W. W. Morgan and the Discovery of the Spiral Arm Structure of our Galaxy
  William Wilson Morgan was one of the great astronomers of the twentieth
century. He considered himself a morphologist, and was preoccupied throughout
his career with matters of classification. Though his early life was difficult,
and his pursuit of astronomy as a career was opposed by his father, he took a
position at Yerkes Observatory in 1926 and remained there for the rest of his
working life. Thematically, his work was also a unified whole. Beginning with
spectroscopic studies under Otto Struve at Yerkes Observatory, by the late
1930s he concentrated particularly on the young O and B stars. His work on
stellar classification led to the Morgan-Keenan- Kellman [MKK] system of
classification of stars, and later - as he grappled with the question of the
intrinsic color and brightness of stars at great distances - to the
Johnson-Morgan UBV system for measuring stellar colors. Eventually these
concerns with classification and method led to his greatest single achievement
- the recognition of the nearby spiral arms of our Galaxy by tracing the OB
associations and HII regions that outline them. After years of intensive work
on the problem of galactic structure, the discovery came in a blinding flash of
Archimedean insight as he walked under the night sky between his office and his
house in the autumn of 1951. His optical discovery of the spiral arms preceded
the radio-mapping of the spiral arms by more than a year. Morgan suffered a
nervous breakdown soon after he announced his discovery, however, and so was
prevented from publishing a complete account of his work. As a result of that,
and the announcement soon afterward of the first radio maps of the spiral arms,
the uniqueness of his achievement was not fully appreciated at the time.
