During the 1880s and up until 1989, a technique of astronomy known as glass plates or photographic plates was widely used to map stars, clusters, and other astronomical objects. The most important collection is held at the Harvard College Observatory and contains more than 500,000 plates with information about stars: data which was collected through telescopes and observation was recorded with detail on these see-through plates and today consist in an archival insight into the stellar past and thus the evolution of our universe.

The techniques used to map the skies

Indeed, there were a few ways in which astronomers used these glass plates to register their observations. Most of the time, simply transparent glass sheets were used as a slate for astronomers to indicate the position of stars with dark spots. However, a fraction of them used another technique: marking greyish smears (resembling the shade declinations of a rainbow) on the plate instead of dots. These represented the spectra of stars instead of their positions.

Now we might wonder what these seemingly arbitrary small black dots or grey smears tell us about astronomy. Well, the explanation lies behind meticulous and utterly precise work. Astronomers used these photographic plates to establish a system of classification by spectral light that categorized stars and attributed their key properties to their emission and absorption spectra: tedious work that benefitted many in understanding the behavior of stellar objects. Therefore, more than drawing the observations through telescopes (which were mainly conducted by men), the value of the photographic plates lay in the classification systems many women astronomers were able to use to sort hundreds of thousands of them.

Look at this Brilliant Technique Used to Map Stars and Galaxies For More Than 100 Years 1
A photographic plate of the 1919 total solar eclipse, taken by Andrew Claude de la Cherois Crommelin and Charles Rundle Davidson during an expedition to Sobral, Brazil | Image: smithsonianmag.com

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Look at this Brilliant Technique Used to Map Stars and Galaxies For More Than 100 Years 2
An image of the moon on one of the photographic plates from the collection of George Ellery Hale, found in the Hale Solar Laboratory. It was taken at Lick Observatory and dated July 19, 1891. The white markings are a result of damage from water. |Image: Carnegie Institution for Science / Dan Kohne

The woman to whom we owe the glass plates/ photographic plates

The technique of photographic plates was first developed by female astronomer Williamina Fleming who put into place a system of spectra classification in sequences of 15 letters. It was valued for its simplicity and efficiency compared to other systems with more complicated sequences containing 22 letters which considerably increased the number of arrangements.

Williamina Fleming
Williamina Fleming is also credited with the discovery of the first white dwarf star

Then, a few years later, Annie Jump Cannon unified all theories of classification in a more harmonious system. In addition, she brought about the founding of stellar classification as we still use it today: using the letters O, B, A, F, G, K, and M. Finally, Cannon also started to spot different types of stars such as variable stars and novae which Fleming had previously classified. In short, Cannon began to label the classifications in more tangible categories.

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What did the glass plates teach us?

Glass plates were at the founding of our understanding of the chemical composition of stars and our universe. This is thanks to the information and interpretation Cecilia Payne-Gaposchkin was able to extract from them. She understood the meaning behind stellar spectra, linking the chemical composition of stars’ atmospheres to the wavelengths they could absorb.

Cecilia Payne
Cecilia Payne

She discovered that despite most of the spectra looking extremely different from one another, the chemical compositions were still extremely similar, consisting mainly of helium and hydrogen. Therefore, the shift in spectra was mostly due to the different temperatures of the stars rather than their composition. Thus, Payne-Gaposchkin laid the basis of a spectroscopic technique still used today, and that through the mere observation of glass plates and the realm of information they hold.

Photographic plates andromeda galaxy
One of the glass photographic plates showing the Andromeda galaxy, taken at the Karl Schwarzschild Observatory in 1965 | Image: Jay Bennett

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