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Why do galaxies and globs have diff magnitude diff

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#1 Darren Drake

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Posted 21 March 2013 - 08:45 AM

Why do they seem to have different magnitude difficulty levels? It seems as the the dimmer globs such as NGC 5053 which is listed at around mag 9.5 is MUCH more challenging to see than a 9.5 mag galaxy. I've come to expect it to be a pretty difficult challenge to spot an 8th or 9th magnitude globular but a similar magnitude galaxy would be easy pickins. Anyone else ever notice this???

#2 blb

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Posted 21 March 2013 - 09:09 AM

It's all about surface brightness. Almost all galaxies have a bright central core region which makes them easier to see, although the outer halo is much fainter. Even though globular clusters are brighter in the center their seems to be less differance and generaly they are larger than most of the galaxies. Hense there light is spread out more, making them harder to see. :bawling:

#3 LivingNDixie

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Posted 21 March 2013 - 09:28 AM

Also magnitudes for Galaxies and Globular Clusters are if the object were stellar, once an object is more then a star that light is spread out. Magnitude is important but knowing the size of an object is just important. Lastly Magnitude can be in various wavelengths, that is why some objects are not as tough as the listed Magnitude suggests.
There is a good article on this, it is by Jay Reynolds Freeman, and it explains how he completed the H400 with a small refractor.

#4 blb

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Posted 21 March 2013 - 10:22 AM

Also magnitudes for Galaxies and Globular Clusters are if the object were stellar

That is the integrated magnitude listed for each object. It is what the magnitude of all the light from the object would be if it were seen as a point source.

once an object is more then a star that light is spread out.

That is surface brightness and the larger an object is the fainter it will appear to be. Two objects that are mag. 8, one is twice as large as the other, the larger one will appear to be about four times as faint. Thats surface brightness.

Lastly Magnitude can be in various wavelengths, that is why some objects are not as tough as the listed Magnitude suggests.

Yes, most of the older information gives photographic magnitudes in blue light. Those photographic magnitudes are about one magnitude fainter than visual magnitudes. Try to make sure you are looking at visual magnitudes in your research.

There is a good article on this, it is by Jay Reynolds Freeman,

I wish we all had Jay's eyes and dark skies. He is a great resource of observing information.

#5 Tony Flanders

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Posted 21 March 2013 - 01:24 PM

It seems as the the dimmer globs such as NGC 5053 which is listed at around mag 9.5 is MUCH more challenging to see than a 9.5 mag galaxy. I've come to expect it to be a pretty difficult challenge to spot an 8th or 9th magnitude globular but a similar magnitude galaxy would be easy pickins.


Actually, by and large I expect a 9th-magnitude globular cluster to be easier to spot than a galaxy of equal magnitude. There are obvious exceptions, like NGC 5053. But there are also many galaxies with truly low surface brightness -- ones that make NGC 5053 look like a blinding spotlight by comparison.

Among the Messier galaxies, M33 and M101 are the most obvious examples. But the real toughies are the Local Group dwarf galaxies. Start with NGC 6822, Barnard's Galaxy, which is one of the easiest. After that you can tackle real toughies like IC 1613 or the Sextans or Draco Dwarfs. All of those are 9th magnitude or brighter, and all of them are utterly invisible unless your skies are pristine -- and quite likely even then.

#6 GlennLeDrew

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Posted 21 March 2013 - 01:52 PM

To illustrate with an even more profound example. Take a nebula like the North America, with a surface brightness of about 24 magnitudes per square arcsecond (15 magnitudes per square arcminute). That's two magnitudes fainter than the darkest night sky. If we take a mean diameter of two degrees, its integrated magnitude is about 5. Yet it can make for a difficult detection in sizeable instruments.

#7 Dave Mitsky

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Posted 21 March 2013 - 02:02 PM

NGC 5053 is a bit of an oddball when it comes to globular clusters.

Dave Mitsky

#8 LivingNDixie

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Posted 21 March 2013 - 03:17 PM

NGC 5053 is a bit of an oddball when it comes to globular clusters.

