Using the Hubble Space Telescope to obtain visual and other spectral data, our knowledge of the 1000+ known PN's has increased immensely. The image shown is of the PN known as the 'Eskimo Nebula', and sometimes as the 'Clownface Nebula'. It is in the constellation Gemini. This is just one example of many forms that PN's take while expelling their outer layers of gas.
PN's are formed when old stars of a particular size, similar to our Sun's size, have consumed most of their hydrogen fuel after billions of years. The hydrogen has mostly been converted to helium, and the star has expanded to become a Red Giant. The star does not so much explode as much as it ejects the gases at much lower speeds and at different times. As the star evolves, the central core of the star evolves into a White Dwarf, a very hot star. The very high temperature radiation causes the ejected gases to become ionized and glow.