Novas in andromeda

The Andromeda nebula was debated among astronomers, with some believing it was part of our Galaxy and others positing it as a separate collection of stars. Curtis observed unusually bright stars called novas which suggested that the nebula contained a large number of very faint stars, and that it was an independent galaxy, but Shapley thought otherwise. The matter remained unresolved.
 Heber Doust Curtis United States 1920 Novas in andromeda

The Andromeda nebula was debated among astronomers, with some believing it was part of our Galaxy and others positing it as a separate collection of stars. Curtis observed unusually bright stars called novas which suggested that the nebula contained a large number of very faint stars, and that it was an independent galaxy, but Shapley thought otherwise. The matter remained unresolved.