Dave Mitsky

Yes it is :lol: .

#9 uniondrone

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Posted 21 March 2013 - 03:54 PM

NGC 5053 is a bit of an oddball when it comes to globular clusters.

Dave Mitsky

Yes it is :lol: .


Or is it an oddglob? :grin:

#10 Tom Polakis

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Posted 21 March 2013 - 05:01 PM

It seems as the the dimmer globs such as NGC 5053 which is listed at around mag 9.5 is MUCH more challenging to see than a 9.5 mag galaxy. I've come to expect it to be a pretty difficult challenge to spot an 8th or 9th magnitude globular but a similar magnitude galaxy would be easy pickins.


Actually, by and large I expect a 9th-magnitude globular cluster to be easier to spot than a galaxy of equal magnitude. There are obvious exceptions, like NGC 5053. But there are also many galaxies with truly low surface brightness -- ones that make NGC 5053 look like a blinding spotlight by comparison.

Among the Messier galaxies, M33 and M101 are the most obvious examples. But the real toughies are the Local Group dwarf galaxies. Start with NGC 6822, Barnard's Galaxy, which is one of the easiest. After that you can tackle real toughies like IC 1613 or the Sextans or Draco Dwarfs. All of those are 9th magnitude or brighter, and all of them are utterly invisible unless your skies are pristine -- and quite likely even then.



Tony,

I'm with the original poster in typically seeing galaxies more easily than globulars of the same magnitude. Let's go a magnitude deeper than his example, and eliminate the outlier Local Group dwarf galaxies, which have low surface brightness.

Around magnitude 10, a few well-known examples with similar angular sizes from each class would be:

Globular clusters:
NGC 6229 (Her)
NGC 2419 (Lyn)
NGC 7006 (Del)


Galaxies:
NGC 5005 (CVn)
M89 (Vir)
NGC 2976 (UMa)


I tried not to cherry pick examples to prove my point. In the typical case, those 10th magnitude galaxies show up in my little 70mm Pronto scope without effort, but those globular clusters are going to take some time.

I'll buy into the explanation given in this thread that the surface brightness of a galaxy is typically more peaked in the center, and that's what gets picked up in small scopes or under a bright sky. Globular clusters are typically more broadly concentrated.

When you get down to the level of 11th magnitude globular clusters, my 18-inch has a problem with many of them, while a galaxy of 11th magnitude is still bright enough to possibly show some structure.

Tom

#11 uniondrone

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Posted 21 March 2013 - 05:32 PM

Tom,

I agree with your assessment. With globular clusters, I find that class really matters a lot, perhaps more than magnitude, size, or overall surface brightness. The Class X, XI, and XII globs generally pose a bigger challenge, unless they're exceptionally bright or the sky is rather dark. The Class I-VII globs, on the other hand, do much better as they peak magnitude per square arcsecond is brighter.

#12 FirstSight

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Posted 22 March 2013 - 07:18 AM

An example of globular clusters which are nominally close in magnitude, yet one is much easier to see than the other, is M9 (mag either 7.7 or 8.4, depending on source) vs M107 (N. 7.9 or 8.8,depending on source). They are nearby each other in southern Ophiuchus, facilitating comparison. M107 is a fairly loose GC, whereas M9 is more densely packed.

Early this morning in predawn skies, I was observing from my suburban Raleigh location with my NP101 and could easily find M9 as a faint puffy smear of light in an 8E and even a 21E, even though skyglow the Raleigh metro area to my south unfortunately significantly impairs viewing in that direction, but I was unable to detect M107 despite any amount of searching with any focal-length eyepiece. Since both these GCs are at nearly the same altitude and less than a degree apart R.A., this pair make a good comparison for the differntial effects of their respective compositions on their visibility. Under darker sky conditions, of course, M107 would likely be almost trivially easy to spot with the aperture of an NP101.

#13 Tony Flanders

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Posted 22 March 2013 - 07:59 AM

I'm with the original poster in typically seeing galaxies more easily than globulars of the same magnitude . . .

