Knut Emil Lundmark (born 14 June 1889 in Älvsbyn, Sweden; died 23 April 1958 in Lund, Sweden) was a Swedish astronomer, a professor of astronomy, and the head of the observatory at Lund University from 1929 to 1955.

Lundmark studied astronomy at the Uppsala University Observatory. His dissertation, completed in 1920, was titled The relations of the globular clusters and spiral nebulae to the stellar system. In the 1920s, he worked at several observatories in the United States, including the Lick Observatory and the Mount Wilson Observatory.

Knut Lundmark was among the first scientists to study galaxies and their distances. He was one of the first to think that galaxies are faraway star systems at great distances, not nearby objects in our own galaxy, the Milky Way. In 1919, he measured the distance to M31, the Andromeda Galaxy, as 650,000 light years (about one-fourth of today’s estimated value). He used brightness measurements of novae in M31 and compared them to novae in nearby objects with known distances. His work helped shape the famous debate over whether nebulae were galaxies or clouds of glowing gas.

Lundmark also studied how light is spread across galaxies. He found that the light distribution could only be explained if galaxies contained large amounts of dark clouds that block light.

From the 1930s onward, Lundmark was the leading writer on popular astronomy among professional astronomers in Sweden. He often appeared on Swedish national radio, sharing programs about astronomy and the history of science. His efforts inspired many Swedes to become interested in astronomy.

A lunar crater, Lundmark, and a minor planet, 1334 Lundmarka, were named after him. The Wolf-Lundmark-Melotte Galaxy is named after Lundmark, Max Wolf, and Philibert Jacques Melotte.

Lundmark is also well-known among Chinese astronomers for his important work on the catalog of novae.

Dark matter

A publication found recently in 1930 shows that Knut Lundmark was the first to realize the universe must have much more mass than what we can see, which is now called dark matter.