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Every astronomy textbook lists stars as O, B, A, F, G, K and M, and almost every student wonders why the letters are in such a strange order. The answer goes back to Annie Jump Cannon, who classified hundreds of thousands of stellar spectra and built the system astronomers still use. This post explains where the order came from, links it to the black-body physics in Class 11 and 12, and ends with what students can learn from her methods.
Who was Annie Jump Cannon?
The MacTutor biography of Annie Jump Cannon records that she was born on 11 December 1863 in Dover, Delaware, in the USA. She graduated from Wellesley College in 1884 as valedictorian, with a degree in physics. After a trip to Spain in 1893, a bout of scarlet fever left her almost deaf.
In 1896 she joined the Harvard College Observatory as an assistant to its director, Edward C. Pickering. She worked in a team of women hired to examine photographic plates of stellar spectra, led by Williamina Fleming.
Why are the star types in the order O, B, A, F, G, K, M?
The letters were first handed out in an older scheme, and when Cannon sorted the spectra into a smooth sequence the alphabet ended up scrambled. MacTutor explains that she "simplified Fleming's scheme by removing and reordering some letters, to give the classes O, B, A, F, G, K, M." A rival scheme by her colleague Antonia Maury used 22 groups with extra subdivisions for line sharpness. Cannon's simpler sequence was the one that lasted.
Cannon arranged the spectra by the patterns of their dark lines. MacTutor notes that it was soon realised her sequence was based on the stars' temperatures, from O the hottest to M the coolest, and the International Astronomical Union adopted her classifications on 9 May 1922. Our profiles of Meghnad Saha and Cecilia Payne explain how physics made sense of her classes.
How many stars did she classify?
The scale was enormous. By 1913, MacTutor reports, she was working at a rate of about 200 stars an hour. The Henry Draper Catalogue, the result of this work, was published between 1918 and 1924 and contains 225,300 stars. The modern machine-readable edition held at the VizieR catalogue service (III/135A) lists Cannon and Pickering as authors of the original catalogue (1918 to 1924) and Cannon as author of the Henry Draper Extension (1925 to 1936). Astronomers still refer to stars by their HD numbers.
Worked example: what Wien's law says about each class
Wien's displacement law gives the wavelength at which a star's light is brightest: λmax = b/T, where the CODATA value is b = 2.898 × 10⁻³ m K and T is the surface temperature in kelvin.
The Sun (class G). The NASA Sun fact sheet gives an effective temperature of 5772 K. So λmax = 2.898 × 10⁻³ / 5772 = 5.02 × 10⁻⁷ m = 502 nm, in the visible band.
A hot star at 30,000 K. λmax = 2.898 × 10⁻³ / 30,000 = 9.66 × 10⁻⁸ m = 96.6 nm, in the ultraviolet.
A cool star at 3000 K. λmax = 2.898 × 10⁻³ / 3000 = 9.66 × 10⁻⁷ m = 966 nm, in the near infrared.
Working backwards. If a star's spectrum peaks at 400 nm, then T = b/λmax = 2.898 × 10⁻³ / (4.00 × 10⁻⁷) = 7245 K, hotter than the Sun.
Notice that ten times the temperature gives one tenth of the peak wavelength. That inverse relation is why hot stars look blue-white and cool stars look orange-red. Our guide to black-body radiation has more problems of this type.
Common mistakes. Using nanometres in the formula without converting to metres, so the answer comes out a billion times off. And assuming the peak colour is the colour you see: the Sun peaks near 502 nm, which is blue-green, yet it looks white because it emits strongly across the whole visible band.
What recognition did she receive?
MacTutor lists honours that were rare for women scientists of her time. In 1925 the University of Oxford gave her an honorary Doctor of Science degree; a news report of the time, quoted by MacTutor, said it was the first time Oxford had given such a degree to a woman. In 1931 the National Academy of Sciences presented her with the Henry Draper Medal. She used the money from another prize to fund the Annie J. Cannon Award for women astronomers, first given in 1934. In 1938 she was appointed William Cranch Bond Astronomer at Harvard. She died on 13 April 1941 in Cambridge, Massachusetts.
What can a student take from how she worked?
Simplify the categories. She cut a messy scheme down to a few classes that everyone could use. When notes grow cluttered, ask which distinctions really matter.
Speed comes from volume. Her rate of hundreds of spectra an hour came after years of looking. Fast recognition in exams comes the same way, from many solved problems.
Classification comes before explanation. She sorted stars before anyone knew why the sequence worked, and the physics came later. A careful table of patterns is often the first step to understanding.
Open doors for others. She turned prize money into an award for women astronomers.
If the link between temperature and colour is still fuzzy, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, including black-body radiation.
Your next step: pick three bright stars, look up their spectral classes, and use Wien's law to estimate where each one's light peaks.
