Surface brightness: Higher = better?

[pipnina]

I've been looking thtough some objects in stellarium (mostly while complaining about daylight savings and how the milky way only shows up properly at dawn  ) and noticed that it (helpfully) states the surface brightness for many DSOs.

Things is, many of these DSOs seem to have very similar surface brightnesses. How does this scale work, is it logarithmic or something else complicated? I also notice how a surface brightness isn't listed for darker DSOs (e.g. the HH nebula, Flame, NGC2023 etc.

So, are the numbers really close together In the case of M31, M42, eagle and crab nebulae because they're a similar brightness or because the rating system just has really small number differences for large brightness differences?

    Maths shows it's face in this hobby often, my B grade GCSE isn't helping me now...

   

    ~pipnina

[ronin]

The higher the surface brightness the better, simply the whole surface is easier to identify.

Not sure how they determine it however.

As you have found M31 (Andromeda) is "bright" but the surface brightness is low and it is difficult to actually pick it out.

[jkwhinfrey]

The apparent magnitude scale you're probably used to when looking at stars and planets is logarithmic, and so if the surface brightness scale.

The difference is that surface brightness relates the total amount of light coming from an object - its integrated magnitude - to the area of the sky over which it's spread. For a given magnitude, the larger the area of sky the object covers, the lower its surface brightness. It's basically the object's brightness per unit area of sky.

Given the relatively low surface brightness of nebulae and galaxies compared to the planets, it doesn't take much difference this log value to make a big difference to visibility. Remember that the sky itself has a surface brightness and it's often contrast between that and the object that makes it visible at all.

That's my basic understanding. It may need correcting ;-)

James

[pipnina]

The apparent magnitude scale you're probably used to when looking at stars and planets is logarithmic, and so if the surface brightness scale.

The difference is that surface brightness relates the total amount of light coming from an object - its integrated magnitude - to the area of the sky over which it's spread. For a given magnitude, the larger the area of sky the object covers, the lower its surface brightness. It's basically the object's brightness per unit area of sky.

Given the relatively low surface brightness of nebulae and galaxies compared to the planets, it doesn't take much difference this log value to make a big difference to visibility. Remember that the sky itself has a surface brightness and it's often contrast between that and the object that makes it visible at all.

That's my basic understanding. It may need correcting ;-)

James

The sky indeed does have a surface brightness. Where I am it's probably mag 2 or higher!

So higher does indeed mean better then, but you also mentioned that it only takes a small difference in numbers to make a big difference in perception. Does this mean the crab nebula at 12.13 surface brightness is WAY dimmer than andromeda at 14.59?

[YKSE]

The sky indeed does have a surface brightness. Where I am it's probably mag 2 or higher!

So higher does indeed mean better then, but you also mentioned that it only takes a small difference in numbers to make a big difference in perception. Does this mean the crab nebula at 12.13 surface brightness is WAY dimmer than andromeda at 14.59?

James' explanation is correct, more detail to read here if you feel for it.

As to surface brightness of crab nebula and andromeda, if andromeda's surface brightness is evenly distributed over its whole area as the crab, it will be dimmer than the crab. But, andromeda has very bright core, which is what we see in our scopes, if you look at Olly's picture here, M32 is actually inside andromeda, but the extra low surface brightness in andromeda's outer part makes it invisible to us.

[ollypenrice]

James' explanation is correct, more detail to read here if you feel for it.

As to surface brightness of crab nebula and andromeda, if andromeda's surface brightness is evenly distributed over its whole area as the crab, it will be dimmer than the crab. But, andromeda has very bright core, which is what we see in our scopes, if you look at Olly's picture here, M32 is actually inside andromeda, but the extra low surface brightness in andromeda's outer part makes it invisible to us.

We did go deeper in the end:

I hold out no hope of seeing the main galaxy contacting its satellites at the eyepiece, though. People with better eyesight than mine might do better.

I don't find published surface brightness values all that much help in predicting observability though they are obviously a rough guide.

Olly

[PeterW]

Anybody imaged the area with a wider view... Andromeda shelf anyone.... And other things people don't see often.

http://www.bbastrodesigns.com/6inchF2.8/Unobserved%20Wide%20Angle%20Nebulae%20Next%20to%20Well%20Known%20Objects.html

Cheers

PeterW

[ollypenrice]

Anybody imaged the area with a wider view... Andromeda shelf anyone.... And other things people don't see often.

http://www.bbastrodesigns.com/6inchF2.8/Unobserved%20Wide%20Angle%20Nebulae%20Next%20to%20Well%20Known%20Objects.html

Cheers

PeterW

Interesting. I see no sign of this on our image, nor have I heard of it other than in the link you provided. (Quick net search only found the same link.) I'll put myself down as sceptical but open for a concerted go at it in the right season!

Olly

Edit, this might be worth a new thread. Apologies to the OP.