1 Iki Piki: {looking out the bridge windows at a glorious coloured nebula} I don't know, I find it hard to grasp the concept of these interstellar clouds.
2 Spanners: They're not that complicated.
3 Iki Piki: But they're so nebulous.

Nebulae are clouds of gas and dust in interstellar space. Many are regions where stars are forming, condensing by gravitational collapse within the clouds. As the new stars "turn on", their radiation illuminates the gas in the nebula, giving rise to the beautiful colours we see. (There are other sorts of nebulae, which I may go into some other time.)

This is the Carina Nebula, which I have used a few times for backgrounds.

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2013-12-13 Rerun commentary: The word "nebula" comes directly from Latin, where it means "cloud", "mist", or "vapour".

Imagine if cloud computing had been called nebula computing instead. You'd save your data to "the nebula" instead of "the cloud".

That would be so much cooler.