When old giant stars come to the end of their life, they often expel copious amounts of their atmosphere into the surrounding space. What remains behind is their extremely hot central cores, which emit a lot of their light in the ultraviolet range of their spectra. This high-energy light makes the ejected atmospheric gases fluoresce in a variety of colours and gives a hint of what elements they contain. That helps determine the chemistry of the star of which these gases were originally a part.
These fluorescent gases are called a "planetary nebula". This is a misnomer, they are nothing like the planets in our solar system.
Here is a picture of the first planetary nebula discovered - the "Dumbbell Nebula", which I obtained via a Slooh.com remotely controlled telescope (located on the Canary Islands):
A concise explanation for the theories regarding the cause and reasons why planetary nebulae exist can be found here:
https://en.wikipedia.org/wiki/Planetary_nebula
Not all stars end their life being surrounded by a planetary nebula. It appears that the mass of a star plays an important role in their generation. The sun is likely too small for this to happen.
A colorful way to go...
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