1758 - Ruđer Josip Bošković developes his Theory of forces, where gravity can be repulsive on small distances. So according to him such strange classical bodies, similar to white holes, can exist, which won't let other bodies to reach their surfaces,
1917 - Paul Ehrenfest gives conditional principle a three dimensional space,
1918 - Hans Reissner and Gunnar Nordström solve the Einstein-Maxwell field equations for charged spherically-symmetric non-rotating systems,
1918 - Friedrich Kottler[?] gets Schwarzschild solution without Einstein vacuum field equations,
1923 - George Birkhoff[?] proves that the Schwarzschild spacetime geometry is the unique spherically symmetric solution of the Einstein vacuum field equations,
1963 - Roy Kerr[?] solves the Einstein vacuum field equations for uncharged symmetric rotating systems,
1964 - Roger Penrose proves that an imploding star will necessarily produce a singularity once it has formed an event horizon,
1965 - Ezra Newman[?], E. Couch, K. Chinnapared, A. Exton, A. Prakash, and Robert Torrence solve the Einstein-Maxwell field equations for charged rotating systems,
... and spectroscopy (i.e., the study of the variation of intensity with electromagnetic wavelength for a given star — its "spectrum"), and observations ...