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.
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