From the March 2022 issue

The sky’s top 10 colorful planetary nebulae

These dying stars are going out in style, showing off rich greens, blues, and reds you can enjoy through your eyepiece.
By | Published: March 8, 2022 | Last updated on May 18, 2023
The compact Lion Nebula (NGC 2392) is surrounded by a fluffy ring of gas akin to the mane framing a big cat’s face.
Douglas J. Struble

For new observers, the night sky appears to be a monochrome scene of black and white. Although astronomy books and websites are replete with objects revealing vivid reds, pinks, blues, and greens, gazing at most of these same objects with a small telescope shows none of that. Light from the deep sky appears mostly as shades of gray.

But those of us with more experience know the universe is a colorful place. It’s just that color in the universe requires intensity — enough photons to stimulate the cones in your eyes. These color receptors are an evolutionary response to humans spending most of our lives in daylight: When light is plentiful, its subtle differences in wavelength convey useful information, which we perceive as color. By contrast, while the eye’s rods are highly sensitive to light to help us see clearly in nocturnal environments, they don’t register color.

In the case of the Sun, its many photons at every wavelength saturate all our color receptors at once, making it appear white. And the Moon consists of dark basalts and gray dust and rock fragments — no color there unless there is an eclipse or its light is reddened by Earth’s atmosphere as it rises or sets. But looking elsewhere in the solar system, Mars, Uranus, and Neptune show us disks that are intensely red, green, and blue, respectively. And their shape and bright color are duplicated by one group of deep-sky objects: planetary nebulae.

Colorful targets

Planetary nebulae are the product of Sun-like stars shedding and then lighting 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 this was known, two 18th-century astronomers, Antoine Darquier de Pellepoix and William Herschel, both considered the shape of these nebulae planetlike. Herschel is widely credited with first calling them planetary nebulae, although there is no definitive answer as to whether the term truly originated with him.

Despite the name, only about 20 percent of planetary nebulae are spherical. The rest occur in a variety of shapes, resulting from the particular way each central dying star sloughs off its outer layers. Their density ranges 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. That’s because denser regions of gas glow more intensely.

NGC 40 is a small but bright planetary nebula in Cepheus the King. Its glow is distinctly red — odd for this type of object. It sits in front of an unrelated supernova remnant, CTA 1.
Douglas J. Struble

Observing tips

The invention of the Oxygen-III (OIII) filter revolutionized observing planetary nebulae because the filter’s peak transparency is the same wavelength as these objects’ strongest emission. However, although a filter dramatically improves image contrast, it also blocks out natural color. To see their true color, it’s best to observe planetaries without a filter. The full intensity of the unfiltered target will stimulate your cones, giving these compact nebulae color and even making them bright enough to spot from suburban skies. By contrast, faint and extended nebulae usually don’t show color, and the special filters are designed for observing when the goal is simply finding the object, not revealing its color.

One common thread between the colorful planetaries on our list is their distance. Most are located between 1,000 and 5,000 light-years away. At these distances, they range from 15″ to 40″ in diameter — except for the Ring Nebula (M57), which is much larger. Bright planetaries can show color in telescopes as small as 6 inches. Larger apertures are better and all bear magnification well. A lot of planetaries have tenuous outer shells from previous eruptions, but these are too faint for most amateur telescopes. However, many on our list do reveal their central star — the white-hot engine lighting up these glowing balls from within.

The list

NGC 40
is the first target — and the most challenging. Located in the circumpolar northern skies at 72° declination in Cepheus, it’s magnitude 10.4 and 35″ across. It sits about 3,500 light-years distant. Some 1,000 light-years behind it is the unrelated, tenuous supernova remnant CTA 1.

With moderate telescopes, this nebula resembles a partial ring that looks more like a parenthesis. It is also called the Bow Tie Nebula. The magnitude 11.4 progenitor star in the center is a rare Wolf-Rayet star — a type of star deficient in hydrogen and producing strong stellar winds. With a full magnitude of difference between the star and its nebula, some observers consider this one of the best targets in which to observe the progenitor. In large apertures (20 inches or more), NGC 40 shows a reddish hue, which is atypical for planetaries as normally, ionized oxygen and nitrogen give these objects green or blue colors.

NGC 2392
was once known as the Eskimo Nebula because it has a round interior surrounded by a ring of “fluffy” gas reminiscent of a head ensconced within a parka hood. However, after considering the term’s derogatory history, NASA decided in 2020 to refer to the object only by its catalog name. It’s recently also been described as the Lion Nebula, comparing the exterior gas to a lion’s mane. Sources disagree on its exact distance: This dying star is located somewhere between 3,000 and 6,000 light-years away.

At magnitude 10.1, it’s the brightest nebula in Gemini and an easy target with small telescopes. Seeing its bluish-green color requires more aperture rather than darker skies. Can you see it in an 8-inch scope?

Also called the Ghost of Jupiter, NGC 3242 appears to the eye as a compact blue-green ball roughly the same angular size as Jupiter. Higher magnification will show it is slightly oblate.
Adam Block/Mount Lemmon SkyCenter/University of Arizona
NGC 3242 is sometimes called the Ghost of Jupiter for its shape and apparent size, reminiscent of our solar system’s fifth planet. It’s a colorful planetary tucked in Hydra, the largest constellation snaking its way through the spring skies. This nebula is 2° south and slightly west of Mu (μ) Hydrae. Binoculars or a finder scope will show it as a magnitude 8.6 star, while a small telescope reveals its nonstellar appearance. Under higher magnification, you’ll see a slightly oblate disk 40″ by 35″ across. The nebula fluoresces greenish blue with a 6-inch scope and becomes bluer with increasing aperture.

