16-01 Galaxies outside the Milky Way

  • The concept of 'island universes' was proposed by Immanuel Kant, 1755
    In 1845, William Parsons observed many 'nebulae' with his telescope.

    [The 1.8-m 'Leviathan', built in 1845, was restored to its original state in 1996-1998. Parsons, Lord of Rosse in Ireland, made this sketch of M51 using this telescope.]

    [A modern view of M51 (NGC 5194) and NGC 5195. M51 is about 8.5 Mpc away, 20 kpc in diameter.]
     
    Many 'nebulae' were known, including 'spiral nebulae', for example in Messier's catalogue. But many astronomers in the 19th century did not agree with the concept of 'island universes'.
     
  • The Great Debate in April 1920
    Harlow Shapley vs Heber Curtis
     
  • 1924, Edwin Hubble discovered that the Andromeda 'Nebula' is about 750 kpc away from us, and extends about 70 kpc, larger than our own Milky Way galaxy. For the first time, the human beings know structures beyond our own galaxy!



    (Hubble used Cepheids as distance indicators.)