I'll buy into the explanation given in this thread that the surface brightness of a galaxy is typically more peaked in the center, and that's what gets picked up in small scopes or under a bright sky. Globular clusters are typically more broadly concentrated . . .

When you get down to the level of 11th magnitude globular clusters, my 18-inch has a problem with many of them, while a galaxy of 11th magnitude is still bright enough to possibly show some structure.


Interesting; I really need to go through my notes and study this systematically. I had sort of assumed that globs are easier to see, but now that I think of it, that's likely because there are a lot more bright globulars than bright galaxies.

#14 Sarkikos

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Posted 24 March 2013 - 12:24 PM

FWIW, I've seen GC NGC 5053 through my 10" f/4.8 Dob at a yellow zone site. So far I've seen 82 globs and 451 galaxies at this site. The Terzans and Pals have been a problem, but I've even bagged some of them.

Mike

#15 Sarkikos

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Posted 24 March 2013 - 12:33 PM

Tony,

Among the Messier galaxies, M33 and M101 are the most obvious examples. But the real toughies are the Local Group dwarf galaxies. Start with NGC 6822, Barnard's Galaxy, which is one of the easiest. After that you can tackle real toughies like IC 1613 or the Sextans or Draco Dwarfs. All of those are 9th magnitude or brighter, and all of them are utterly invisible unless your skies are pristine -- and quite likely even then.


I've tried 6822 a couple times with no success. If you have the info, what instrument did you use to see it, the eyepiece, magnification and how were the skies (color zone)? The best dark site I have ready access to is a yellow zone. I'd like to give it another try this summer.

Mike

#16 Tony Flanders

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Posted 24 March 2013 - 03:35 PM

I've tried 6822 a couple times with no success. If you have the info, what instrument did you use to see it, the eyepiece, magnification and how were the skies (color zone)?


Like M33, the visibility of NGC 6822 depends a lot more on sky darkness than on aperture.

I've seen it from my country home in 15x70 binoculars, my 7-inch Dob, and my 12.5-inch Dob, but it's quite challenging to detect in the two smaller instruments. Not visible, by the way, in my 70-mm refractor -- a classic case where two eyes beat the ability to vary magnification.

This is in the yellow zone according to the original light-pollution map and the green zone according to the snow-corrected version.

Under dark skies, NGC 6822 is visible though not especially easy through a 60-mm refractor and quite impressive through bigger telescopes.

My best views have been at surprisingly low magnifications, exit pupils 2.5 to 5 mm.

No doubt it's easier from more southerly locations. I suspect it would be fairly easy from a yellow zone if it were high in the sky -- which it never is from the northern U.S.

#17 Jim Nelson

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Posted 24 March 2013 - 10:01 PM

NGC 6822 really is all about the darkness of the sky. From a borderline yellow/green zone, I find it tough but possible in a 4.5inch scope. From really truly dark skies (desert skies in New Mexico), I'm almost tempted to call it easy in a 4.5 inch scope, and I even suspected I saw it in 8x42 binoculars - it was a little too crowded in the region, especially in such dark skies, for me to be sure I wasn't just seeing some knot in the Milky Way; however, given it's size and integrated magnitude, I wouldn't be surprised if it's possible.

#18 blb

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Posted 25 March 2013 - 09:33 AM

Hey Barnard's galaxy (NGC 6822) was discovered by Edward Emerson Barnard (1857 - 1923) in 1884 using a 5-inch refractor. My point is that it can be seen with a small telescope if you have dark enough skies. Dark skies and low magnification with a wide field-of-view eyepiece should make the galaxy pop with some tube tapping, averted vision, patience, etc., etc. This galaxy is probably not going to be seen with very much light pollution though.

#19 LivingNDixie

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Posted 25 March 2013 - 10:31 AM

Barnard's Galaxy was pretty easy from West Texas with every instrument I looked at it with. I haven't tried for it here in Alabama.

