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
Because it contains a spherical interior ringed by a ring of "fluffy" gas like a head nestled beneath a parka hood, NGC 2392 was previously known as the Eskimo Nebula. However, in 2020, NASA chose to refer to the asteroid simply by its catalogue name, after considering the term's pejorative history. It's also been dubbed the Lion Nebula, because the exterior gas resembles a lion's mane. The actual distance of this fading star is unknown, however it is estimated to be between 3,000 and 6,000 light-years away.
It's the brightest nebula in Gemini, with a brightness of 10.1, and a simple target for modest telescopes. Rather than darker skies, larger aperture is required to see its bluish-green tint. Is it visible via 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
NGC 3242 is known as the Ghost of Jupiter because of its form and apparent size, which is similar to that of our solar system's fifth planet. It's a bright planet nestled away in Hydra, the greatest constellation winding its way across the spring sky. Mu () Hydrae is 2° south and slightly west of this nebula. It appears as a magnitude 8.6 star through binoculars or a finder scope, while a tiny telescope shows its nonstellar appearance. A slightly oblate disc 40" by 35" across can be seen with higher magnification. With a 6-inch scope, the nebula appears greenish blue and becomes bluer when the aperture is increased.
The Ghost of Jupiter is at least 1,400 light-years away and has a diameter of around 2 light-years. With larger telescopes, the structure is apparent as a thin ring encircled by a larger, tenuous envelope. The centre star is visible with moderate apertures and glows at magnitude 11.7.
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.
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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?
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.
The Ring Nebula (M57) in Lyra features a broad halo of almost 4' across and glows at magnitude 8.8. It was my first encounter with a planetary nebula. At the time, I was around ten years old, and my interest in astronomy was restricted to spotting Echo satellites. The telescope I used was a Louisville Astronomical Society-owned 21-inch reflector that I built myself. Because of its vivid green colour, M57 left a lasting impression.
A decade later, the University of Louisville received that telescope, which featured glass cast using the same formula as the 200-inch Hale telescope. The green colour was more muted when I watched M57 with that same telescope at Moore Observatory. It's never been as brilliant green as it was when I was a kid. It makes me question if youngsters perceive colours differently than adults. Or am I just seeing things through the lens of my 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.
The brightest planetary in Hercules is NGC 6210, which is around 5,400 light-years distant. Its position in our sky places it about 3,000 light-years above the disc of the Milky Way, far away from the majority of its relatives. It measures 20" by 16" and has a magnitude of 9.7. In small telescopes, its blue-green tint is seen. It bears magnification well, like most tiny planetaries, yet locating the magnitude 12.7 central star embedded within the dazzling nebula is difficult.
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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.
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The Cat's Eye Nebula (NGC 6543) in Draco has a diameter of 18" and a magnitude of 8.8. This nebula is known as the Atom Nebula because it includes two intersecting ellipses that resemble electron orbits around the centre star, which serves as the nucleus of the atom. Its blue-green glow elongates north-south and can be seen with a small telescope. As the aperture is increased, more information inside the nebula becomes available. The central star is ten times hotter than the Sun, and some observers report that seeing it with lesser magnification is easier. About 3,300 light-years away is this fading star.
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.
In Cygnus, the famed Blinking Planetary Nebula (NGC 6826) provides a one-of-a-kind experience. The weird optical illusion you receive while switching between normal and averted vision inspired the term. When I looked at this object via the telescope, I noticed that while the green colour was evident, the central star appeared white when viewed straight on. The nebula seemed to vanish with averted vision, and the green colour of the nebula compressed into the bright central star, giving it an intense greenish glow. Is that a green star? That's exactly what I saw! Give it a shot.
The Blinking Planetary is 27" by 24" in size and has a magnitude of 9.8. The centre star is fainter by around 0.5 magnitude. It is approximately 2,200 light-years away.
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NGC 7027 shows its boxy shape in larger telescopes. Its dense concentration of gas and dust hide the central star from view.
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At a distance of 3,000 light-years, NGC 7027 in Cygnus is a youthful nebula that is both compact and dense. Small telescopes can see its 10th-magnitude, strong green glow, but larger telescopes can see its 15-inch-wide boxy shape better. For the most part, the core star is deeply entrenched in the gas cloud and out of sight. This is an excellent target for your high-power eyepieces.
At the extremities of its Saturn-like rings, the Saturn Nebula (NGC 7009) has dazzling ansae, or handles. William Herschel couldn't explain how this thing developed when he discovered it in 1782.
The Saturn Nebula (NGC 7009) is located about a degree west of Nu (v) Aquarii and is simple to find. It's a brilliant magnitude 8.3 object that measures 41" by 35" across and appears green or yellow in tiny telescopes. Structures termed ansae, symmetrical clumps of gas at each end of the nebula's long axis, give the nebula its Saturn-like "rings." They're not exclusive to NGC 7009; it's just the brightest example you'll see through your telescope. The nebula's form perplexed Herschel when he found it in 1782. He was baffled as to how it had formed.
(Ansae are now thought to be linked to a star's activity as it evolves into a planetary nebula.) The name was later coined by Irish observer William Parsons, Earl of Rosse. It's about 2,000 to 4,000 light years away.
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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.
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NGC 7662
The Blue Snowball, NGC 7662, is Andromeda's brightest planetary nebula, with a simple and aesthetically appropriate name. It's magnitude 8.3 and spans 32" by 28" at a distance of 2,200 light-years. At the University of Louisville's on-campus observatory, I made one of my most memorable observations of this object using a 12-inch Alvan Clark reflector. The Blue Snowball, seen from atop a four-story Natural Sciences building in a city, lived good to its moniker, demonstrating that deep-sky viewing may be done even when light pollution is prevalent. Compact planets are great targets for mediocre sky due to their high contrast.
Of fact, there are numerous different coloured planetary nebulae. This is only the start of the list. This group of deep-sky objects presents a wide range of problems regardless of whether you use a large or small telescope. These vivid and intriguing sights give opportunities to test any stargazer's mettle, from identifying colour, detail, and the centre star to simply separating the distant, tiny nebula itself from the rich background of the Milky Way.
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