The Ghost of Jupiter lies at least 1,400 light-years away and is about 2 light-years in diameter. The structure is a thin ring surrounded by a larger, tenuous envelope visible with larger telescopes. The central star shines at magnitude 11.7 and is visible in moderate apertures.

One of just a few planetary nebulae in Charles Messier’s catalog, the Ring Nebula (M57) looks just as its name indicates: a ring-shaped cloud in space, glowing green.
Mark Hanson
The Ring Nebula (M57) in Lyra glows at magnitude 8.8 and has an extended halo nearly 4′ across. It was the first planetary nebula I ever observed. I was about 10 years old at the time, and my interest in stargazing was limited to locating Echo satellites. The telescope I used was a homemade 21-inch reflector owned by the Louisville Astronomical Society. Through it, M57 left an indelible memory because of its bright green color.

A decade later, that telescope, which included glass cast from the same formula as the 200-inch Hale telescope, was donated to the University of Louisville. When I observed M57 with that same telescope again at Moore Observatory, the green color was more muted. I’ve never seen it as bright green as in my childhood observation. It makes me wonder whether children see colors more intensely than adults. Or am I just remembering that vivid vista with green-tinted glasses?

Tiny NGC 6210 hangs amid the stars of Hercules, showing off its stunning color. Challenge yourself to spot its 12th-mangitude central star.
Chris Schur
NGC 6210 is the brightest planetary in Hercules and lies roughly 5,400 light-years away. Its location in our sky puts it more than 3,000 light-years above the Milky Way’s disk, away from the bulk of its kin. It is a compact 20″ by 16″ and magnitude 9.7. Its blue-green color is apparent in modest telescopes. Like most small planetaries, it bears magnification well, although spotting the magnitude 12.7 central star embedded within the bright nebula is a challenge.
The author thinks of the Cat’s Eye Nebula (NGC 6543) as the Atom Nebula because it contains intersecting rings that look like electron orbits in simple atom illustrations.
Adam Block/Mount Lemmon SkyCenter/University of Arizona
The Cat’s Eye Nebula (NGC 6543) in Draco is a compact 18″ in diameter and shines at magnitude 8.8. I personally call this the Atom Nebula because it has two intersecting ellipses that remind me of electron orbits around the central star as the atom’s nucleus. Its blue-green glow elongates in a north-south direction and is visible in a modest telescope. Even more detail within the nebula becomes available with increasing aperture. The central star is some 10 times hotter than the Sun and some observers claim it is easier to see with lower magnification. This dying star lies about 3,300 light-years away.
The Blinking Planetary Nebula (NGC 6826) does just as its name suggests: Switching between direct and averted vision makes it appear to blink in and out of your visual awareness.
Peter Goodhew
The famous Blinking Planetary Nebula (NGC 6826) in Cygnus offers a unique experience. The name comes from the odd optical effect you get when switching between normal and averted vision. When observing this object, I noted that while its green color was visible in the telescope, the central star appeared white when viewed straight-on. With averted vision, the nebula seemed to vanish — and the green color collapsed into the bright central star, giving it an intense verdant glow. A green star? That’s what I saw! Try it

The Blinking Planetary is a compact 27″ by 24″ and magnitude 9.8. The central star is about 0.5 magnitude fainter. It lies about 2,200 light-years distant.

NGC 7027 shows its boxy shape in larger telescopes. Its dense concentration of gas and dust hide the central star from view.
NGC 7027 in Cygnus is a young nebula, both compact and dense at its distance of 3,000 light-years. Its 10th-magnitude, intense green glow is visible in small telescopes, but larger scopes show its 15″-wide boxy shape better. The central star is deeply embedded in the gas cloud and beyond viewing for the most part. This is a great object to observe with your high-power eyepieces.
The Saturn Nebula (NGC 7009) features bright ansae, or handles, at the ends of its Saturn-like rings. Upon finding this object in 1782, William Herschel couldn’t explain how it had formed.
Daniel Verschatse
The Saturn Nebula (NGC 7009) is easy to find, lying about a degree west of Nu (ν) Aquarii. It is a bright magnitude 8.3, 41″ by 35″ across, and shows green or yellow color in small telescopes. Its Saturn-like “rings” are formed by structures called ansae: symmetrical knots of gas at each end of the nebula’s long axis. And they aren’t unique to NGC 7009 — this object is simply the brightest example you’ll see through your telescope. When Herschel discovered it in 1782, the nebula’s shape stumped him. He couldn’t figure out how it had formed. (Today, researchers believe ansae are related to a star’s behavior as it ages into a planetary nebula.) The Irish observer William Parsons, Earl of Rosse, later coined the name. It lies between 2,000 and 4,000 light-years away.
NGC 7662 is also called the Blue Snowball — and that’s exactly what you’ll see through your scope. This relatively bright target is good even under light-polluted skies.
Derek Santiago

NGC 7662 is Andromeda’s brightest planetary nebula and sports a simple and visually accurate moniker: the Blue Snowball. At 2,200 light-years away, it is an easy magnitude 8.3 and spans 32″ by 28″. One of my most memorable observations of this object was with a 12-inch Alvan Clark reflector at the University of Louisville’s on-campus observatory. Viewed from atop the four-story Natural Sciences building in an urban area, the Blue Snowball lived up to its name, proving that deep-sky observing can be done even with light pollution present. The high contrast of compact planetaries makes them ideal targets for mediocre skies.

Of course, there are plenty of other planetary nebulae that show color. This list is just the beginning. Whether you use a large or small telescope, this group of deep-sky objects offers a broad spectrum of challenges. From discerning color, detail, and the central star to simply separating the distant, tiny nebula itself from the rich background of Milky Way, these colorful and compelling targets present opportunities to test any stargazer’s mettle.