Planetary nebulae have nothing to do with planets, and the Sun will make one
Through early telescopes, these glowing rings looked like faint planetary disks, so the name stuck. They are actually the shed outer layers of dying stars, lit up by the exposed core. A mysterious green line in their light was once blamed on an unknown element, nebulium, which turned out never to exist.
Charles Messier logged the first one, the Dumbbell Nebula in Vulpecula, on 12 July 1764, as M27. In 1779 Antoine Darquier de Pellepoix described the Ring Nebula as a sharply outlined smudge the size of Jupiter, like a fading planet. William Herschel, who had discovered Uranus, found the Saturn Nebula in 1782 and admitted he hardly knew what to call it. He grouped such objects as planetary nebulae, eventually listing 78, most of which are really galaxies. Nobody knows exactly why he chose the term, but astronomers kept it.
Spectroscopy cracked the puzzle. On 29 August 1864 William Huggins turned a prism on the Cat's Eye Nebula and, instead of the dark-lined continuum typical of stars, saw bright emission lines. The strongest, at 500.7 nanometres, matched no known element. Helium had been found in sunlight in 1868, so an undiscovered nebulium seemed plausible. Henry Norris Russell suggested instead a familiar element in unfamiliar conditions, and in the 1920s physicists showed that in extremely thin gas, oxygen and nitrogen ions can emit so-called forbidden lines that collisions would suppress at higher densities. The nebulae were revealed as extraordinarily rarefied gas.
They mark the end of stars between roughly 0.8 and 8 solar masses; heavier stars explode as supernovae. After billions of years fusing hydrogen, such a star swells into a red giant, later burns helium into carbon and oxygen, and as an asymptotic giant branch star may lose 50 to 70 percent of its mass in winds. When the exposed core tops about 30,000 kelvin, its ultraviolet light makes the ejected shell glow. The phase is brief, perhaps a few tens of thousands of years, and the Sun is expected to go through it.
Hubble images from the 1990s showed that only about a fifth are roughly spherical; the rest take elaborate shapes that may owe something to companion stars, winds or magnetic fields, though the causes remain unclear. By returning freshly made elements to interstellar space, these nebulae probably play a big part in the chemical evolution of the Milky Way.
Source: Planetary nebula
