Mind your elders the next clear night and pay a visit to some of Spring's biggest and most ancient planetary nebulae.
I love looking at planetary nebulae and have ferreted out more than 200 of these shelly stellar remains in the past 20 years. Their delicate shapes and pastel colors not only delight the eye, but they possess subtle details that test the limits of vision: outer rings, darker centers, and those frequently elusive central stars that play peekaboo from behind pale veils of nebulosity.
Planetaries, so called because their generally round shapes reminded early observers of planets, represent a late stage in the evolution of Sun-like stars from red giant to white dwarf. Powerful stellar winds emanating from the star's core blow away its outer layers, creating an expanding shell of gas and dust.
The remaining core, compressed and heated to more than 46,000°F (26,000°C) becomes a white dwarf. Bathed in copious UV light from the dwarf, the former giant's atmosphere lights up like a neon donut. Some planetaries have binary central stars; their interaction with each other and the expanding bubble add further complexity to a nebula's shape and evolution.
The planetary nebula phase is brief, lasting only around 10,000 years before the cast-off cloak become so distended it fades from view. Only the lonely white dwarf and whatever planets it might still possess soldier on. Such will be the fate of the Sun, one of the reasons that observing planetaries gives pause to reflect on the future of our own Solar System.
There are an estimated 10,000 planetary nebulae in our galaxy, of which roughly 1,500 have been cataloged. Many are extremely tiny and look identical to stars. The only way to tell them apart is to "blink" them with a nebula filter such as a Lumicon O III or Orion Ultra-Block. Nebula filters pass the light of ionized oxygen, prominent in planetary nebulae, while suppressing skyglow and manmade light pollution. To "blink" a planetary, slide the filter back and forth between your eye and eyepiece while gazing at the nebula. The filter will cause the object to sharply brighten compared to the neighboring field stars, immediately identifying it as the nebula.
Planetaries come in all sizes from stellar to more than a degree across. We'll focus on the ones I like to call giants, the most familiar of which is the Helix Nebula in Aquarius, with a diameter more than half that of the full Moon.
The smallest of our featured objects spans 5.8′ and the largest 17′. For comparison, the Ring Nebula in Lyra measures 1.4′×1.0′ across. Large apparent size can mean an object is close by, but all ten of our featured nebulae lie from many hundreds to up to 2,000 light-years away. They're huge because they're ancient.
Jumbo size also means their light is spread over a large area, making some of these stellar windbags faint and difficult to see. For our featured nebulae, I recommend a 10-inch (25-centimeter) or larger telescope, dark skies and an O III-style filter. My observations were made from a rural site with a 15-inch (37-cm) reflector. Two were beyond my range, but the others were not what I would call difficult, only faint. Some, like Jones-Emberson 1, in Lynx reveal striking details while others, like Abell 31 in Cancer, look like huge, diffuse comets. I used a magnification of 64× and an O III filter for all observations.
All make for great hunting on spring nights, when you can't help but feel a time warp between these venerable smoke rings and the fresh green world sprouting up around you.
* Abell 21 in Gemini: A remarkable object! Catch it right at the end of evening twilight in the western sky. Also known as the Medusa Nebula, it was immediately obvious and shaped like a puffy, backwards "C". With averted vision I could discern a brighter patch in either arm of the nebula. No luck seeing the "braided hair" texture that suggest Medusa's mane of snakes. Easy to find near Beta (β) Canis Minor. Diameter = 12′×9′.
* Abell 31 in Cancer: When many of us think of the constellation Cancer, the Beehive or M67 clusters come to mind. Who knew this big balloon of a planetary resided there? I see a nearly featureless, large glow almost centered on an 11th-magnitude star and elongated slightly east-west. The brightest part of the planetary lies very close west of the star. Large and diffuse but surprisingly easy to see using an O III filter. Mottled texture suspected with averted vision. Diameter = 17′×16′.
* Jones-Emberson 1 in Lynx: What a contrast to Abell 36! You'll see lots of structure here. The gaping dark hole is obvious, but further observation will bring to view a pair of bright lobes or knots symmetrically placed on either side of the ring. Their neat resemblance to headphones or earbuds has earned it the nickname of The Headphones Nebula. One of the brightest of the dim objects in our spring sampler. Named for astronomers Jones and Emberson who discovered the nebula in 1939. Diameter = 7′×6′.
* Ellis-Grayson-Bond (EGB) 6 in Leo: Despite a thorough search I never turned up anything but suspected nebulosity in all the wrong places. Deep-sky observer and author Reiner Vogel sees an extremely faint patch of light. Photos reveal a faint smoke ring with a dark center. A large and very ancient nebula. Diameter = 13′×11′.
* Abell 35 in Hydra: Low declination, but this is an easy-to-see patchy glow with an amazingly bright +9.6 magnitude central star. Two other slightly fainter stars are involved. Although I can't distinguish the bar south of center, other observers have spotted it. I also couldn't make out the bow shock arc of nebulosity that wraps around the central star like a scarf flying in the wind. Diameter = 16′×11′.
* Abell 36 in Virgo: To my eye a featureless, circular haze with a bright 11.5 magnitude central star. Faint but not difficult. I had hoped to see some of the whirls pictured in photos but perhaps the object's –20°declination added enough extinction to scrub them from view. Observers in the southern U.S. and points south should do much better. Diameter = 8′.
* NGC 6543 in Draco: An astonishing, multi-faceted planetary also known as the Cat's Eye. Most of us stop by for the bright 9.8-magnitude interior nebula that looks like a miniature blue egg. A beauty in its own right. Next time, slap on the O III and you'll see an entirely different aspect. The egg lies at the center of a large, slightly textured disk of previously cast-off material 5.8′ across! A patch of nebulosity within the halo bright enough to get its own designation, IC 4677, lies about 2′ due west of the egg.
* Jacoby 1 in Boötes: Too tough for me in part because of several bright field stars that mess with dark-eye adaption. I suggest a 20-inch or larger for this one. If you do seek this fainty, the brightest portion of the ring lies immediately east-southeast of the 7.5-magnitude star set inside the southern half of this delicate ring-shaped nebula. Diameter = 11′.
* Longmore-Tritton (LoTr) 5 in Coma Berenices: Very easy object appearing as a round, glowing patch offset from the bright 8.8-magnitude star at its center. The star is a binary — we see the G5 giant but tucked in close there's an invisible hot dwarf responsible for ionizing the nebula. Diameter = 9′.
* Ellis-Grayson-Bong (EGB) 1 in Cassiopeia: Vague and very faint, but I see a round patch about 5′ in diameter. Deep photos reveal a faint smoke ring with a dark center. Diameter = 5′.
I normally would include a basic table and finder charts for each of our 10 planetaries, but German amateur astronomer Reiner Vogel has already done so in his excellent Large Planetaries Observing Guide, which is available as a free .pdf book. Check it out!
- Catalog of Galactic Planetary Nebulae
- Reiner Vogel's Large Planetaries, Observing Guide (download)
- Steve Gottlieb's Southern Planetaries
- The Abell Planetaries
- Astronomical League Planetary Nebula Program