Sure, comets are awesome, so are solar and lunar eclipses but for night to night sky watching pleasure, give me planetary nebulas. Like finding a prize in the bottom of a box of Cracker Jacks, I get a little thrill every time one comes into view in the telescope. Not only are they beautiful to look at, but I see the sun’s fate right in front of my eyes.
They’re called “planetary” because their circular and oval shapes reminded early astronomers of planets. “Nebula” means cloud as in a cloud of celestial gas and dust. Other than appearance, they have nothing to do with planets.
Reminiscent of a chrysalis or cocoon, a planetary nebula is the cast-off atmosphere of a star like the sun. Exposed at its center is the former star’s core, now reborn as a tiny, Earth-sized object called a white dwarf star.
When a supergiant star runs out of fuel to burn in its core, it collapses and explodes as a supernova. Stars like our sun reach their golden years more gracefully by puffing their atmospheres into space like an aging Gandalf blowing smoke rings in the movie series The Lord of the Rings. The huff and puff comes from the final throes of fuel-burning in their cores as the stars settle into stable white dwarfs.
During its life sun-mass stars burn hydrogen to create the heavier element helium. When that’s used up, the star’s core contracts and gets hotter – hot enough to burn helium into carbon. The increased heat produced by burning helium causes the star to expand into a red giant. Five billion years from now, our sun may bloat up big enough to completely incinerate the inner planets including Earth.
Bigger stars can burn carbon and other even heavier elements. A sun-sized one stops at carbon and then expels its expansive outer atmosphere, exposing a tiny, dense, fiercely-hot core – here at last we have our white dwarf.
All that gas expelled into space fluoresces in the intensely energetic ultraviolet light pouring from the dwarf and glows like a neon sign in space advertising the final days of another star. Different elements in the shell glow different colors: green is prominent and comes from excited oxygen. Nitrogen and hydrogen provide brush strokes of red and helium blue.
Stars with masses like the sun and up to 8 times its mass will eventually evolve into planetary nebulae. Our middle-aged sun still has about 5 billion good years left before becoming a colorful balloon like IC1295 and its kin. Want to see more images of these gems? Click HERE.