Robert Julius Trumpler (October 2 1886 – September 10 1956) was a Swiss-American astronomer.
He was born in Zurich. In 1915, he came to the United States to take a position at Allegheny Observatory , and later went to Lick Observatory. In 1921, he became a naturalized citizen of the United States.
He is best known for his work on galactic clusters, demonstrating the phenomenon of absorption of light (extinction) by interstellar dust.
A crater on Mars is named in his honour.
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