How do I - Magnitudes of DSO's?

I'm a bit confused here, or maybe I'm really not, you can tell me.

Since it is nice and cloudy here, I was planning out my next viewing
sessions. I was looking at some of the Messier objects comimg up
(some neat Clusters!) and noticed some Galaxies.

"Hmmm...Magnitudes in the 9's. That's going to be tough," I think.

To check, I go back and look, at M81 and M82, the only two galaxies
I've found so far.

This is when things begin to go downhill.

According to what I found, M81 has a magnitude of 6.9 and M82 of 8.4.
Now, through my eyepiece, M82 looks MUCH brighter. Thinking about it
though, I realize that Andromeda has a magnitude in the high 3's but
it is definitely dimmer than 4th magnitude stars.

So, how do they figure the magnitude of a DSO? The nearest I can
guess is that they must take the total illumination over the entire
area of the object. That would tend to make M82 appear brighter in
the eyepiece because it is much smaller than M81, even though M81
would have the higher total brightness because it is so much larger.

This seems inherently confusing to me since the lower the magnitude,
the brighter the object should be, but I'm sure there is a reason for
it...

Thanks.

Clear, Dark Skies.

Mark

According to what I found, M81 has a magnitude of 6.9 and M82 of 8.4.
Now, through my eyepiece, M82 looks MUCH brighter. Thinking about it
though, I realize that Andromeda has a magnitude in the high 3's but
it is definitely dimmer than 4th magnitude stars.


What you are looking for here is 'Surface Brightness" which is some sort of
normalized measure of magnitude per unit surface area.

M81 Magnitude 6.9 Surface Brightness 13.2

M82 Magnitude 8.4 Surface Brightness 12.5

Most star mapping software provides both the surface brightness and the visual
magnitude.

Of course one has to be careful in interpreting this information, surface
brightness is really an average value so
that M31 has a visual magnitude of somewhere around 4 but a surface brightness
of something over 14. However the core is much brighter so one can see part of
M31 even when there is significant light pollution.

However Andomeda is about 1 degree x 3 degrees in size and it takes some quite
dark skies coupled with a scope capable of a wide field view to see the entire
galaxy.

jon

According to what I found, M81 has a magnitude of 6.9 and M82 of 8.4.
Now, through my eyepiece, M82 looks MUCH brighter. Thinking about it
though, I realize that Andromeda has a magnitude in the high 3's but
it is definitely dimmer than 4th magnitude stars.


What you are looking for here is 'Surface Brightness" which is some sort of
normalized measure of magnitude per unit surface area.

M81 Magnitude 6.9 Surface Brightness 13.2

M82 Magnitude 8.4 Surface Brightness 12.5

Most star mapping software provides both the surface brightness and the visual
magnitude.

Of course one has to be careful in interpreting this information, surface
brightness is really an average value so
that M31 has a visual magnitude of somewhere around 4 but a surface brightness
of something over 14. However the core is much brighter so one can see part of
M31 even when there is significant light pollution.

However Andomeda is about 1 degree x 3 degrees in size and it takes some quite
dark skies coupled with a scope capable of a wide field view to see the entire
galaxy.

jon

So, how do they figure the magnitude of a DSO? The nearest I can
guess is that they must take the total illumination over the entire
area of the object.

Hi:

Yes. The "visual magnitude" you see listed is the "integrated" magnitude for
the object. IOW, it's how bright that galaxy would be if it were squished down
to a point source. That's why the difficult (from less than prisitine skies)
M101 can sound so bright at 8.3, but be so tough. That mag 8.3 light is spread
out over the surface of a large, face-on Sc galaxy.

Many programs, like Skytools 2, will list surface brightness, and I'm sure many
of the catalogs on the net wil have it too. Take a look at the data in the NASA
Extragalactic Database (NED).

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

So, how do they figure the magnitude of a DSO? The nearest I can
guess is that they must take the total illumination over the entire
area of the object.

Hi:

Yes. The "visual magnitude" you see listed is the "integrated" magnitude for
the object. IOW, it's how bright that galaxy would be if it were squished down
to a point source. That's why the difficult (from less than prisitine skies)
M101 can sound so bright at 8.3, but be so tough. That mag 8.3 light is spread
out over the surface of a large, face-on Sc galaxy.

Many programs, like Skytools 2, will list surface brightness, and I'm sure many
of the catalogs on the net wil have it too. Take a look at the data in the NASA
Extragalactic Database (NED).

