A question regarding magnitudes

[nocturnality]

Tonight I was trying to see M101 which with tonight's reasonably clear sky I was expecting to be able to see with my 5" Mak, as it is listed as mag 7.7. I have previously seen M57 (mag 9) quite clearly.

After a little messing around with alignment, I found the spot where M101 is located but could not see any trace of it even with averted vision, despite being able to see the nearby stars of magnitude 10.3 and even the 11.45 star just to the south.

Why is this the case? If I can clearly see objects in the EP that are magnitude 10.3 how is it that there is no trace of the mag 7.7 object? Does magnitude mean something other than the brightness of the object as seen by an earthbound observer?

For info I was using a SW 127 Mak with 25mm ep.

Cheers

Bryn

[GazOC]

Hi Bryn,

Roughly speaking the listed magnitude is worked out as being across the whole area of the object. So a large mag 7 object can be a lot harder to see and is fainter than a small mag 10 object like a star.

[Demonperformer]

Bryn,

For diffuse objects (galaxies, nebulae, etc) you will be better looking up the 'surface brightness' rather than the 'visual magnitude', because that is a measure of the brightness over a unit area.

HTH

[NGC 1502]

Hi Bryn, post #2 is correct. When I joined my astronomy club, a member did a talk

on this very subject that cleared up my confusion about this.

M101 is quite tough to spot. You can be staring at it and still not see it !

But when you do see it (perhaps under a better sky) it is a large fluffy blob.

Regards, Ed.

[nocturnality]

Thanks all, that makes perfect sense. I am quite sure that I was indeed staring straight at M101 yet could not see it, judging by the distribution of the stars that I could see.

I'll try again once I'm in a better location than my back garden.

Thanks again

Bryn