21 January
1908: On 21 January 1908, Bengt Strömgren was born in Copenhagen, Denmark.
Strömgren made important research in stellar structure in the 1930s, but he is best known for his theory of the ionised gas clouds around hot stars.
He found relations between the gas density, the luminosity of the star, and the size of the 'Strömgren sphere' of ionised hydrogen around it.
