Swedish physicist Angstrom discovered this in 1958. He later used this to determine that the sun is made from Hydrogen. During a full solar eclipse, when the moon blocks out the brightness of the sun, you can only then see the sun’s corona enough to determine its composition.
Interesting about this fact, is that the moon is currently at a distance from the earth to actually block out the sun entirely. If it wasn’t, it would have taken people a lot longer to figure that the sun is mostly made of Hydrogen.