Leavitt’s Standard Candles

Title: 1777 variables in the Magellanic Clouds (1908)
Periods of 25 Variable Stars in the Small Magellanic Cloud (1912)
Authors: Henrietta Swan Leavitt (with Edward C. Pickering in 1912 paper)
First Author’s Institution: Harvard College Observatory
Status: Published in Harvard College Observatory Circular, vol. 173 (1908) and Annals of Harvard College Observatory, vol. 60 (1912) open access

Henrietta Swan Leavitt, Credit: AAVSO

Today we take it for granted that our galaxy, the Milky Way, is just one galaxy of billions in the universe. We also have a good idea of the Milky Way’s size. However, just a century ago this was not obvious at all. Two equally bright stars could be at the same distance and equally bright, or more likely, one was much closer but dimmer. Similarly, fuzzy ‘blobs’ named nebulae were not believed to be other distant galaxies but much smaller and closer objects. Only the distances to the closest stars could be found, by measuring their parallaxes (the relative change in the position of close stars against an unmoving background of stars).
This changed in part due to the technological development of astronomical photographic plates. Realising the power of being able to permanently record the night sky using this relatively new technology, Harvard College Observatory director Edward C. Pickering initiated a large campaign to monitor the night sky using wide angle cameras to catalogue the stars. As astronomers today appreciate, it is much easier to use computers to look at lots of data, but at the time the computers were human and often women. The author of today’s classic papers, Henrietta Swan Leavitt, was one of the ‘computers’ employed at the Harvard College Observatory along with Williamina Fleming and Annie Jump Cannon. Leavitt’s job was to analyse the many photographic plates.

Finding variable stars

Henrietta Swan Leavitt measured the positions and brightnesses of stars in the Magellanic Clouds as recorded in the photographic plates. Leavitt stacked two photographic plates of the Small Magellanic Cloud taken on different nights on top of each other, and noticed that a number of stars changed brightness. Examining further plates, she found 57 new variable stars in the Small Magellanic Cloud (SMC). Excited by this discovery, more photographic plates were taken at the southern Harvard College Observatory in Arequipa, Peru before being shipped to Cambridge, USA for analysis, along with plates of the Large Magellanic Cloud. Through painstaking measurement of the positions and brightnesses in these frames (check out an example frame here), an astounding 1777 new variable stars were found in the Magellanic Clouds, with positions and minimum and maximum brightnesses reported in Leavitt’s 1908 paper.

For 16 variable stars in the SMC which were bright enough to be seen in a sufficient number of photographic plates, Leavitt was able to determine their periodicity, i.e. the time from minimum brightness to maximum and back to minimum again. She recorded the range of their brightnesses along with their periods in her 1908 paper, noting, ‘It is worthy of notice that in Table VI the brighter variables have the longer periods’. Leavitt also commented that ‘those having the longest periods appear to be as regular in their variations as those which pass through their changes in a day or two’, indicating that a similar process was causing the variation in brightness at these different periods.

Figure 1: Example lightcurve of a classical Cepheid variable star showing the characteristic periodic sharp increase and slower decrease in brightness. Credit: Wikipedia.

Confirming suspicions

While Leavitt initially felt that a sample of 16 SMC variable stars was not large enough to draw general conclusions, when her 1912 paper increased this sample to 25 she confirmed that period and measured (apparent) brightness were related. By using the simplification that all the variable stars in the SMC are at approximately the same distance she reasoned that the stars’ intrinsic luminosities were directly related to their periods. Now these stars are called Cepheid variables. Figure 2 shows that the log of period is directly proportional to the log of luminosity (measured in magnitudes), in what we now call the ‘Period Luminosity Relation’ or ‘Leavitt’s Law’.

Figure 2: Luminosity in magnitudes (y axis) plotted against period (x axis). Left figure shows the period in days, the right figure scale is log(period). The upper points, plotted with a straight line represent the maximum observed luminosity while the lower points show the minimum observed luminosity. Figures 1 and 2 in the 1912 paper.

Building on Leavitt’s legacy

If the distances of some Cepheid variable stars could be measured using another method then the distance could be calibrated. As Leavitt had hoped, measuring the distances to nearby Cepheid variable stars using parallaxes provided the direct relationship between period and luminosity. If you could simply measure the period of a Cepheid, you could find its intrinsic luminosity and therefore its distance, no matter how far!
Unsurprisingly, being able to determine the distance to stars is very useful in astrophysics, with Cepheids acting as ‘standard candles’. Studying Cepheids was key for American Astronomer Harlow Shapley to measure the distances to globular clusters and determine the size of our galaxy, the Milky Way. Edwin Hubble’s detection of Cepheids in the ‘spiral nebulae’ Andromeda and the Triangulum was key to determine their distances and discover they are in fact their own galaxies, drastically increasing the scale of the known universe. Cepheids continue to act as a step in the ‘cosmic distance ladder’, allowing us to calibrate and measure progressively larger distances to determine the Hubble constant and the age and expansion of the universe.

While Leavitt’s job was as a ‘computer’ at the time, her work was key to Hubble’s discovery and she truly was an astronomer in her own right. Perhaps if she had lived longer she would have received more credit in her lifetime. Her work certainly impressed Swedish mathematician Gösta Mittag-Leffler, who wrote to her, expressing his wish to nominate her for the Nobel Prize in Physics, not realising she had died. I wanted to finish with this quote about Leavitt from CfA Wolbach Librarian Maria McEachern (to the Harvard Gazette):
“One of the profound truths which comes to mind when I think of Henrietta, her work and its influence, is best expressed in a quotation attributed to Sir Isaac Newton: ‘If I have seen further, it is by standing on the shoulders of giants.’ Beginning with Edwin Hubble’s discovery in 1921, and continuing on to the present day, Henrietta Swan Leavitt’s slender shoulders have continued to support truly groundbreaking research.”

About Emma Foxell

I am a PhD student at the University of Warwick. My project involves searching for transiting exoplanets around bright stars using telescopes on the ground. Outside of astronomy, I enjoy rock climbing and hiking.

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