Initially, astronomers debated whether objects like M51
were simply odd structures in our own galaxy or separate
objects outside of our own galaxy. In fact, an important
discussion in astronomy at the time questioned whether our
galaxy was unique in the universe. Some scientists believed
that our galaxy took up the entire universe, and everything
in the sky was part of our galaxy. Other scientists insisted
that the Milky Way was only one of many galaxies in the
universe and that spiral nebulae like M51 were actually
galaxies similar to our own galaxy, but outside of it. It was
not until the twentieth century that Edwin Hubble determined
the distance to another “spiral nebula” known as M31
or the Great Andromeda Nebula. Based on this distance,
Hubble determined M31 and others like it had to be outside
of the Milky Way