PX158 - J1 - introduction

gravitationally bound collection of stars, dust, gas, planets, etc

in the $1920s$, there was a great debate as to what galaxies were:

• gas and stars within the milky way
• 'island universes', other milky way-like structures

edwin hubble managed to measure the distance to a nearby galaxy ($M31$, andromeda) using cepheid variables, which have absolute magnitudes proportional to their pulsation period

he measured the distance to be $\sim 100kpc$ (cf: diameter of the milky way: $\sim 30kpc$)