Robert Julius Trumpler

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Robert Julius Trumpler (October 2, 1886 – September 10, 1956, Berkeley, California) was a Swiss-American astronomer.

He was born in Zürich, Switzerland and obtained his early education in that country. He then studied in Germany where he earned his Ph.D. in 1910. In 1915, during World War I he emigrated to the United States, and joined the University of California. He took a position at Allegheny Observatory, and later went to Lick Observatory. In 1921, he became a naturalized citizen of the United States.

He is most noted for observing that the brightness of the more distant globular clusters was lower than expected, and the stars appeared more red. This was explained by the interstellar dust scattered through the galaxy, resulting in the absorption (extinction) of light. Since globular clusters had been used to estimate the size of the milky way, this led to a reduction in the estimated size of our galaxy by about 40%. The absorption of light by interstellar dust was discovered independently by Boris Vorontsov-Velyaminov.

Trumpler also studied and catalogued open clusters in order to determine the size of the Milky Way galaxy. At first he thought his analysis placed an upper limit on the Milky Way's diameter of about 10,000 parsecs with the Sun located somewhat near the center although he later revised this. While catalogueing open clusters he also devised a system for their classification by according them designations for their central concentration, the range of brightness of their individual stars and the number of stars in the cluster. This system of classifying open clusters is still in use today.