In their youth and middle age, medium-sized stars like our sun generate energy from nuclear fusion, the transmutation of light elements into heavier elements. But when the fuel runs out, stars spend their last days catapulting their outer layers into space to make a planetary nebula. Here’s how it works.
• As gravity pulls in on a newly-formed star, the core heats up to 10 million degrees or so and burns hydrogen into helium, creating a massive amount of energy that makes the star shine and prevents the star from contracting further. The hydrogen burns for tens of millions to billions of years. The more massive the star, the faster it burns its fuel.
• Eventually, nearly all the hydrogen in the core gets turned into helium. Fusion burning slows and the core shrinks, heating up to over 100 million degrees.
• If it gets hot enough, helium starts burning into carbon and oxygen, which generates more energy and stops the core from contracting. The hot core pushes out the star’s outer layers. The star balloons in size by a hundred times or more and becomes a cool and luminous red giant.
The Helix Nebula, one of the closest planetary nebulae to Earth
A Deeper Look
• Eventually, the helium runs out and the core shrinks again. But in small and mid-sized stars, the core does not get hot enough to burn carbon and oxygen, so fusion stops.
• But a thin shell of helium around the core continues to burn for a short time. The hot shell drives the star’s outer layers into interstellar space where they escape forever. We see this glowing shell of ejected gas– heated and ionized by the star’s scorching-hot core– as a planetary nebula.
• The nebula ejects trace amounts of heavier elements like carbon, nitrogen, and oxygen into space. Some of these atoms may coalesce into dense clouds that form new stars and planets. In a way, this is how the galaxy recycles itself.
• Some atoms of the lighter elements in your body may have been shed by a planetary nebula billions of years ago. Most heavier elements (iron, calcium, magnesium) were likely produced in a supernova explosion, not in a planetary nebula. But that’s a story for another day.
A Bit of History
The name “planetary nebula” came from William Herschel, who suggested these disk-like nebulae looked like planets, especially the blue-green disk of Uranus. Herschel understood these objects weren’t planets, but he had no idea what they were. Not until the early 20th century was the mystery solved.
The fact that bits and pieces of me, you, and all of us were once inside of burned-out and long-forgotten stars never ceases to amaze me. When you know things like this, you can never get bored.