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

M81 Magnitude 6.9 Surface Brightness 13.2

M82 Magnitude 8.4 Surface Brightness 12.5

Most star mapping software provides both the surface brightness and the visual
magnitude.


OK. I found Surface Brightness (thanks to both You and Rod), now what
does that mean? Is a surface brightness of 12.5 equivalent to a 12.5
magnitude star? Does a difference of 1 in Surface Brightness equate
to a difference in 1 of Magnitude?

Thanks.

Clear, Dark Skies
Mark

M81 Magnitude 6.9 Surface Brightness 13.2

M82 Magnitude 8.4 Surface Brightness 12.5

Most star mapping software provides both the surface brightness and the visual
magnitude.


OK. I found Surface Brightness (thanks to both You and Rod), now what
does that mean? Is a surface brightness of 12.5 equivalent to a 12.5
magnitude star? Does a difference of 1 in Surface Brightness equate
to a difference in 1 of Magnitude?

Thanks.

Clear, Dark Skies
Mark

Hi there Edward. Well, since DSO's are generally "extended" objects with some
appreciable size, the magnitudes often quoted for them are what are known as
"total integrated magnitudes". This is effectively the magnitude the object
would have if all of its light were concentrated into a single point source
like a star. At a low enough power, galaxies may appear more point-like,
which accounts for some people actually picking up M81 with the unaided eye
(magnitude 6.9). In 7x35 binoculars, M81 still seems to look a bit brighter
than M82, but in a telescope, the larger size and slightly lower mean surface
brightness of M81 can make M82 (magnitude 8.4) seem to stand out a bit more.
What is generally more useful is what is known as the mean surface brightness
figure, usually quoted in "magnitudes per square arc minute". This is
generally calculated by a formula based on the angular size of the galaxy down
to some brightness level ("isophote"), which allows one to calculate the
surface area. It is then fairly easy to calculate the mean surface brightness
and thus give an idea of how bright the object actually will appear. M81 is
27.1' x 14.2' in size and has a mean surface brightness of 12.4 magnitudes
per square arc minute, while M82 is only 11.3' x 4.2' in size and has a mean
surface brightness of 12.5 magnitudes per arc minute. Thus, even though M81
is brighter, its larger size may make it seem to be less prominent. I tend to
let the brighter core of M81 dominate, so even in a telescope, I still feel
M81 to look like the brighter of the two galaxies. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Hi there Edward. Well, since DSO's are generally "extended" objects with some
appreciable size, the magnitudes often quoted for them are what are known as
"total integrated magnitudes". This is effectively the magnitude the object
would have if all of its light were concentrated into a single point source
like a star. At a low enough power, galaxies may appear more point-like,
which accounts for some people actually picking up M81 with the unaided eye
(magnitude 6.9). In 7x35 binoculars, M81 still seems to look a bit brighter
than M82, but in a telescope, the larger size and slightly lower mean surface
brightness of M81 can make M82 (magnitude 8.4) seem to stand out a bit more.
What is generally more useful is what is known as the mean surface brightness
figure, usually quoted in "magnitudes per square arc minute". This is
generally calculated by a formula based on the angular size of the galaxy down
to some brightness level ("isophote"), which allows one to calculate the
surface area. It is then fairly easy to calculate the mean surface brightness
and thus give an idea of how bright the object actually will appear. M81 is
27.1' x 14.2' in size and has a mean surface brightness of 12.4 magnitudes
per square arc minute, while M82 is only 11.3' x 4.2' in size and has a mean
surface brightness of 12.5 magnitudes per arc minute. Thus, even though M81
is brighter, its larger size may make it seem to be less prominent. I tend to
let the brighter core of M81 dominate, so even in a telescope, I still feel
M81 to look like the brighter of the two galaxies. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

What is generally more useful is what is known as the mean surface brightness
figure, usually quoted in "magnitudes per square arc minute". This is
generally calculated by a formula based on the angular size of the galaxy down
to some brightness level ("isophote"), which allows one to calculate the
surface area.

I think I more or less understand now how it is calculated, and
perhaps understand why. My big questions now a

1. "Magnitudes per square arc minute." Does this mean that something
with a surface brightness of 12.5 would have an average magnitude of
12.5? Or that it's average over any square arc minute would be be an
average of 12.5? In other words, is the "per square arc minute" a sum
of the total brightness in that area concentrated into a point source
or does that area glow with that magnitude?

2. I think I know the answer to this one but... A difference of 1 in
Surface Brightness is the same as a difference in 1 of magnitude
(2.5x)?

Thanks for your help.

Clear, Dark Skies

Mark