X-ray astronomy in the new millennium: a summary

Recent X–ray observations have had a major impact on topics ranging from proto–stars to cosmology. They have also drawn attention to important and general physical processes that currently limit our understanding of thermal and non–thermal X–ray sources. These include unmeasured atomic astrophysics data (wavelengths, oscillator strengths, etc.), basic hydromagnetic processes (e.g. shock structure, reconnection), plasma processes (such as electron–ion equipartition and heat conduction) and radiative transfer (in discs and accretion columns). Progress on these problems will probably come from integrative studies that draw upon observations, throughout the electromagnetic spectrum, of different classes of source. X–ray observations are also giving a new perspective on astronomical subjects, like the nature of galactic nuclei and the evolution of stellar populations. In addition, they are helping us to address central cosmological questions, including the measurement of the matter content of the Universe, understanding its overall luminosity density, describing its chemical evolution and locating the first luminous objects. X–ray astronomy has a healthy future with several international space missions under construction and in development.