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Planetary Nebula: Definition & Facts Video

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  • 0:02 Planetary Nebula Defined
  • 0:46 Life of a Planetary Nebulae
  • 2:10 Locations, Sizes & Shapes
  • 3:19 Planetary Nebulae &…
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Planetary nebulas are perhaps one of the most spectacular sights in the night sky. Learn the definition of a planetary nebula and take a look at some of the most famous ones in the universe in this lesson.

Planetary Nebula Defined

Peering through a small telescope, one might notice the similarities between the glow of a planetary nebula and the light from a planet; they both look like small disks. Besides likeness in name, this is the only real similarity between a planetary nebula and a planet. Planetary nebulas are much larger and perhaps far more spectacular.

A planetary nebula is the immense, glowing cloud of dust that is forcefully ejected from a low mass star as it enters the last phase of its life. The dust cloud glows because of energy released by the remaining core of the dying star. Planetary nebulae occur in a variety of shapes, sizes and colors.

Life of Planetary Nebulae

As a low mass star, which is a star with a mass less than twice the mass of our sun, approaches the end of its life, it begins to use up the last of its fuel. Initially, it will use up all of its hydrogen. When this happens, the star expands into a red giant and begins burning helium. As the red giant burns through its helium supply, inert carbon begins building up at the star's core. When the last of the helium has been used, the star sheds its outer layers into space, leaving the small inert carbon core at the center of a huge cloud of dust. Energy from the cooling carbon core energizes the dust cloud, making it glow. This is the birth of a planetary nebula.

The dying little star is known as a white dwarf. Initially the star is very hot and glows with white light. The surrounding dust cloud is very tightly packed around the white dwarf. As the white dwarf cools, the strength of the energy it emits diminishes and the dust cloud begins to spread out.

This phenomena can be seen in this black and white photo of the Ring nebula, which clearly shows the white dwarf in the middle of the nebula. The dust cloud is visible as the gray ring surrounding the star.
Black and white photo of Ring nebula.

In a million years or less the star will have cooled down too much to illuminate the dust and the nebula will disappear.

Locations, Sizes & Shapes

Cosmically speaking, planetary nebulae have relatively short life spans. Because they are so short lived, planetary nebulae are considered rare features of our galaxy, the Milky Way. Within the Milky Way, approximately 3,000 planetary nebulae have been identified, most of which are clustered around the center of our galaxy.

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