Astronomical challenges for the twenty-first century

Unsolved astronomical problems are summarized. The chief problem areas are the solar system, nearby stars, stellar evolution, interstellar matter, and the extragalactic Universe. Individual questions of great interest include: the formation of the solar system; the structure of the sun; the existence of planetary systems around nearby stars; the evolution of stars into white dwarfs, neutron stars, black holes, pulsars, novas, or supernovas; the birth of stars; biochemistry in space; elemental abundances in interstellar gas; properties of galaxies and active extragalactic objects. Other vital issues are: the hierarchical large-scale structure of the Universe; the value of the Hubble and deceleration constants; cosmological models; the interface between astronomy and high energy physics. The progress achieved in the twentieth century and future projects are briefly reviewed.