A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the Andromeda Nebula and spiral galaxies in general as "spiral nebulae" before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Nebulae are often star-forming regions, such as the Eagle Nebula.