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Observing >> Deep Sky Observing

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Darren Drake
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Is M27 the brightest pn? new
      #5715998 - 03/06/13 09:07 AM

Its listed in skysafari as 7.09 mag and I don't see any on a quick search that are brighter. Does anyone know if this is indeed the brightest pn in all the celestial sphere???

Edited by Darren Drake (03/06/13 09:08 AM)


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Tony Flanders
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Re: Is M27 the brightest pn? new [Re: Darren Drake]
      #5716025 - 03/06/13 09:20 AM

NGC 7293, the Helix Nebula, probably has roughly the same integrated magnitude as M27. Which is brighter might well depend on what wavelengths you measure.

I can't think of any other contenders. They're the only two planetary nebulae that seem like plausible naked-eye targets in a super-dark sky.

Edited by Tony Flanders (03/06/13 09:21 AM)


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JakeSaloranta
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Re: Is M27 the brightest pn? new [Re: Tony Flanders]
      #5716153 - 03/06/13 10:43 AM

Quote:

I can't think of any other contenders. They're the only two planetary nebulae that seem like plausible naked-eye targets in a super-dark sky.




Under those super-dark skies one might be able to see even down to, say magnitude 8 without optical aid. This would then include NGC 3242 (7.7) and NGC 7009 (7.9) as possible, stellar objects visible to the naked eye. Stretching this even more would net you NGC 6752 (8.1) and NGC 6543 (8.1). Those would be some serious naked eye challenges...

/Jake


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David Knisely
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Re: Is M27 the brightest pn? new [Re: Darren Drake]
      #5716197 - 03/06/13 11:19 AM

Quote:

Its listed in skysafari as 7.09 mag and I don't see any on a quick search that are brighter. Does anyone know if this is indeed the brightest pn in all the celestial sphere???




It can be a little difficult to precisely measure the magnitude of diffuse objects like planetary nebulae, although I find Doug Snyder's Planetary Nebula web page a good source for data on them. As such, M27 (mag. ~ 7.1) and the Helix NGC 7293 (mag. ~ 7.6) are fairly close to the same integrated magnitude (depending on the source a person might quote). I have seen M27 when it was near the meridian with my unaided eye on an excellent night at the Nebraska Star Party, where the zenith limiting magnitude can run from 7.5 to 8.1 depending on the observer and the local conditions. Using averted vision, M27 made a nice pairing with the magnitude 5.7 star 14 Vulpeculae which sits about 24 arc minutes to its north. However, at that same site later on in the night, I was unable to see NGC 7293 with my unaided eye, probably due to it being larger and just a bit too low in the sky at the time (only 26 degrees above the southern horizon). My usual stellar limiting magnitude at NSP ranges from 7.2 to 7.6 typically, so maybe the Helix is just a bit too faint for me to see. Clear skies to you.


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Kraus
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Re: Is M27 the brightest pn? new [Re: David Knisely]
      #5716591 - 03/06/13 02:52 PM


Hmmm....I'd say M-57 is brighter as my Frau akins its appearance to an engagement ring. (We were already married so she wasn't hinting at something.) M-27 to her was "I think I see a puff of smoke." So I know she saw the ring better than other objects she's tried to see.

I don't let the magnitude number scare me from trying to seek, find and observe the night sky's awe. And if this darned Middle-Georgia wind lets up, the Eskimo will get my attention. I have important Eskimo nebula work leftover from last Spring to get done.


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Tony Flanders
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Re: Is M27 the brightest pn? new [Re: Kraus]
      #5716719 - 03/06/13 04:00 PM

Quote:

Hmmm....I'd say M-57 is brighter ...




M57 has higher surface brightness (intensity) than M27 and considerably lower total (integrated) brightness. In other words, it's much smaller, so its light is concentrated in a smaller area.

If your main obstacle is light pollution, something with higher surface brightness and lower integrated brightness is often easier to see.


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GlennLeDrew
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Re: Is M27 the brightest pn? new [Re: Tony Flanders]
      #5717033 - 03/06/13 06:41 PM

Where extended objects are concerned, surface brightness is more important than integrated brightness. Compare the huge Helix with most tiny planetaries. The former is a ghost which disappears with even slight light pollution, whereas many of the latter, in spite of being several magnitudes fainter in *total* light, punch through even fairly bad light pollution.

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Kraus
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Re: Is M27 the brightest pn? new [Re: GlennLeDrew]
      #5717229 - 03/06/13 08:34 PM

Just playing, through a blue filter the dumbbell was still there. Through a red filter, it disappeared.

Hmm...


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azure1961p
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Re: Is M27 the brightest pn? new [Re: Kraus]
      #5717269 - 03/06/13 09:01 PM

The Helix I think has more total light than M27 but its surface brightness is a fraction. The brightest planetary I've ever seen in terms of surface brightness was NGC7027. And with a filter its beaming.

