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Help me understand Magnitude

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#1 Oyster74

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Posted 31 March 2020 - 05:38 PM

Hi everyone!  Finally got a clear night in over a month. I wasn't expecting it, but I had to take advantage. I went outside without a plan. Still had a blast seen the Cateye nebula for the first time, and things that I have seen before that are still awesome like the Hercules cluster.

I am in a bortle 5 area according to some maps, and there are objects I can't see that I think should be visible. I can clearly see M81, which is a mag 6.8. Also, I can see M66, which is a mag 8.9. But, I can't see M51, which is a mag 7.9. I have a goto 8" SCT, but is not visible in the piece of sky is pointing. I have scanned around the area to make sure is not outside my field of view, but I can't see it. Seems like the magnitude never corelates to what I can see or not see under my skies. Am I misunderstanding what the magnitude number is?

thanks,



#2 Jon Isaacs

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Posted 31 March 2020 - 05:53 PM

There are two basic types of magnitude.

 

Surface brightness.  This is measured in units of Magnitudes per square arc-second.  

 

Total integrated brightness or visual brightness.  If you add up all the light from a star, this is the visual brightness.  

 

Surface brightness determines the contrast, how it compares to the sky glow or light pollution.

 

Integrated brightness determines how much light you have to work with in magnifying the object. 

 

Both M66 and M51 have similar surface brightnesses, 21.7mpsas versus 21.8 mpsas.

 

So they should be very similar in how easily they're seen.  

 

I suspect what is happening is that in the evening, M66 is higher up in the sky than M51 and so it's easier to see because you're looking through less atmosphere and less light pollution.

 

I generally find M51 easier to see than M66,

 

Jon


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#3 stevenrjanssens

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Posted 31 March 2020 - 05:54 PM

If galaxies were point sources like stars, your experience would match your understanding, i.e. visibility is tied to magnitude and whatever magnitude limit your skies and equipment are capable of on a given night. But galaxies are extended sources and so it is instead the surface brightness that determines visibility. Surface brightness is the amount of light per unit area and is typically expressed in units of magnitudes per square arcsec, or magnitudes per square arcmin. The surface brightness of the object compared to the surface brightness of the sky (the sky is another extended object) determines whether an object is visible or not.


Edited by stevenrjanssens, 31 March 2020 - 05:54 PM.

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#4 Sketcher

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Posted 31 March 2020 - 06:49 PM

The magnitudes you have of various galaxies, etc. are an indication of the total amount of light those objects emit.

 

If a galaxy is listed as being magnitude 7.9, that means that if you were to look at a 7.9 magnitude star and de-focus the star until it appeared to be the same apparent size as the galaxy, then that galaxy would appear to be of about the same brightness as the de-focused star.

 

Now let's assume you've found two different 7.9 magnitude galaxies. and one is 60x60 arc-minutes in size while the other is 30x30 arc-minutes in size.  Then if you were to de-focus a 7.9 magnitude star until it appeared to be 60x60 arc-minutes in size, that de-focused star would appear to be of similar brightness as the 60x60 arc-minute galaxy.  But when you de-focus the same star until it's a smaller, 30x30 arc-minutes in size, you'll see a smaller, but (seemingly) brighter 30x30 arc-minute patch of light that more closely matches the view of the smaller galaxy.  Both of these galaxies have magnitudes of 7.9, but the larger galaxy would be more difficult to see (will appear to be fainter) than the smaller galaxy because the same 7.9 magnitudes of light would be spread out over a larger area.

 

In other words, the size of the object, along with its magnitude, must be taken into account in determining how easy or difficult a particular galaxy might be to see.

 

A real-world example:  M33 has a size of 62x39 arc-minutes and a magnitude of 5.7.  Meanwhile, M81 has a size of 26x14 arc-minutes and a magnitude of 6.8.  So, M33 is the brighter galaxy; but M81 is smaller.  With a telescope under a light-polluted sky many people who can see M81 cannot see M33.  Yes, M33 puts out more light than M81, but M81's light is more concentrated, making it easier to see.


Edited by Sketcher, 01 April 2020 - 12:02 PM.

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#5 Dave Mitsky

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Posted 01 April 2020 - 12:26 AM

The following articles discuss the concept of surface brightness:

https://astronomy.co...face-brightness

 

https://tonyflanders...ace-brightness/

 

https://www.astrobuy...com/paul/sb.htm

Here's a list of the Messier galaxies, their integrated visual magnitudes, and their surface brightness figures in magnitudes per square arc minute:
 

