This is a set offering the Springer books "Pioneer of Galactic Astronomy: A Biography of Jacobus C. Kapteyn" (ISBN 978-3-030-55422-4) and "Master of Galactic Astronomy: A Biography of Jan Hendrik Oort" (ISBN 978-3-030-55547-4). These accessible and entertaining biographies written by Piet van der Kruit chronicle lives and achievements of two pioneering astronomers of their time. KAPTEYN-BIOGRAPHY: This non-technical biography of Jacobus Cornelius Kapteyn (1851-1922) presents to the general reader the scientific life of the astronomer who pioneered the studies of the structure of the Milky Way Galaxy. In telling Kapteyn's story the author weaves in astronomy basics and uses modern astronomical images to show the developments of astronomical research from Kapteyn's times to the present. In particular the study of the distribution of stars in space has now culminated with spectacular new insights coming from the astrometric satellite GAIA, which is receiving much public attention today. The biography shows how Kapteyn's ideas influenced prominent astronomers worldwide. He is prominent as designer of the Kapteyn Universe, the alternative to the large system found by Harlow Shapley. He is the discoverer of Kapteyn's Star, still the second fastest moving star in the sky, which is now one of the nearest stars with a planet in the habitable zone. This fascinating hybrid of astronomy history and popular astronomy tells the story of the astronomy professor without an observatory who founded the first astronomical laboratory specializing in measuring photographic plates exposed elsewhere. Kapteyn took astronomy out of cataloguing stars to measuring distances and velocities in order to study their spatial distribution, systematic motions (Kapteyn's Star Streams) and the equilibrium between their gravity and motions. His legacy includes, in addition to the first application of Galactic structure and dynamics, Jan Hendrik Oort, the famous astronomer from Leiden, who as a student was so impressed by Kapteyn's lectures that he decided to become an astronomer. OORT-BIOGRAPHY: This accessible and entertaining biography chronicles the life and triumphs of astronomer Jan Hendrik Oort, who helped lay the foundations of modern astronomy in the 20th century. The book puts into context some of Oort's most significant achievements, including his discovery that the Milky Way rotates, as well as his famous hypothesis that our Solar System is surrounded by a reservoir of comets -- now simply known as the Oort Cloud. Written by Oort's former student, this fascinating story also delves into Oort's pivotal role in the foundation of major astronomical facilities, including radio telescopes in the Netherlands and the European Southern Observatory (ESO), which now operates the most successful astronomical observatories in the world. The book draws extensively on new archival research through the Oort Archives, along with personal reminiscences by Oort's son and astronomer-grandson, to paint a more detailed picture of Oort's life not just as an astronomer, but also as a husband, father, and citizen. The strong public interest in comets triggered by the Rosetta mission to comet 67P/Churyumov-Gerasimenko and the recently discovered interstellar comet in the Solar System make this book particularly timely.