A planetary nebula is created when a star blows off its outer layers after it has run out of fuel to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble. A planetary nebula (PN, plural PNe), is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets. A nebula is an enormous cloud of dust and gas occupying the space between stars and acting as a nursery for new stars. The roots of the word come from Latin nebula, which means a “mist, vapor, fog, smoke, exhalation.” Nebulae are made up of dust, basic elements such as hydrogen and other ionized gases. A nebula is a distinct body of interstellar clouds. Originally, the term was used to describe any diffused astronomical object, including galaxies beyond the Milky Way. The four types of nebulae are: emission, reflection, dark, and planetary. Primarily the birth place of stars, nebulae are clouds of gas and dust in space. Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by. The temperature of the gas in the nebula is about 10,000 degrees Celsius, and the central stars of planetary nebulae are among the hottest stars in the Universe, with temperature in the range of 25,000 to over 200,000 degrees Celsius. Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes as a protostar. Eventually the temperature reaches 15,000,000 degrees and nuclear fusion occurs in the cloud's core. The nebula is illuminated by a central star, which sometimes is too faint to be seen. Although initially grouped with galaxies and star clusters under the class of “nebulae”, we now know that galaxies and star clusters are made up of stars, whereas planetary nebulae are gaseous. Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium).

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