Knut Lundmark was one of the pioneers in the modern study of the galaxies and their distances. He was one of the first to suspect that the galaxies are remote stellar systems at vast distances and not nearby objects belonging to our own galaxy, the Milky Way. In 1919 he measured the distance to M31 - the Andromeda Galaxy, to 650,000 light years (about a fourth of the present day value) using magnitudes of novae found in M31 and comparing them to nearby ones with known distances. Lundmark's work contributed to the later famous Great Debate.
Lundmark also studied the light distribution in the galaxies, and discovered that the distribution could only properly be explained if the galaxies contained vast amounts of light-blocking dark clouds.
He was the leading writer of popular astronomy among the professional astronomers in Sweden from the 1930s and onwards. He also often appeared in the Swedish national radio with programs on popular astronomy and the history of science. He made generations of Swedes fascinated and interested in astronomy.