Dutch astronomer who, in 1952, discovered that the zeta-Persei star cluster is relatively young (about 1.3 million years old). This was an important discovery because it showed that star formation is still taking place in the Milky Way galaxy and, presumably, in other similar galaxies. This, in turn, means that galaxies continue to evolve with time, rather than remaining static from the time of their formation until the last stars die billions of years later.