M31 3.4 13.6
M32 8.1 12.7
M33 5.7 14.2
M51 8.4 12.6
M58 9.7 13.0
M59 9.6 12.5
M60 8.8 12.8
M61 9.7 13.4
M63 8.6 13.6
M64 8.5 12.4
M65 9.3 12.4
M66 8.9 12.5
M74 9.4 14.4
M77 8.9 13.2
M81 6.9 13.0
M82 8.4 12.8
M83 7.6 13.2
M84 9.1 12.3
M85 9.1 13.0
M86 8.9 13.9
M87 8.6 12.7
M88 9.6 12.6
M89 9.8 12.3
M90 9.5 13.6
M91 10.2 13.3
M94 8.2 13.5
M95 9.7 13.5
M96 9.2 12.9
M98 10.1 13.2
M99 9.9 13.0
M100 9.3 13.0
M101 7.9 14.8
M104 8.0 11.6
M105 9.3 12.1
M106 8.4 13.8
M108 10.0 13.0
M109 9.8 13.5
M110 8.1 13.9

 

The Messier galaxies with the lowest surface brightness figures are M33, M74, and M101, all of which can be very difficult to detect in a light-polluted sky.  Many observers consider M74 (the Phantom Galaxy) to be the toughest of the Messier objects to detect in a less-than-pristine sky.


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#6 Redbetter

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Posted 01 April 2020 - 02:36 AM

At 8.4 magnitude, M51 has good surface brightness of around 21.8 MPSAS, but Bortle 5 skies can make it difficult to see, especially if one is on the brighter side of Bortle 5.  I gave it a go tonight (first clear night in weeks) with a 60mm refractor in town with half moon in the sky, 18.95 MPSAS sky measured in that direction.  I could just barely identify M51 and the companion, mostly the central brightening of each, but with a general wider expanse to the primary galaxy in averted vision.   A novice would find it more difficult, even with a mid aperture scope.  

 

Galaxies aren't much fun to look at in town, and Bortle 5 qualifies as in town.  Higher surface brightness targets such as open clusters and some of the more condensed globulars will be more rewarding than galaxies in Bortle 5 skies.


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#7 Jon Isaacs

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Posted 01 April 2020 - 05:54 AM

Here's a list of the Messier galaxies, their integrated visual magnitudes, and their surface brightness figures in magnitudes per square arc minute:

 

 

Daves list is in units of Magnitudes Per Square Arc-Minutes, I used Magnitudes Per Square Arc-Second. To convert mpsam to mpsas second,  just add 8.9.

 

The advantage of mpsas is that sky brightness is usually measured in mpsas so contrast can easily be determined. Magnitudes are logarithmic so ratios are found by subtraction. If the sky is magnitude 20.0 mpsas and the object 21.8 mpsas, the object has a surface brightness that 1.8 magnitudes dimmer.

 

It's also worth noting that these surface brightnesses are average surface brightnesses. Many galaxies have a bright core with a much higher surface brightness. Andromeda, M31, is listed at 13.6 mpsam but has a very bright core so it can be easily seen in hand held binoculars from most urban skies.

 

Also important is using magnification. Most galaxies are small and best seen at exit pupils from about 1.5 mm to 2.5 mm. In an 8 inch (200 mm) this would be from about 80x to 135x. It never hurts to experiment.. 

 

Jon


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#8 Tony Flanders

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Posted 01 April 2020 - 06:42 AM

Here's my take on this: Numbers only get you so far. First of all, you will get different figures for both galaxies's brightness depending what source you consult. The data are especially suspect for M51, which is actually a galaxy pair rather than a single galaxy.

 

Anyway, regardless of what source you consult, you're likely to find that both M51 and M66 are in the same ballpark, but M51 should be a bit easier to see than M66. My own experience is just the opposite: I find M66 much easier than M51 in badly light-polluted skies, and a tad easier under dark skies.

 

I suspect that has to do with the galaxies's morphology. Both of them are spirals, but M51 is a very unusual spiral, due no doubt to the fact that it's interacting with its companion. The core is relatively modest, and the spiral arms are much, much brighter than normal. The net result is that through a small telescope, it appears more or less like a uniformly bright circle extending all the way out to its theoretical size. With M66, by contrast, the core is very bright, and the brightest parts of the spiral arms are right next to the core. The net result is that through a small telescope, especially under brightish skies, it appears like a very bright, extended core surrounded by a very faint halo whose brightness drops off quite rapidly. I think the fact that M66's light is so heavily concentrated toward the center is what makes it a bit easier to spot, despite the fact that the numbers suggest the opposite.


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#9 Jon Isaacs

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Posted 01 April 2020 - 11:08 AM

I find M66 much easier than M51 in badly light-polluted skies, and a tad easier under dark skies.

 

My own experience is just the opposite, I see the central core of M51 quite readily from my light polluted skies, M66 seems more difficult.  

 

Jon



#10 Oyster74

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Posted 01 April 2020 - 12:13 PM

Thanks for the replies and links. All the different explanations on how surface brightness is the true indication of contrast make perfect sense. It helps paint a clearer picture on how the numbers work. I like it!!!
On the practical side. I use a 35mm, which gives me around 60x. I start a low mag to try to spot things easier. I am going to experiment with different magnifications to see if I have better luck with M51. It sounds like it should be visible at the right place in the sky and maybe I have not learned to spot what it looks like in real life.



