What is a Nebula?

We received this question from listener, Francesca. Chris Smith put it to Astrophysicist Matt Bothwell... 

Matt - So a nebula is just the name we give for a cloud of gas and dust in space. The name nebula comes from the Latin meaning clouds and there are all different kinds of nebula. And the reason they look so different is for the same reason that clouds on earth look different, just because there are different types and they form in different ways. There is a type called H2 region. So a H2 region is a star forming cloud in space to it's like a big cloud of molecular gas that is collapsing under gravity and turning into stars, and it gets illuminated by the light from the young stars and glows very nicely. So the Orion Nebula is a nice example of this. There are other nebulae called planetary nebula. These are actually a completely different formation mechanism, so planetary nebula are clouds of gas that are blown out by stars like towards the end of their life. The names are a bit of a misnomer. When early astronomers saw them, because they tend to be round, they thought they resembled planets. They called them planetary nebula but of course they’re dying stars. Galaxies as well, before we understood what galaxies actually were they were called Spiral Nebula. So about 100 years ago astronomers incorrectly thought that galaxies were, kind of, whirly clouds inside of our own Milky Way. But now we of course understand that they are you know much much further away than that.