. On this day in 1887 – October 25th – American astronomer Henry Norris Russell was born in Oyster Bay, New York.
Russell was one of the major figures of early 20th century astronomy. He's best known today for his part in creating what's now called the Hertzsprung-Russell diagram which demonstrates stellar evolution. (Danish astronomer Ejnar Hertzsprung independently made the same discovery.)
. On this day in 2006 – October 25th – NASA launched STEREO.
STEREO was a solar observation mission using two identical spacecraft in two slightly different orbits. That permitted stereoscopic imaging of the Sun. The mission lasted longer than the two years for which it was planned, but in 2014 STEREO-B malfunctioned and couldn't be recovered. STEREO-A is still operational.
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