#11 Starman1

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Posted 01 April 2020 - 04:34 PM

Thanks for the replies and links. All the different explanations on how surface brightness is the true indication of contrast make perfect sense. It helps paint a clearer picture on how the numbers work. I like it!!!
On the practical side. I use a 35mm, which gives me around 60x. I start a low mag to try to spot things easier. I am going to experiment with different magnifications to see if I have better luck with M51. It sounds like it should be visible at the right place in the sky and maybe I have not learned to spot what it looks like in real life.

A couple thousand years ago, Ptolemy decided to divide the stars in the sky into "bins" of magnitudes.  The brightest stars he called "First magnitude" because they were of primary importance.

The faintest stars he saw with his eyes were binned into the 6th magnitude and others in between.

When instruments started measuring these brightnesses, we discovered that First magnitude covered a wide range (1+ to -1.42) and that there was approximately a ratio of 100:1 between the faintest and brightest.

 

Since our eye sees brightness logarithmically, that meant the 5 magnitude spread was 100, so each magnitude represented the 5th root of 100, or 2.512, i.e. 5th magnitude is 2.512x as bright as 6th magnitude, 4th magnitude is 2.512x as bright as magnitude 5, and so on.

 

On that same scale, the sun is magnitude -26.7, the full Moon is about magnitude -12.5, and Vega is magnitude 0.03 (pretty close to mag. 0).

So, magnitude 10 would be 2.512 to the tenth power dimmer than magnitude 0, or 1/10000 times as bright.  By calculation, the brightest star I've looked at through my scope is about 37 million times 

brighter than the faintest star I've seen.  Quite a range.

 

Surface brightness is calculated from the size of the object, so a magnitude 10 galaxy 1' across would appear quite bright, while a 10th magnitude galaxy 15' across would appear quite dim.

So you need to not only know the surface brightness, but also the size and total integrated magnitude figure to get an idea if the object will be hard to see or not.

And, galaxies in particular have cores that are brighter than the outskirts.  You might not see spiral arms, but the cores of very faint galaxies are usually visible.

 

Your technique of starting at low power, then bumping it up to view the object is a sound one, though this might be hard with planetary nebulae, which are usually small.  With those, starting at a medium power might aid in finding the objects.

 

As for M51, try a magnification around 15x/inch of aperture to see it well.  Lower powers will show the galaxy fine, but you'll not see much detail.


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#12 knUk23

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Posted 01 April 2020 - 09:27 PM

I just wanted to pop in and say I've found this discussion enormously beneficial - it is answering questions I was still trying to figure out how to ask!  I've been spending time so far looking at a lot of open clusters and have had similar experiences to the OP, where a cluster with a dimmer magnitude somehow seems easier to see than a cluster with a brighter magnitude - I am assuming it is basically the same phenomenon being described above for galaxies?


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#13 Tony Flanders

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Posted 02 April 2020 - 04:21 AM

My own experience is just the opposite, I see the central core of M51 quite readily from my light polluted skies, M66 seems more difficult.


So there you are -- it just proves that there's no possible formula to calculate whether an one object is "easier" than another, since the answer can vary from one person to another.
 
I might also say that when I look back over my notes, there have been occasions when I thought object A was easier to see than object B, and other times when I thought B was easier than A -- even using the same instrument from the same location, with both objects at similar altitudes. So you don't even need to resort to two different observers; a single observer may have varying experiences at different times, for heaven knows what reason.


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#14 JoshUrban

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Posted 02 April 2020 - 07:33 AM

Yeah, try those higher powers, and also, I find the sky  conditions in terms of transparency make a huge difference for my observations.  It can be helpful to have a test object to see how it varies night by night.  

 

  Magnitudes can be misleading at first glance (so thanks to the crew here for some great explanations - enjoyed reading!), as well as classifications.  I was looking for M101 the other night with a 12.5" dob, and had such an easier time of seeing obscure NGC galaxies than a "showpiece" Messier object.  I always like trying my luck.  The worst that happens is I see empty space, and that's pretty cool, too.  


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#15 sonny.barile

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Posted 12 April 2020 - 11:24 AM

So there you are -- it just proves that there's no possible formula to calculate whether an one object is "easier" than another, since the answer can vary from one person to another.
 
I might also say that when I look back over my notes, there have been occasions when I thought object A was easier to see than object B, and other times when I thought B was easier than A -- even using the same instrument from the same location, with both objects at similar altitudes. So you don't even need to resort to two different observers; a single observer may have varying experiences at different times, for heaven knows what reason.

Those may be the days we ate our carrots 🥕  lol.gif

 

We are complex creatures and our vision and perception is not going to be the same on each day or even each moment of a session.

 

Some of our detection ability can be diminished by the brightness of what we viewed moments earlier. 


Edited by sonny.barile, 12 April 2020 - 11:27 AM.



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