Philip Childs Keenan
Encyclopedia
Philip Childs Keenan was an American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

.

Keenan was an American spectroscopist who collaborated with William Wilson Morgan
William Wilson Morgan
William Wilson Morgan was an American astronomer.The principal theme in Morgan's work was the study of stellar and galaxy classification. Along...

 and Edith Kellman (1911–2007) to develop the MKK stellar spectral classification system between 1939 and 1943. This two-dimensional classification system (temperature & luminosity) was further revised by Morgan and Keenan in 1973. The MK system remains the standard stellar spectral classification system used by astronomers today.

During their long collaboration, Keenan tended to focus his research on stars cooler than the Sun, while Morgan emphasized the hotter stars. Keenan had a long and productive career, publishing his final scientific paper in 1999, seventy years after his first.

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