April 13 marks the passing of Annie Jump Cannon. Cannon was an American astronomer who developed the star classification system in use today.
Astronomy students learn the system using the mnemonic “Oh! Be A Fine Girl — Kiss Me!” The system ranks stars by their temperature, color and spectrum lines by seven categories.
- O type stars are the hottest stars. They are often blue-white in color and have strong helium spectral lines.
- B type stars are blue-white hot stars with strong helium II lines and strong hydrogen I lines.
- A type stars are white stars with strong Balmer hydrogen lines.
- F type stars are yellow-white stars with strong calcium II lines and weaker Balmer hydrogen lines. They also contain weak metal element spectral lines like iron and chromium.
- G type stars are yellow stars with distinct calcium II lines and stronger metal element lines. The Sun is a G type star.
- K type stars are cooler orange stars with strong calcium II, hydrogen, and potassium lines. The overall spectrum contains several metal element lines.
- M type stars are red stars containing molecular spectral lines and strong metal spectral lines.
This is a general sketch of the system Cannon used. She introduced subcategories of each type ranging from 0 to 9. For example, a white star with very strong Balmer lines would be an A0 star and a white star with weak Balmer lines would be classified as an A9 star.
Cannon earned a physics undergraduate degree but could not obtain a position which utilized her skills. Women were generally not accepted in positions in advanced scientific work. The director of the Harvard University Observatory, Edward Charles Pickering started hiring women as “computers” and “recorders”. He began hiring women after proving his maid could do a better job at the finicky work of star classification than the men who previously held the positions. As the department filled up with women employees, they were collectively known as Pickering’s Harem.
Cannon joined Harvard to cataloging variable stars and classifying stars in the Southern sky. She proved to be exceptionally quick at any task handed to her. She could identify a star on a blurry photographic plate, match it to its spectra on another sheet, classify the star, and record her decision at a rate of three a minute. She averaged 5,000 stars a month and managed to catalog 225,300 stars by the time Harvard published the Henry Draper Catalog in 1924.
Cannon achieved many firsts in astronomy. She was the first woman to receive an honorary doctorate from Oxford University, the first woman to win a Henry Draper medal, and the first woman to be elected as an officer in the American Astronomical Society. The AAS even established an Annie Jump Cannon Award to be awarded to female astronomers who make distinguished contributions to astronomy within five years of receiving their doctorate.
Notable Science History Events for April 13
2008 – John Archibald Wheeler died.
Credit: E. Mielke
Wheeler was an American theoretical physicist who made many contributions to nuclear physics and the Unified field theory. One of his early contributions was the Breit-Wheeler process where particles are created from photons. In particular, when photons create positron-electron pairs.
He coined the term “black hole” to describe the effect of gravitational collapse in the theory of general relativity. He also coined the term “neutron moderator” to describe materials that slowed down fast neutrons in nuclear reactors. When Fermi was building the Chicago-1 nuclear reactor, the term for this type of material was “slower downer”. Neutron moderator sounds a little more scientific.
Other well known terms he added to modern physics are “quantum foam” to describe a possible structure of spacetime at the smallest levels, “wormhole” for a mathematically predicted connection of two separate points in spacetime, and “it from a bit” for his theory where a particle or field (it) derives its existence from the information (bit) derived from observing the “it”.
1960 – First navigational satellite launched.
US Navy
NASA and the US Navy launched the first of the Transit series of navigational satellites. Transit-1B was designed to help Navy ships determine their position at sea. The Transit system would be able to pinpoint a ship’s location within 500 feet in the early days of the program to 80 feet by the end of the program in 1996. These satellites lead to the current GPS (Global Positional System) satellite technology.
1941 – Michael Stuart Brown was born.
Brown is an American biologist who shares the 1985 Nobel Prize in Medicine with Joseph Goldstein for their discoveries on how cholesterol metabolism is regulated. They found that cells remove cholesterol from the bloodstream by low-density lipoproteins. This discovery would lead to the statin drugs to lower cholesterol used by many people today.
1941 – Annie Jump Cannon died.
1905 – Bruno Rossi was born.
Rossi was an Italian-American physicist who pioneered cosmic ray research and x-ray astronomy. He showed cosmic ray intensity from the West is significantly higher than the intensity from the East. This demonstrated cosmic rays are mostly composed of positively charged particles.
Rossi designed the instrumentation on NASA’s Explorer 10 discovery of Earth’s magnetopause. His instruments also detected the first extra-solar X-ray source, Scorpius X-1.
1890 – Eugène Melchior Péligot died.
Péligot was a French chemist who first isolated the element uranium. He produced the metal sample by heating a salt of uranium with potassium.
He also discovered the methyl radical (-CH3) with Jean-Baptiste Dumas while experimenting with methanol.