In 1895 Henrietta Swan Leavitt volunteered as an unpaid assistant at Harvard College Observatory. Her work on Cepheid variable stars led to the discovery of galaxies beyond our Milky Way, the expansion of the universe, and the big bang. Leavitt’s Law for determining astronomical distances ushered in the modern age of cosmology – yet she received little recognition in her lifetime.
Early Women Astronomers: The Early Years of Leavitt
Born on July 4, 1868, Henrietta Swan Leavitt was the daughter of the minister at Pilgrim Congregational Church in Cambridge, Massachusetts. She graduated from what is now Radcliffe College with an Bachelor of Arts degree in 1892.
In her senior year, she took a course in astronomy at Harvard College Observatory, and attended graduate courses in astronomy the following year. However, an illness which began shortly after college left her almost completely deaf.
Frustrated by the work of his male assistants, Observatory director Edward Charles Pickering decided to hire females – a progressive step for its time. The tedious job of the women was to count and catalog stars from photographic plates from telescopes all over the world. In 1895, twenty-seven-year-old Henrietta Swan Leavitt volunteered for “Pickering’s Harem,” as it was called – at no salary.
Despite continuing health issues, Leavitt was made a permanent staff member in 1903, for 30 cents an hour. Along with the other females, however, she was forbidden to use the telescopes.
Cephid Variables: Twinkle, Twinkle
Amongst the star photographs Leavitt examined were Cepheid variables – super giant stars which pulsate at regular intervals (typically from a few days to a few months): a rapid brightness phase is followed by gradual dimming.
Using a laborious process called superposition, Leavitt studied the changing brightness of nearly two thousand Cepheid variables in the Large and Small Magellanic Clouds (nearby satellite galaxies of our Milky Way). Leavitt noticed a pattern – a correlation between a star’s brightness and its frequency of pulsation; the brighter a Cepheid variable is, the slower it pulsates.
Leavitt published preliminary findings in the Annals of the Astronomical Observatory of Harvard College in 1908. Four years later, she wrote a follow-up paper confirming her discovery: there is a direct correlation between the time a Cepheid variable star goes from bright to dim, its period, and its brightness.
” A remarkable relation between the brightness of these variables and the length of their periods will be noticed,” reported Leavitt.
What became known as Leavitt’s Law states that there is a straight line relation between a Cepheid variable star’s intrinsic luminosity (true brightness) and the log of its period. (She assumed the Cepheids in the Magellanic clouds were all the same distance away.)
Parallax and Leavitt’s Law: Measuring With a Cosmic Yardstick
Before Leavitt’s discovery, astronomers used parallax to determine distance to a star. To measure by this method, telescopic images of the same star are taken at different times, for example, six months apart. Using the distance between the two Earth positions, and the angles to the star, astronomers calculate the distance to the star. This method is accurate for stars up to about 100 thousand light-years away.
Using the parallax method, Danish astronomer Ejnar Hertzsprung determined the distance to several Cepheid variable stars in 1913. From this, he calibrated Leavitt’s Law. Now, by measuring a Cepheid’s period and apparent brightness, astronomers could determine its true brightness, and as a result, they could calculate its distance.
How does this work? A star’s apparent brightness – how bright it appears to be – drops off as the square of its distance from us. By comparing a Cepheid’s apparent brightness (from observation) to its actual or true brightness (from its observed period and Leavitt’s Law), astronomers could then determine how far away it is. This new standard candle worked for any Cepheid variable star, and extended astronomical distance measurements to some 10 million light-years!