- Astrophysics california Institute of Technology and Astronomer, Mount Wilson and Palomar Observatories, USA
Galaxies, or extragalactic nebulae, as we now know are more or less vast systems of stars. The galactic nebulae on the other hand are masses of gas illuminated by hot stars whose radiations excite the gases to fluorescent emission. The great nebula in Andromeda which is an extragalactic stellar system visible to the naked eye was already known to the Arabs. With the advent of the telescope it was soon recognized that there exist very many similar nebulae. For centuries, however, with the telescopes then available one could not distinguish between the galactic and the extragalactic nebulae. The story of how astronomers finally learned to recognize the true characters of these objects is a long and confused one. It is beyond the scope of this book to go into any details but, if some of the great names are to be mentioned, the Herschels perhaps come first because of their extensive surveys of the skies and their suggesting the idea of island universes. Lord Rosse discovered that some of the nebulae have spiral structure and the pholosopher Kant must be mentioned because of his remarkable speculations and his arguments supporting these speculations that the numerous nebulae in high galactic latitudes are extragalactic stellar systems.