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Introduction: Late in the 19th century, Harvard astronomer Edward Pickering wanted to sort and catalog the thousands of star spectra that had been collected by the Harvard Observatory. He hired several women to do the work, paying them 25 cents a day. The most prominent of these women was Annie Jump Cannon, who devised a classification system still used today.

Harvard Classification Scheme
Class
Color
Prominent spectral lines
Surface Temp. (K)
O
Blue
Ionized helium, hydrogen
> 25,000
B
Blue-white
Neutral helium, hydrogen
11,000 – 25,000
A
White
Hydrogen, ionized sodium, ionized calcium
7,500 – 11,000
F
White
Hydrogen, ionized sodium and calcium; neutral sodium and calcium
6,000 – 7,500
G
Yellow
Neutral sodium and calcium, ionized calcium, ionized iron, ionized magnesium
5,000 – 6,000
K
Orange
Neutral calcium, neutral iron, neutral magnesium
3,500 – 5,000
M
Red
Neutral iron, neutral magnesium, and neutral titanium oxide (not shown)
< 3,500

1 answer

The Harvard Classification Scheme was an innovative system developed by Annie Jump Cannon and others to categorize stars based on their spectral characteristics. It classifies stars into different spectral classes from O to M, each with distinct colors, spectral lines, and surface temperatures. Here’s a summary of the classification:

  1. Class O

    • Color: Blue
    • Prominent Spectral Lines: Ionized helium, hydrogen
    • Surface Temperature: Greater than 25,000 K

  2. Class B

    • Color: Blue-white
    • Prominent Spectral Lines: Neutral helium, hydrogen
    • Surface Temperature: 11,000 to 25,000 K

  3. Class A

    • Color: White
    • Prominent Spectral Lines: Hydrogen, ionized sodium, ionized calcium
    • Surface Temperature: 7,500 to 11,000 K

  4. Class F

    • Color: White
    • Prominent Spectral Lines: Hydrogen, ionized sodium and calcium; neutral sodium and calcium
    • Surface Temperature: 6,000 to 7,500 K

  5. Class G

    • Color: Yellow
    • Prominent Spectral Lines: Neutral sodium and calcium, ionized calcium, ionized iron, ionized magnesium
    • Surface Temperature: 5,000 to 6,000 K

  6. Class K

    • Color: Orange
    • Prominent Spectral Lines: Neutral calcium, neutral iron, neutral magnesium
    • Surface Temperature: 3,500 to 5,000 K

  7. Class M

    • Color: Red
    • Prominent Spectral Lines: Neutral iron, neutral magnesium, and neutral titanium oxide
    • Surface Temperature: Less than 3,500 K

Annie Jump Cannon's work not only advanced the field of stellar astronomy but also highlighted the significant contributions of women in science during a time when their roles were often overlooked. The classification system continues to be a cornerstone in stellar astrophysics, helping astronomers understand the properties and evolution of stars.