Pete


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BillFerris
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Re: Is M27 the brightest pn? new [Re: GlennLeDrew]
      #5717490 - 03/06/13 10:59 PM

Yes, there are a small number of objects so large in size that their integrated magnitudes do not accurately convey the challenge of observing them. However, on the whole, integrated magnitude is a far more reliable indicator of an object's visibility than surface brightness. Ordering the 65 planetary nebulae I've observed by magnitude, the four Messier planetaries are included among the first 16 objects. Ordering by surface brightness, they fall in a range from 23 to 39 and there is certainly no rational case that can be made for 23 planetaries being easier to observe than M57. The results are similar when arranging galaxies and globular star clusters in the same manner. Integrated magnitude is far more indicative of visibility.

Bill in Flag


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GlennLeDrew
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Re: Is M27 the brightest pn? new [Re: BillFerris]
      #5717525 - 03/06/13 11:37 PM

When objects are small, integrated brightness *tends* to become a more reliable indicator of visibility. Most planetaries have sizes measured in mere arcseconds. Most galaxies are a few arcminutes in diameter. And for reasons of nature and observational selection, most of these objects have reasonably high surface brightness, with a not so large range in their class. And so there is a pretty good correlation between size and integrated brightness, which goes nicely hand in hand with the gains afforded by increases in aperture.

But when we look to the larger objects, especially diffuse nebulae, we find a larger variation in surface brightness. Even an excessively low surface brightness object extending over several square degrees (the California, for example), can have an integrated brightness of brighter than magnitude 6. But *no* telescope could make an easy meal of it, certainly not without filtration.

Furthermore, our catalogs are biased in a way so as to reinforce the misconception that integrated brightness is a useful indicator of visibility. The smaller objects, and those of higher surface brightness, are the ones for which measurements/estimates of integrated brightness are available. The larger and dimmer ones tend to not have magnitude information, integrated nor surface.

For all these reasons I am against the *blanket* statement of integrated brightness being a good indicator of visibility. Such a claim must be qualified.


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Tony Flanders
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Re: Is M27 the brightest pn? [Re: BillFerris]
      #5717745 - 03/07/13 05:21 AM

Quote:

On the whole, integrated magnitude is a far more reliable indicator of an object's visibility than surface brightness.




It's a little silly to say that either one is more reliable than the other. It depends on the observer's skill, the darkness of the skies, the instrument used, and the range of objects in consideration.

Surface brightness gains in relative importance if the observer is unskilled -- not used to noticing "lumpy darkness" -- and even more so if bright skyglow is washing out the sky.

Integrated magnitude becomes more important if you're using low magnification -- for instance, conventional binoculars. Using binoculars under a dark sky, the Helix is arguably more obvious than M27 (not that either is hard to see at all!), and both are much more obvious than M57.

Like Bill, I find integrated magnitude generally a better indicator of galaxy visibility. But that's because our galaxy catalogs are biased toward galaxies with high surface brightness. There's every reason to believe that for each galaxy cataloged there are several more whose surface brightness is too low for even the professionals to pick them up in deep images.

Likewise, there are quite a lot of huge, faint planetary nebulae that are rarely considered to be visual targets because of their ultralow surface brightness.


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BillFerris
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Re: Is M27 the brightest pn? new [Re: Tony Flanders]
      #5718366 - 03/07/13 12:46 PM

Quote:

Quote:

On the whole, integrated magnitude is a far more reliable indicator of an object's visibility than surface brightness.




It's a little silly to say that either one is more reliable than the other.




Go through the exercise of sorting a list of planetary nebulae, galaxies or globular star clusters, first by magnitude and then by surface brightness. The lists ordered by magnitude will much more closely match a subjective ranking by ease of visibility. The lists ordered by surface brightness will feature a lot of inherently faint objects at the top, objects any experienced observer would describe as challenging under the best of observing conditions.

This is a natural result of the fact that the vast majority of deep-sky objects are small. Referring again to the list of planetary nebulae I've observed: Precisely 3 are larger than 10 arcminutes in diameter, 14 are between 2 and 6 arcminutes in size, and 31 are 1 arcminute or smaller in size. Let's look at galaxies: In my list of 630 observed galaxies, 2 are larger than a degree in size, 39 are 10 to 30 arcminutes in size, 475 are 5 arcminutes or smaller in size, and 110 are less than 1 arcminute in size. Globular star clusters: 49 of 77 are smaller than 10 arcminutes in diameter, with 27 being 5 arcminutes or smaller in diameter.

Most of the above objects can be found in the Messier catalog and Herschel 400 list. They're garden variety DSO's observed by amateur astronomers throughout the world. And they're tiny. So tiny that much of their light is concentrated into a very small central area. Their perceptibility is more akin to that of a star than to one of the few large celestial objects people trot out in defense of the defenseless notion that, as a general rule, surface brightness is more indicative--or even equally indicative--of visibility than integrated magnitude.

