When a star dies, it does not erupt into a supernova or collapse into a black hole. Instead, it softly loses its outer layers, forming a magnificent cloud known as a "planetary nebula," while the dying star's core transforms into a white dwarf. Gravity and other effects form the cloud into intriguing shapes, and when illuminated by the white dwarf, the complex molecules inside the nebula light in beautiful hues. As a result, we frequently call planetary nebulas by their appearance: the Ring Nebula, the Dumbbell Nebula, the Stingray Nebula, and so on.

Planetary Nebulas


The Beautiful Death of Sun-like Stars

Nuclear fusion is what makes a star a star: the collision of atomic nuclei deep within the star's core. Every star, however, will eventually exhaust its capacity to combine nuclei into heavier elements. When this happens to a star with fewer than 8 times the mass of the Sun, it grows to enormous proportions, large enough to swallow some of the planets circling it. Meanwhile, its core contracts, and it loses enough gravity to cling to the star's outer layers.

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The dying star then loses its layers, resulting in a planetary nebula. The new nebula comprises both old and new chemicals: atoms created by nuclear fusion and molecules generated in the dying star's outer layers as they cooled and migrated into space. Many of those atoms are "metals," or elements heavier than helium on the periodic table. Because new stars and planets can emerge from these atoms and molecules in a planetary nebula, dying stars affect the chemistry of the galaxy. When our Sun runs out of nuclear fuel in around 5 billion years, it may also become a planetary nebula.

Planetary nebulas are beautiful, but they only last a few thousand or tens of thousands of years before dissipating into interstellar space. In human words, that's a long period, but in cosmic terms, it's a little time. Because of this, given their modest size, we only know around 3,500 planetary nebulas in our galaxy.

However, because of their limited lifespan, we can see certain planetary nebulas grow, expand, and fade over decades. This paints a clear picture of how dying stars disseminate their atoms and molecules around the cosmos. The intricate and gorgeous structure of planetary nebulas offers information about the innards of a dying star.

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Despite their name, planetary nebulas have nothing to do with planets: when astronomer William Herschel first found them in the 1700s, he thought their blue-green radiance resembled the recently discovered planet Uranus.