In the late 1800s, William Pickering, the director of the Harvard College Observatory, had a huge number of photographic plates of star spectra. Long ago, photosensitive chemicals were deposited on plate glass, loaded into cameras attached to telescopes. These telescopes had circular prisms in front to cause star light to be dispersed into a rainbow or spectra of colors.
These spectra were crossed by dark lines caused by the absorption of atoms in the star’s atmosphere. So by the stars’ spectra, one could tell what elements were present in the atmospheres of stars.
Pickering had men to classify the spectra of stars, but the work was tedious. One day, Pickering stated that his maid could do a better job than the men. So Pickering employed his maid, a Scottish immigrant named Williamina
Fleming, to analyze plates of stellar spectra. Fleming had worked as a teacher in Scotland and had a natural curiosity about science. Fleming realized that the spectra were sort of a stellar fingerprint, depending not just on the composition of the stars but also their surface temperature. Fleming developed a stellar classification scheme using the letter of the alphabet — A for the stars with the strongest hydrogen lines all the way down to O for the weakest hydrogen lines.
In 1896, Annie Jump Cannon joined the Harvard classifiers (known as the computers) and became a rapid classifier of stellar spectra. During her career Cannon classified 330,000 stars. Cannon also rearranged the spectral classes, starting with O (hottest stars), then B stars, A stars with the strongest hydrogen lines, followed by F stars, G stars (our sun is one), then K stars, finishing with M stars (the coolest stars). The mnemonic to remember the seven spectral classes in order is: “O Be A Fine Girl, Kiss Me.”
When Cannon’s classification was placed on a vertical plot of luminosity or power versus surface temperature, most stars fall on a broad line called the main sequence. Our sun is near the middle of the main sequence, both in power and surface temperature. There are four types of stars: 90% are in the main sequence.
The next most populous stars are giant stars, typically dozens of times as wide as our sun and hundreds of times more powerful. In the lower left corner are the white dwarfs, exhausted star cinders about the size of the Earth. At the top of the plot are the supergiants, stars hundreds of times wider than our sun and many thousands of times more powerful.
The next pioneer, Cecilia Payne, came to America in 1923 as English universities did not give graduate degrees in astronomy to women. In 1920, the Indian astrophysist Meghnad Saha discovered the connection between stellar temperature and the intensity of spectral lines of different elements.
Before Payne, most astronomers believed that stars had compositions similar to the Earth’s crust. Payne, using Saha’s equation showed that stars were primarily hydrogen, a lesser amount of helium and relatively small amounts of the other elements. Payne became the first woman to receive a Ph.D. in astronomy. She married fellow astronomer Sergey Gaposchkin, collaborating on a huge catalog of stellar variable stars still in use today. By weight, the atmospheres of stars are 75% hydrogen, 23% helium and 2% of other elements.
Sky sights for the coming week
Sunrise is about 7:22 a.m., sunset about 6:41 p.m., giving us 11 hours and 16 minutes of daylight.
On Oct. 12, the evening moon will be half full (like a “D”) in the southwestern evening sky, offering fine views of its craters through a telescope.
The brilliant planet Venus is setting in the Southwest about 8:35 p.m. The bright planet Jupiter and Saturn are in the South about 8:30 p.m.
Bob Doyle is a retired science teacher at Frostburg State University who is available to talk to adult and student groups about matters related to his columns. Contact him at email@example.com.