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SET Women - Leicestershire Women in Science Engineering & Technology

e-Book - Discoveries by hearing impaired women

Work at the Harvard College Observatory has led to several major discoveries about the universe, and many of them by women. It was the practice there from 1880 through 1920 to hire women on a half-pay scale to analyse data taken by Harvard's many telescopes around the world. Called "The Calculators", these women proved to be more than data crunchers when they interpreted what they were measuring, and developed theories and methods to explain the universe that are still used today.

Two of these famous women were deaf, and here are the stories of their long-lasting contributions to our understanding of the universe:

The Secret of Periods
Until the mid 20th century, we didn't know the size of the galaxy, let alone the universe. Measuring distances that you couldn't walk or fly to was and still is not easy. Sure, the dimmer a star looks, the farther away it might be -- but astronomers quickly learned that many stars were actually shining more than others. Local dim stars whose distances were measurable turned out to be shining ten times brighter than our Sun if you could put them side-by-side! It took some clever female intuition to measure beyond our local stars, and this foresight helped astronomers learn there were many galaxies of stars in our vast universe.

Henrietta Swan Leavitt was an astronomer who searched photographic plates for stars that changed their brightness, known as variable stars. There are several types of variable stars, but she is most famous for her work on Cepheid variables, named after the star Delta Cephei. Cepheids become much brighter for a short period of time, and then go back to their original brightness just as predictably. We now know this is because they swell up as quickly as once a day to once every 50 days, producing up to 1000 times more light than our Sun, before relaxing again. We should be glad we don't orbit a Cepheid.

Leavitt concentrated her work on 25 Cepheids in the Small Magellanic Cloud (SMC), a haze of dim stars visible in the Southern Hemisphere night sky. Her leap of faith was to decide that the SMC was a true clump of stars that were actually all at the same distance away from her, so that any differences in brightness she saw among its stars were their true brightness differences and not just because some were closer or further away from her. This meant she could compare these stars side-by-side where she wanted them, a powerful tool where light is concerned.

Light gets dimmer as the square of the distance change away from you. So, if you were a metre away from a torch and then walked another metre from it, you will have doubled your distance from the torch: two metres are twice than one metre, so you've changed your distance by a factor of two. Since the light falls off by the square of the change, two times two is four, and so the torchlight will thus appear four times dimmer to you. In this way, if you had two identical torches and put them at different but unknown distances from you, you would see them at different brightnesses. If you could measure exactly how much light was coming out of the torch bulbs and exactly how little you were receiving at your distance away, you could use the maths above to figure backwards how far away each torch must be. Hoorah for maths, but what does this have to do with Miss Leavitt?

Cepheids, Leavitt learned, held a secret: the brighter the Cepheid, the longer its period of flaring up. Her calculations were so good, that if you told her the period of any Cepheid, she could tell you how bright it appeared to you. But until she knew exactly how much light was coming from its surface, she couldn't know exactly how far away that light had travelled.

Astronomers after Leavitt finished the job she started in 1912 by learning the distances of nearby Cepheids with different periods. Using a calibrated version of Leavitt's function, astronomers eventually calculated that the SMC was 210,000 light years away - that's 1,200,000,000,000,000,000 miles. Learning the distance to the SMC started a huge debate, because it meant that there were large objects outside of our own galaxy and therefore, the universe was a much larger place than we thought. We now know the SMC is a tiny clumpy galaxy of its own orbiting our larger spiral galaxy like a moth to flame.

Leavitt image courtesy of Wisconsin State University, SMC photo courtesy of Anglo-Australian Observatory/Royal Obs. Edinburgh.

The Colour of Kisses
If you look up to the sky on a clear night, you should notice the stars come in different colours. A great example is to look at Orion, a winter constellation. Even since ancient times these differences were known and used to divide the sky into beasts and men, with the bloodiest red stars marking their fiery eyes and beating hearts. It took scientific investigation to learn that the colour of stars tells you their temperature, and that temperature can reveal composition and mass. It's amazing how much we can learn about something just from the light it gives off!

Annie Jump Cannon came to the work of classifying stars in 1896 after completing her Masters degree in astronomy from Wellesley College in Massachusetts, USA. Three women had preceded her, accumulating data on tens of thousands of stars and several complicated explanations of why they were the colour they were. Cannon worked on the Southern Hemisphere stars, and quickly created a classification scheme all on her own that was simpler but worked on much of the same strong scientific principles of her predecessors.

In her work, Cannon used the spectra, or the spread rainbow of light, from over 400,000 stars she catalogued to develop her new classification system. Like the women before her, she knew the spectra of a star could tell you very specifically what is glowing in the star. By spreading out bright light with a prism, it is possible to actually see the individual colours of light (wavelengths) that make up the brightness.

The resolution can be so good, that you can even see which wavelengths are brighter or dimmer than the others, and the pattern of these selected colours tells you which elements are present. If there is anything like a gas between you and the glow, you will see gaps in the gas spectrum where the gas has absorbed those wavelengths that were coming at it. By burning a bunch of elements on Earth and comparing the pattern of wavelengths to those seen in star spectra, it is a fairly straightforward task to match them up and hence learn what is in a star many trillions of miles away.

What Cannon perfected was a means of ranging the stars based on those patterns in their spectra. This may sound simple, but with the hundreds of thousands of stars she was observing, there were many different permutations of arrangement possible. (You can try your hand at classification by cutting out the next page and arranging the seven spectra in some logical way. The answer key is in the back of the book.) Cannon created the classification scheme known as "O,B,A,F,G,K,M" into which all known stars could be placed. It ranges from bluest to reddest, with seven sliding points within each of the letter categories to refine the temperature range.

Astronomers still learn the spectral classes today as "Oh, Be A Fine Girl, Kiss Me!" and have added since Cannon's time the additional classes of R, N, and S -- "Right Now, Smooch!" Although it sounds awfully silly, this scheme tells an astronomer how fearsomely a star is shining. With that knowledge of what makes it shine, she can tell what's happening deep inside its nuclear powered core, how long it will shine for, and what will happen when it runs out of fuel.

It was with this understanding that we know now our Sun is a G2 type star, a yellow dwarf that is unimpressive in the universe. It will shine for another 5 billion years before bloating like a Cepheid and boiling Earth's atmosphere into space. It will bloat so large as to spill its outer self out across the Solar System, leaving behind its core, a small white-hot star, and the planets will continue to orbit it as if little had happened. Cannon's work helped us to see into the future.

Orion image copyright David Malin, Cannon image courtesy of the AAVSO.