What is extragalactic astronomy?
The dawn of extragalactic astronomy was in 1917, when American astronomer Heber Curtis observed a stellar nova within M31, the formal name for what was then called the Great Andromeda Nebula. At the time, spiral nebulae such as Andromeda were thought to lie within our own galaxy, with a size only several times larger than that of our solar system and a distance less than 50,000 light-years. They thought the Milky Way Galaxy represented the entire universe. After observing the nova in M31, Curtis searched the photographic record, noticing 11 additional novae in the region. If M31 was just a stellar nebulae, why were there so many novae within it, and why were these characteristically fainter than other novae? Reasoning from the observation that these novae were about 10 magnitudes fainter than novae known to occur in our own galaxy, Curtis declared that the Great Andromeda Nebula was in fact an “island universe,” distinct from the Milky Way and located 500,000 light-years away. Astronom