For every M104 and M27 (high surface brightness objects with high integrated magnitudes), or for every M33 and Helix Nebula (lower surface brightness objects with high integrated mangitudes) there are dozens of PGC 53930's and Jonckheere 320's; objects with deceptively high surface brightnesses that in no way, shape or form indicate the challenge that awaits. For such objects, integrated magnitude is the one characteristic you need to know to gauge their inherent visibility.

Bill in Flag


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BillFerris
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Re: Is M27 the brightest pn? new [Re: Darren Drake]
      #5718502 - 03/07/13 01:56 PM

This may be one of those questions where the answer is the unsatisfying, "It depends." Searching online, the Webb Society catalog lists NGC 7293 (Helix Nebula) as being 0.1 magnitude brighter than NGC 6853 (M27, Dumbbell Nebula). By contrast, the Strasbourg-ESO Catalog of Galactic Planetary Nebulae data table compiled by Kent Wallace and Steve Gottlieb would rank M27, NGC 3242 (Ghost of Jupiter) and the Helix as the three brightest planetary nebulae.

The wild card in this discussion is Hewett 1, the largest known planetary nebula. At approximately 2 degrees in size with a halo extending to 10 degrees across, Hewett 1 could be as bright as 6th magnitude and still be too subtle in contrast to be visually observable.

So, if M27 isn't the brightest planetary nebulae, it is undoubtedly among the most visually impressive.

Bill in Flag


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Achernar
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Re: Is M27 the brightest pn? new [Re: Darren Drake]
      #5718544 - 03/07/13 02:22 PM

Based on integrated magnitude, that is assuming all the light comes from a point source, the brightest planetary nebulae is NGC-7293, or the Helix Nebula. However, that is spread out over an area half as wide as the full moon, and that makes individual portions of it very dim. M-27 is huge in apparent size, but the surface brightness is much higher than NGC-7293's, making it possible to see it from an urban area while the Helix is utterly impossible from a large city.

Taras


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Tony Flanders
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Re: Is M27 the brightest pn? new [Re: BillFerris]
      #5718925 - 03/07/13 05:42 PM

Quote:

Go through the exercise of sorting a list of planetary nebulae, galaxies or globular star clusters, first by magnitude and then by surface brightness. The lists ordered by magnitude will much more closely match a subjective ranking by ease of visibility.




I agree with respect to galaxies and globular clusters -- at least assuming that you're observing under reasonably dark skies. I'm not so sure about planetary nebulae.

Of those three classes, globular clusters are in fact fairly similar to each other morphologically. There are some outliers, such as NGC 5053 because it's abnormally sparse or some of the Palomar globs, which are very heavily obscured by dust. But globulars have a relatively small range of surface brightnesses.

Galaxies are a different story. As I said in an earlier note, they actually have a huge range of surface brightnesses, but the catalogs don't reflect that because most of the low-surface-brightness galaxies remain undiscovered.

Most of the known low-surface-brightness galaxies are in fact very nearby, so this bias is smaller if you consider only nearby galaxies, just as the bias against brown dwarfs is much smaller if you consider only nearby stars.

If you were to prepare a list of nearby galaxies and sort it by surface brightness, you might well find that surface brightness is a better predictor of visibility than integrated brightness. There are a lot of Local Group Dwarfs that are exceptionally hard to spot despite having fairly high integrated brightness.

What's true of galaxies is probably even truer of planetary nebulae; presumably they just grow and grow and get lower and lower in surface brightness until they become completely undetectable.

If you prepare a list of Herschel planetaries or NGC planetaries, the visibility will be well predicted by integrated brightness, but that's because visual observers of the 18th and 19th century -- having no narrowband filters -- were unable to detect the ones with low surface brightness.


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stevecoe
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Re: Is M27 the brightest pn? new [Re: Tony Flanders]
      #5718957 - 03/07/13 05:57 PM

Tony, et al;

Yes, my hero (William Herschel) did a pretty good job of sweeping up the visible planetaries with a modest telescope. Just stand out in the cold night after night in southern England!!! Then tell Caroline what you saw. It does make me appreciate my Sky Commander.

Clear skies;
Steve Coe


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Starman1
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Re: Is M27 the brightest pn? new [Re: stevecoe]
      #5718996 - 03/07/13 06:20 PM

I typically view M27 without a filter and at magnifications up to 228X.
I typically view NGC7293 with a filter and rarely over 140X and I prefer it at 89X.
The difference is surface brightness and the total integrated magnitude for these objects simply isn't relevant for observability.

For nearly all other planetaries, including M57, total integrated magnitude is the more relevant factor because they are all close enough to the same size so TIM matters. TIM and SB are often close to the same figures.

When I look for small PK planetaries of, say, 10" or less, TIM is the only relevant figure because they are so small, the SB figures for each overstate the brightness of the nebulae. Many small 14-16 magnitude planetaries have SB figures of 9-10, and that simply doesn't describe their visibilities at all.


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