Nebulae and external galaxies

In the 1920s Hubble showed that most “nebulae” were separate galaxies (1924, Cepheids in Andromeda) and classified them by shape: elliptical (E0–E7; old stars, reddish, little gas), spiral (S and barred SB; young blue stars and gas in the arms), lenticular (S0) and irregular (Irr). Our Galaxy belongs to the Local Group (≈ 80 galaxies): the largest is Andromeda (M31, ≈ 2.5 million light-years away, approaching us at ≈ 110 km/s and possibly merging with the Milky Way in billions of years), then Triangulum (M33) and the Large and Small Magellanic Clouds. Galaxies have masses of ≈ 10⁷–10¹³ M☉. The observable Universe is estimated to contain hundreds of billions of galaxies; the Hubble Ultra Deep Field shows tens of thousands of galaxies in a tiny patch of sky. Distances are estimated with Cepheids, type Ia supernovae and redshift; the James Webb Space Telescope has found galaxies with redshifts above 14, from when the Universe was only a few hundred million years old.