21/01/1908 Danish astronomer Bengt Strömgren is born
Strömgren sphere - Video Learning - WizScience.com
He found that the chemical composition of stars was very much different than previously assumed. In the late 1930s, he found the relative abundance of hydrogen to be nearly 70%, and helium to be about 27%. He also discovered the so-called Strömgren Spheres — huge interstellar shells of ionized hydrogen around stars.