#20 deepskydarrell

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Posted 25 March 2013 - 11:15 AM

In gray zone skies I often use Barnard's galaxy as an indicator of conditions. Hop up from the handle of the teaspoon and see if it's there, keep going north another 50' to 6818 --the Little Gem and enjoy, plus confirm if I'm in the right area. Then head back down to Barnard's for another look. If Barnard's is easy then the evening is ripe for other faint pickings.

My globular nemesis is Pal 5 just two degrees south of M 5. At Mag 11.8, brightest stars: 15.5, size 8', class XII, I've been stymied for several years. It's strange when I can get globs in the Fornax Dwarf but not the much closer / larger Pal 5. It would seem class is the big difference.

Now with my new optics I should be able to get Pal 5 on a good Barnard's night.

DSD.

#21 Sarkikos

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Posted 25 March 2013 - 11:30 AM

Tony,

Like M33, the visibility of NGC 6822 depends a lot more on sky darkness than on aperture.


In my experience, M33 is not so difficult. I've seen it several times through 10x50mm binos at home in a red zone. At my yellow zone site, unless it is in the western haze for a Messier Marathon, M33 is easy. I think NGC 6822 must be appreciably more difficult. But I'll take your advice and give it another try this summer.

Mike

#22 Tony Flanders

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Posted 25 March 2013 - 01:20 PM

In my experience, M33 is not so difficult ... I think NGC 6822 must be appreciably more difficult.


Oh yes, M33 is a piece of cake compared to NGC 6822.

My point was simply that since they're both quite large and diffuse, with no highly concentrated centers, their visibility is limited almost entirely by their surface brightness rather than by their absolute brightness.

M33 is indeed fairly easy for an experienced observer to spot with 7x35 binoculars from the red zone -- and not a whole lot easier through an 8-inch telescope.

Likewise, NGC 6822 is fairly easy for an experienced observer to spot with 15x70 binoculars from a black zone -- and not a whole lot easier through an 8-inch telescope. But it's totally out of the question from the red zone.

#23 Sarkikos

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Posted 25 March 2013 - 01:28 PM

Yes, I would not even try for NGC 6822 here in a red zone. Somethings are not worth the time and effort. On the other hand, IMO, all blue/gray/black zones are way too far away for me to travel to unless I plan on spending several days there. Not gonna happen'. But since you have seen NGC 6822 in a yellow zone, it is worth the effort for me at my usual dark site.

Mike

#24 Bill Barlow

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Posted 25 March 2013 - 09:49 PM

This is one galaxy that I have been trying to observe for a few years now, but with no success. I have viewed the Little Gem nebula many times, but never Barnards galaxy..at least I don't think I have. Does it show just a more densely packed group of stars? Is there any nebuloscity? I have used scopes up to a Meade 12 and C14 in a yellow zone. I don't have goto, as I star-hop to my targets. Thanks..

Bill

#25 deepskydarrell

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Posted 26 March 2013 - 01:11 AM

This is one galaxy that I have been trying to observe for a few years now, but with no success. I have viewed the Little Gem nebula many times, but never Barnards galaxy..at least I don't think I have. Does it show just a more densely packed group of stars? Is there any nebuloscity? I have used scopes up to a Meade 12 and C14 in a yellow zone. I don't have goto, as I star-hop to my targets. Thanks..

Bill


Bill, it's usually just a drift by kind of thing. No real stars but a glow that at first is noticed on the move and on better nights can be recognized when still. I get it with my lowest powers: 63X and 70X. At 15.5' by 13.5' it's a good size. Once I got the emission neb IC 1308 as a brighter patch NNE of its centre.

There are quite a few field stars since it's only about 20° from the galactic equator and that can make the hop messy. Perhaps a deep print out would help navigate that. Sometimes not knowing exactly where it is helps me confirm that I've really seen it --- if that makes any sense.

Your latitude is 11° farther south of my dark sky site so it should be that much higher in your sky and that should help. My 16.5 inch optics were only fair+ so a 14" or 12" should catch it eventually. I have gotten it with my old 8" but darkness could be the true key? I waited 3 or so years for the perfect conditions for finally getting the Draco dwarf -- in that instance transparency was very important.

Hope you can get it this summer.

Darrell.






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