As you have discovered in the last sections, stars are born in emission nebulae like the Great Orion Nebula, grow up in open star clusters like the Pleiades, then disperse into the galaxy where they glow for tens of millions to tens of billions of years with energy created from atoms fusing in their hot cores. But much like living things, stars also have an end. Eventually, nearly all the hydrogen atoms in the core get fused into helium atoms. When the hydrogen in the core runs out, fusion burning slows and the core shrinks, heating up to over 100 million degrees. If it gets hot enough, helium starts fusing into heavier atoms like carbon and oxygen. This new burning 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 stars Arcturus, Aldebaran, and Gacrux in the constellations Bootes, Taurus, and Crux are examples of red giants (or orange giants… same idea) you can see with your unaided eye. 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. Only a thin shell of helium around the core continues to burn for a short time. This 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 name “planetary nebula” came from William Herschel, the 18th-century astronomer who suggested these disk-like nebulae looked like planets, especially the blue-green disk of the planet Uranus. Herschel understood these objects weren’t planets, but he had no idea what they were at the time. A planetary nebula ejects hydrogen gas and trace amounts of heavier atoms 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. In fact, some atoms of the lighter elements in your body may have been shed by a planetary nebula billions of years ago. At the center of a planetary nebula, the expose cored of the star glows with a temperature of 50-100,000 degrees. The core, which has stopped burning, will settle down as a dim, hot, Earth-sized lump of carbon and oxygen called a white dwarf. These stars are very dim and hard to see, even with a small telescope. Most mid-sized stars in the galaxy will go on to become planetary nebula. So why can’t we see more of them in the sky? Because they don’t last very long. Computer models of stars show a planetary nebula lasts just 50,000 years, a tiny fraction of a star’s 1-10 billion life span.