Time-line

1610

Galileo resolves the MW into stars

1750

Immanual Kant suggests that some of the other Nebulae are other galaxies, similar to the MW.

end of 1700s

Messier and Herschel catalogue hundreds of Nebulae. Herschel counts stars, and deduces that the Sun lies near the centre of an elliptical distribution with axes ratio 5:5:1

1900-1920

Kapteyn counts stars, decides wrongly that extinction is unimportant, and deduces the MW to be $5{\hbox{\rm kpc}}\times5{\hbox{\rm kpc}}\times1{\hbox{\rm kpc}}$ big, with the Sun at 650 from the centre.

1912

Leavitt discovers the $P(L)$ relation for Cepheids.

1914

Slipher measures large (1000s km s$^{-1}$) velocities for some Nebulae, and finds evidence for rotation. The spectra he takes suggests presence of stars, not of gas. A clear indication these are not proto-planetary structures in the MW, but other galaxies.

1915

Shapley finds the centre of the MW's globular cluster system to be far away from Kapteyn's MW centre.

1920

van Maanen claims (erroneously) that some spiral nebulae have a large proper motion, suggesting they are within the MW.

1920

Shapley and Curtis debate publicly over the size of the MW, but the matter is not settled.

1923

Hubble resolves M31 (Andromeda) into stars, using the newly commissioned 100-inch telescope. Given the large inferred distance means that M31 must be outside the MW. He also discovers Cepheids, and the distance to M31 is estimated at 300kpc. So Andromeda us indeed another galaxy.

1926

Lindblad computes that Kapteyn's MW is so small, it cannot gravitationally bind its Globular Clusters. But Shapley's much bigger MW could.

1927

Jan Oort shows that several aspects of the local motion of stars can nicely be explained if the Sun (and the other nearby stars), is on a nearly circular motion around a position 12kpc away in the direction of Sagittarius. Nearly the same position as found by Shapley, and implying a much larger MW than Kapteyn's.

1927

larger MW picture, where many of the nebulae are extra-galactic MWs, gains general acceptance.

1929

Hubble discovers his expansion law. His derived value is a factor of 10 too large!

1930

Trumpler uses open clusters to show the importance of extinction, and explains why Kapteyn's measurement were faulty

1930-35

Hubble's new data confirm the modern picture of galaxies, and demonstrates van Maanen's measurements must have been wrong.

And so in a very short time indeed, three sweeping changes in our view of the visible universe took place:

• an 8-fold increase in the size of the MW
• the acceptance that many of the nebulae were external galaxies
• the universe is expanding