The objective of this post is not to provide a physical or scientific classification of planetary nebulae, but rather to encourage observing these facinating objects.
Planetaries vary greatly in size and surface brightness, and also - to some extent - in shape.
From the impression I get of the nebulae through the eyepiece I have made myself seven main groups. In most cases I find it easy to readily identify the right group for the object I observe, but certainly there are grey zones, and sometimes one object can belong to more than one group.
Observing these objects with larger telescopes will probably make classification more difficult, since more intricate structure becomes evident.
Group 1: Small, bright and colourful planetaries
I see these nebulae vividly green and bluish. Good examples are NGC7662 in Andromeda and NGC6543 in Draco. Also NGC7009 (The Saturn nebula) fits quite well although it has the peculiar extensions giving the "Saturn" shape. The "Ghost of Jupiter" (NGC3242)also belongs to this class.
The Eskimo nebula (NGC2392) and NGC1535 in Eridanus form a small subgroup where a fainter outer layer is obvious in addition to the bright central zone.
Group 2: Bipolar planetaries
These are dumbbell or peanut shaped nebulae. M27 and M76 belong to this group. Another fine example is NGC2371/72 in Gemini. The small planetary, NGC7027 in Cygnus also turns out to be biploar, but more assymetrical than the previous ones. The Bug nebula, NGC6302 in Scorpius, could be classified as "peculiar", but could also be considered a member of this group.
Group 3: Annular planetaries.
Many planetaries are annular, but this isn't always easy to see visually. In some cases, however, the annular appearance is obvious at first glance. The best example is M57 in Lyra. But also the Helix nebula, NGC7293 is a good example. I find the small planetary in M46 (NGC2437) in Puppis to be clearly annular, and would finally like to mention Shapley-1 far south in Norma as a good example.
Group 4: Stellar or near-stellar planetaries
These objects are easy to overlook even though many of them can be seen in very small telescopes. At low magnification (in some cases even at higher power) they look like stars. In order to identify them, blinking with an UHC or O-III filter is recommended. Stars will become significantly fainter when the filter is held in front of the eyepiece, while the nebula will remain nearly unchanged.
Good examples are IC2149 (near Beta Aurigae), IC1747 (near Epsilon Cassiopeiae) and IC2003 in Perseus.
Group 5: Large ill-defined planetaries
I haven't observed many such nebulae, simply because they are very difficult to see and require extremely dark and clear skies. They can be impossible to see without an UHC or O-III filter. The Medusa nebula in Gemini (PK205+14.1)is a good example (although its shape suggests that it is "peculiar").
Group 6: Round (or nearly round), colourless planetaries
Maybe the less interesting planetaries to observe visually. NGC2022 in Orion is an example.
Group 7: Floating glass spheres
My favourite group. These nebulae normally have a relatively bright central star. The nebulae themselvs are very delicate, often with bright condensations and dark fields. Sometimes a filter is needed to see the nebula well. NGC1514 in Taurus, NGC1501 in Camelopardalis and NGC246 in Cetus are the best examples. NGC40 in Cepheus and the blinking planetary, NGC6826 in Cygnus should also gives me much of the same impression when observed at higher magnification under good seeing conditions.
I have to admit that some planetaries are difficult to put into any of these classes. Where does the Eight-burst planetary, NGC3132 in Vela belong? And what about NGC1360 in Fornax?
Any suggestions. Other ways to classify the planetaries? Anyhow, I recommend all of you to spend some time observing these nice objects.
Inge S
Planetaries vary greatly in size and surface brightness, and also - to some extent - in shape.
From the impression I get of the nebulae through the eyepiece I have made myself seven main groups. In most cases I find it easy to readily identify the right group for the object I observe, but certainly there are grey zones, and sometimes one object can belong to more than one group.
Observing these objects with larger telescopes will probably make classification more difficult, since more intricate structure becomes evident.
Group 1: Small, bright and colourful planetaries
I see these nebulae vividly green and bluish. Good examples are NGC7662 in Andromeda and NGC6543 in Draco. Also NGC7009 (The Saturn nebula) fits quite well although it has the peculiar extensions giving the "Saturn" shape. The "Ghost of Jupiter" (NGC3242)also belongs to this class.
The Eskimo nebula (NGC2392) and NGC1535 in Eridanus form a small subgroup where a fainter outer layer is obvious in addition to the bright central zone.
Group 2: Bipolar planetaries
These are dumbbell or peanut shaped nebulae. M27 and M76 belong to this group. Another fine example is NGC2371/72 in Gemini. The small planetary, NGC7027 in Cygnus also turns out to be biploar, but more assymetrical than the previous ones. The Bug nebula, NGC6302 in Scorpius, could be classified as "peculiar", but could also be considered a member of this group.
Group 3: Annular planetaries.
Many planetaries are annular, but this isn't always easy to see visually. In some cases, however, the annular appearance is obvious at first glance. The best example is M57 in Lyra. But also the Helix nebula, NGC7293 is a good example. I find the small planetary in M46 (NGC2437) in Puppis to be clearly annular, and would finally like to mention Shapley-1 far south in Norma as a good example.
Group 4: Stellar or near-stellar planetaries
These objects are easy to overlook even though many of them can be seen in very small telescopes. At low magnification (in some cases even at higher power) they look like stars. In order to identify them, blinking with an UHC or O-III filter is recommended. Stars will become significantly fainter when the filter is held in front of the eyepiece, while the nebula will remain nearly unchanged.
Good examples are IC2149 (near Beta Aurigae), IC1747 (near Epsilon Cassiopeiae) and IC2003 in Perseus.
Group 5: Large ill-defined planetaries
I haven't observed many such nebulae, simply because they are very difficult to see and require extremely dark and clear skies. They can be impossible to see without an UHC or O-III filter. The Medusa nebula in Gemini (PK205+14.1)is a good example (although its shape suggests that it is "peculiar").
Group 6: Round (or nearly round), colourless planetaries
Maybe the less interesting planetaries to observe visually. NGC2022 in Orion is an example.
Group 7: Floating glass spheres
My favourite group. These nebulae normally have a relatively bright central star. The nebulae themselvs are very delicate, often with bright condensations and dark fields. Sometimes a filter is needed to see the nebula well. NGC1514 in Taurus, NGC1501 in Camelopardalis and NGC246 in Cetus are the best examples. NGC40 in Cepheus and the blinking planetary, NGC6826 in Cygnus should also gives me much of the same impression when observed at higher magnification under good seeing conditions.
I have to admit that some planetaries are difficult to put into any of these classes. Where does the Eight-burst planetary, NGC3132 in Vela belong? And what about NGC1360 in Fornax?
Any suggestions. Other ways to classify the planetaries? Anyhow, I recommend all of you to spend some time observing these nice objects.
Inge S
