We have pointed out in §6.1 that astronomers in the early twentieth century thought that our Milky Way Galaxy is the entire Universe! Even a small telescope shows many nebulous objects in the sky. The great German philosopher Kant already conjectured in the eighteenth century that some of these nebulae could be island universes outside our Galaxy (Kant, 1755). However, astronomers at that time knew no way of either establishing or refuting this conjecture. In 1920 the National Academy of Sciences of USA arranged a debate on this subject – Shapley arguing that these nebulae are within our Galaxy and Curtis arguing that they are extragalactic objects (Shapley, 1921; Curtis, 1921). We discussed in §6.1.2 how the distances of Cepheid variable stars can be determined. Using the newly commissioned Mount Wilson telescope, which was much more powerful than any previous telescope, Hubble (1922) resolved some Cepheid variables in the Andromeda Galaxy M31 and estimated its distance, clearly showing that it must be lying far outside our Milky Way Galaxy. Our current best estimate of the distance of M31 is about 740 kpc. It soon became clear that many of the spiral nebulae are galaxies outside our Galaxy, heralding the subject of extragalactic astronomy and establishing that galaxies are the building blocks of the Universe.
Light coming from a typical simple galaxy seems like a composite of light emitted by a large number of stars.