variable star whose distance was known by a different method. This was quickly provided by measuring the distance of a variable star that was close enough for the parallax method to work.

Before Leavitt’s finding the only method available to astronomers to determine the distance to a star was the use of parallax, the same way our two eyes estimate distance but on a much larger scale. This method is limited to the measurement of distances up to about 100 light years. Leavitt’s method, known as Leavitt’s Law, allowed astronomers to measure the distance to remote galaxies by finding Cephid variables in them, allowing them to understand that the universe was a lot bigger than previously assumed.

Edwin Hubble used Leavitt’s Law to estimate the distance to distant galaxies containing Cepheid variables and to correlate these distances with the galaxy’s recession speed based on its red shift to show that the universe was expanding, Hubble’s Law. Hubble’s findings forever changed our understanding of the scale of the universe. The knowledge that the universe was expanding posed the question of what it was expanding from, leading to the theory of the Big Bang as the origin in space and time of our universe.

Hubble often said that Leavitt deserved the Nobel Prize for her work, but by the time she was being nominated she had died of cancer three years earlier. (The Nobel Prize is not now awarded posthumously.)

Leavitt’s scientific work was frequently interrupted by illness. She never married and had no children. She died at 53 of stomach cancer and was greatly missed by her astronomer colleagues. She is buried in Cambridge, MA, in her family’s plot.