Magnitude and Diffuse Objects

[NAFK]

A query from a novice...

Standing in the garden tonight I can resolve stars down to magnitude 5 with my naked eye. However, I'm unable to make out the andromeda galaxy, which is apparently a magnitude 3.5 object, at all. I can see 35v-And with reasonable clarity, which, according to the Skymap app, is 4.5.

My question, which I'm sure you will have anticipated, is: is there a difference in practical visibility between point sources like stars and diffuse sources like Andromeda? If so, why?

The obvious answer would be that diffuse sources are, well, diffuse. But I thought the point of apparent magnitude was to give the relative brightness of an object as seen from Earth. So what am I misunderstanding?

[Demonperformer]

The magnitude quoted for diffuse obects is the total light from the entire spread of the object. The figure you want for a comparison is the surface brightness, which is a measure of brightness per unit area.

HTH

[Merlin66]

Think of a 3rd magnitude star image put out of focus until the size of the disk is more than twice the diameter of the moon......pretty faint surface brightness.

HTH

[swamp thing]

Hi

As you can see the magnitude values given are not a great way of determining what is and isn't visible as far as DSO's are concerned.

As Demonperformer said the surface brightness is a far more accurate description as to whether an object is visible or not, but not every list or catalogue gives a value of this.

It can be quite an eye opener as some high mag objects are ridiculously difficult if not impossible to see visually. Whilst other much lower mag objects are ridiculously simple.

Good hunting and clear skies

Regards Steve

[NAFK]

Thanks! All very interesting.

So, what is apparent magnitude a measure of, if not relative brightness to an observer on Earth?

[Merlin66]

It's the relative brightness of a point source ie a star image.

Diffuse objects are different......

[NAFK]

It's the relative brightness of a point source ie a star image.

Diffuse objects are different......

So, if apparent magnitude refers to the relative brightness of point sources, why are diffuse objects routinely given apparent magnitudes?

[Merlin66]

It's an easy way of using the apparent magnitude scale....it's applied to Comets, and even the Moon and the Sun.

[NAFK]

It's an easy way of using the apparent magnitude scale....it's applied to Comets, and even the Moon and the Sun.

But, as discussed above, it doesn't work as a relative brightness measure when applied to diffuse objects, so it's not an easy way of doing it at all (at least not for diffuse objects).

I must be misunderstanding something...

[Merlin66]

No, I don't think you're misunderstanding....we haven't got any better magnitude measurement scales which can be applied to diffuse objects...such is life.

[NAFK]

I guess what I'm asking now is, if surface brightness is a more useful measure of relative visibility for diffuse objects then why is the apparent magnitude the value that's ubiquitously quoted?

Also, how is the apparent magnitude of a diffuse object calculated? What is the value actually telling us?

[Merlin66]

have a read of this....may help understanding...

Surface brightness - Wikipedia, the free encyclopedia

[NAFK]

have a read of this....may help understanding...

Surface brightness - Wikipedia, the free encyclopedia

Okay, I think I understand. My mistake was thinking that apparent magnitude is, fundamentally, a measure of relative visibility but it's not; it's actually a measure of the amount of light from an object that reaches Earth's surface.

Everyone's answers make more sense now.

[Capricorn]

Do a search of Messier Objects and Surface Brightness, probably one on Wikipedia, but they are a few around. Thought I had one but cannot see it anywhere.

Makes it interesting as it is often a trade off between size and brightness.

[Demonperformer]

CdC quotes the SB for DSOs.

[pete_l]

But I thought the point of apparent magnitude was to give the relative brightness of an object as seen from Earth. So what am I misunderstanding?

I've never found that to be a particularly helpful attribute of a DSO. A lot of things in astronomy are there just because they've always been there (e.g. the magnitude scale, itself). You could lead a long and happy life as an amateur astronomer and completely ignore apparent magnitude numbers, I certainly do.