Abstract
In this first lecture, we take a brief historical look at the fact that it was only in the 20thcentury that astronomers became aware of the existence of galaxies, worlds apart from our own, the Milky Way. Previously, galaxies appeared to them as nebulae, which is why the Andromeda galaxy, for example, bore the name "Andromeda Nebula". Around 1920, in a debate between Harlow Shapley and Heber Curtis, the former argued that nebulae are part of our Galaxy, as are globular clusters. The size of the Milky Way seemed very large to him (100kpc), and the stars in globular clusters appeared giant. However, in 1909, Henrietta Leavitt discovered the period-luminosity relationship of Cepheids, and showed that they are very interesting distance indicators. Vesto Slipher also measured the velocities of nebulae using the Doppler effect: they are mostly in recession, but this must be due to radiation pressure. Heber Curtis, for his part, believes that the size of the Milky Way is closer to 10kpc, and that distant nebulae are galaxies outside our own. Edwin Hubble identifies Cepheids in M31 and M33: these are galaxies also composed of billions of stars, not gas. They are ~ 1000kpc apart. He interprets positive velocities as expansion of the Universe in 1929.
Galaxy dynamics are based on gravity and gas hydrodynamics. Stars constitute a collision-free medium, and their density in phase space is conserved. We can define stability criteria (Toomre's criterion), and determine which spiral density waves will be able to develop in the disks, depending on their velocity dispersion. Critical regions are those of Lindblad resonances, where there is a rational relationship between rotation period and epicyclic oscillation. Globular clusters are stellar systems within galaxies, and are dark matter-free by virtue of their formation. These clusters are more compact systems than galaxies of comparable mass, although there is a point of continuity between compact dwarf galaxies and globular clusters.
