About: Galaxies and surface brightness.

[pipnina]

In my last observing session I had a look at M31, and unfortunately only saw the core. Just now I had a look on stellarium and saw its surface brightness is 14.59 but that it's companion, M110, has a surface brightness of 14.04... Brighter?

Then I looked down at the Triangulum galaxy... 14.5??? I thought that one was meant to be a relatively hard object!

This has left me wondering about either the usefulness of surface brightness or the accuracy of stellarium's stats.

Also, how are these things measured? Would it be possible to measure it myself with a camera? Would it be possible to make more useful graphs for objects that change colour based on the surface brightness of each point, instead of having an average brightness for the whole object?

This whole thing has left me confused.

    ~pip

[Knight of Clear Skies]

Could that be the average surface brightness maybe? Andromeda has a bright core but the extended arms are faint.

[Stu]

Could that be the average surface brightness maybe? Andromeda has a bright core but the extended arms are faint.

That makes total sense. Surface brightness is useful to a degree, probably more so in smaller objects. On the larger ones as KoCS says there can be a large variation across the object so the average measurement is less useful.

[pipnina]

That makes total sense. Surface brightness is useful to a degree, probably more so in smaller objects. On the larger ones as KoCS says there can be a large variation across the object so the average measurement is less useful.

So andromeda's surface brightness is somewhere between the brightness of the core and the outer arms... So if I went looking at, say, Mirach's ghost, it would be easier, as a whole, than andromeda since it's a much smaller object but also listed as surfave brightness 13 as opposed to 14.5?

[acey]

Surface brightness is effectively the total brightness within a given area, averaged over that area. A camera-plus-telescope sees a galaxy extending to a bigger area than the eye-plus-telescope can, and this skews things. Also galaxies aren't uniform in brightness across their area, which skews things too.

You also need to be careful about what sort of magnitude figure is being given (it should be V-magnitude, though even V-magnitude does not match the human eye's actual perception of brightness perfectly). Published figures (e.g. in Stellarium) can come from a variety of sources. To compare like with like you'd really want everything to have come from one source - I don't know if that's the case for Stellarium. And note that while surface brightness is the best indicator of visibility for large targets, visibility is itself a function of surface brightness. Whether or not you can see a target with a surface brightness of 14 mag per square arcmin depends on how large the target is, and the surface brightness of the background sky. It's complicated.

Bottom line: take all figures with a pinch of salt. The important thing is whether or not you can actually see what you're looking for. You only find that out by trying.

[swamp thing]

 Bottom line: take all figures with a pinch of salt. The important thing is whether or not you can actually see what you're looking for. You only find that out by trying.

That just about sums it up perfectly.