The 10 Most Colorful Planetary Nebulae in the Sky
To new observers, the night sky appears as a monochromatic black and white scene. Although astronomy books and websites are full of objects revealing vivid reds, pinks, blues, and greens, looking at most of those same objects with a small telescope shows none of that. Deep sky light appears primarily as shades of gray.
But those of us with more experience know that the universe is a colorful place. It’s just that color in the universe requires intensity – enough photons to stimulate the cones of your eyes. These color receptors are an evolutionary response to the fact that humans spend most of their lives in daylight: when light is abundant, its subtle differences in wavelength transmit useful information, which we perceive as the colour. In contrast, while the rods of the eye are highly sensitive to light to help us see clearly in nighttime environments, they do not register color.
In the case of the Sun, its many photons at each wavelength saturate all of our color receptors at once, causing it to appear white. And the Moon is dark basalts and fragments of dust and gray rocks – no color there unless there is an eclipse or its light is reddened by the earth’s atmosphere as it rises or lie down. But looking elsewhere in the solar system, Mars, Uranus, and Neptune show us intensely red, green, and blue disks, respectively. And their shape and vivid color are duplicated by a group of deep sky objects: planetary nebulae.
Planetary nebulae are the product of Sun-like stars that break off and then light up their outer layers late in life. Their blues, greens and reds come from glowing gases such as hydrogen, helium, nitrogen and oxygen.
Long before it was known, two 18th century astronomers, Antoine Darquier de Pellepoix and William Herschel, both thought of the shape of these nebulae as a planet. Herschel is widely credited with first calling them planetary nebulae, although there is no definitive answer as to whether the term really originated with him.
Despite its name, only about 20% of planetary nebulae are spherical. The others come in various forms, resulting from the particular way each dying central star sheds its outer layers. Their density varies from 100 to 10,000 times that of empty interstellar space. The more colorful nebulae, which appear on this list, tend to have higher densities and appear round or oval in a telescope. This is because denser regions of gas glow more intensely.