On this day in 1918 - November 19th - Dutch astronomer and mathematician Hendrik C. van de Hulst was born.

Hulst predicted the existence of the 21 cm hyperfine line of neutral interstellar hydrogen. When it was discovered, he worked with Jan Oort and others to use radio astronomy to map out the neutral hydrogen in the Milky Way, which first revealed our galaxy's spiral structure.


Mona Evans
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