<human>:  What are the main differences between a galaxy and a nebula?
<bot>: The main differences between a galaxy and a nebula are that a galaxy is a large grouping of stars, gas, and dust bound together by gravity, while a nebula is a cloud of gas and dust in outer space. Galaxies can range in size from a few dozen light years across to over a million light years across, while nebulae are much smaller and are typically only a few light years across. Additionally, galaxies typically have a central hub or nucleus, while nebulae don’t typically have any such structure.