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Is it a galaxy or is it a planetary nebula

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#1 Keith Rivich

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Posted 08 November 2019 - 12:40 PM

A friend is working on an online astronomy course and asked me this question based on a part of the lesson:

 

Is there a way to visually confirm a DSO is either a planetary nebula or a galaxy? What test can you perform using the commonly available tools in your visual toolbox?

 

The example given is Hoag's Object (a galaxy) https://apod.nasa.go...d/ap040815.html vs. a similar ringed planetary. Example: http://annesastronomynews.com/photo-gallery-ii/nebulae-clouds/the-fine-ring-nebula/

 

I know one can definitively test for one type but the other is not definitive as it is a process of elimination which cannot lead to a definitive answer. 

 

I'm curious to read your thoughts...



#2 John Miele

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Posted 08 November 2019 - 12:43 PM

Apply OIII filter. Usually PN's stay bright whereas a galaxy would go dim.


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#3 Keith Rivich

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Posted 08 November 2019 - 12:53 PM

Apply OIII filter. Usually PN's stay bright whereas a galaxy would go dim.

But not all PN respond to the OIII. Another gotcha is some SNR's will also respond to the OIII.

 

I think the only definitive answer is: It responded to an OIII, so it is not a galaxy. But I may be wrong...



#4 John Miele

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Posted 08 November 2019 - 01:01 PM

True. That's why I said "usually". I sort of missed you are looking for a "definitive" visual test. That will be tough...

 

You are right on the SNR's too. I was amazed how well the veil nebula responds to an OIII. But the shape of the veil will tell you it's not a galaxy.

 

Maybe the shape can play a role?


Edited by John Miele, 08 November 2019 - 01:04 PM.


#5 Keith Rivich

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Posted 08 November 2019 - 01:09 PM

True. That's why I said "usually". I sort of missed you are looking for a "definitive" visual test. That will be tough...

 

You are right on the SNR's too. I was amazed how well the veil nebula responds to an OIII. But the shape of the veil will tell you it's not a galaxy.

 

Maybe the shape can play a role?

That's what I was thinking. I haven't seen any ring PN that have a central condensation that Hoag's has. 



#6 Jon Isaacs

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Posted 08 November 2019 - 01:24 PM

Kevin:

 

I think one can guess.  But even a faint wispy globular cluster and seem galactic.  There are characteristics that experience suggests an object is probably a galaxy.  Until Astronomers had telescopes capable of resolving a star in Andromeda, it was not really known.. 

 

Jon


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#7 junomike

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Posted 08 November 2019 - 01:46 PM

I just cheat and check Skysafari.  grin.gif


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#8 Keith Rivich

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Posted 08 November 2019 - 02:37 PM

Kevin:

 

I think one can guess.  But even a faint wispy globular cluster and seem galactic.  There are characteristics that experience suggests an object is probably a galaxy.  Until Astronomers had telescopes capable of resolving a star in Andromeda, it was not really known.. 

 

Jon

Agreed. I think the point of the exercise was to think about how each DSO behaves visually then create a hypothesis and design an observational test. As I noted above the only *difinitive* proof comes when the DSO responds to a narrow band filter. If it responds it is not a galaxy. I do not believe there are any OIII bright galaxies. Parts of galaxies, yes, but not as a whole.


Edited by Keith Rivich, 08 November 2019 - 03:01 PM.

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#9 Dwight J

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Posted 08 November 2019 - 07:46 PM

The prism test should work.  A direct vision prism will disperse the stars into streaks while the planetary nebula will stay starlike.  A galaxy would do as the stars do.  


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#10 j.gardavsky

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Posted 09 November 2019 - 05:06 AM

The prism test should work.  A direct vision prism will disperse the stars into streaks while the planetary nebula will stay starlike.  A galaxy would do as the stars do.  

The Amici straight view spectral prisms are often difficult to find on budget.

I have this one,

https://www.ebay.de/...kYAAOSwduxbJ7~F

 

Best,

JG



#11 Keith Rivich

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Posted 09 November 2019 - 09:18 AM

The prism test should work.  A direct vision prism will disperse the stars into streaks while the planetary nebula will stay starlike.  A galaxy would do as the stars do.  

If I am not mistaken doesn't that only work on stellar PN's?



#12 Achernar

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Posted 09 November 2019 - 10:42 AM

Apply OIII filter. Usually PN's stay bright whereas a galaxy would go dim.

In general, this is good advice even though I have personally seen planetary nebulae that disappear through an O-II, but don't through a narrowband or H-beta filter. Probably if one uses both a H-beta and O-III on an object, and it's not enhanced by either of them, it's a good bet that it's not a nebula. A diffraction grating would be definitive since planetary nebulae emit only a few wavelengths of visible light, but galaxies emit across the whole visible spectrum.

 

Taras


Edited by Achernar, 09 November 2019 - 10:43 AM.


#13 j.gardavsky

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Posted 09 November 2019 - 01:37 PM

If I am not mistaken doesn't that only work on stellar PN's?

Not only,

here is the spectrum of the Ring Nebula, which is truly not stellar,

http://www.astrosurf...spe6/planet.htm

 

Best,

JG


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#14 AstroVPK

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Posted 12 November 2019 - 07:35 PM

Its a deep question all right. Here's a good write-up on the Great Debate between Harlow Shapeley & Herber Curtis about the true nature of galaxies - https://apod.nasa.go...e/debate20.html


Edited by AstroVPK, 12 November 2019 - 07:39 PM.

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