[ngcles]

Hi Alex & all,

Thanks Alex for such an informative set of posts but may I add just a little to them to tie it up with a neat little bow?

The key concept Alex has touched upon here without actually using the term is "surface brightness" and surface brightness -v- integrated magnitude. As Alex pointed out, the total brightness of a galaxy can sometimes be misleading when it comes to how much "punch" it has in the eyepiece.

Surface brightness magnitude is how bright, on average, each square arc-minute of the area a thing occupies is measured to be. Suburban skies (just the background sky with nothing in it) has a surface brightness of about 18 magnitudes per Sq arc-second. A rural sky 21 and a really top notch site about 22 magnitudes per arc-second. Outside Earth's atmosphere, the value is about 25 magnitudes per arc-second.

Obviously, the darker the native sky brightness is, the better the contrast there is going to be, between the galaxy and its background (all other relevant factors assumed to be equal). Many guides and planetarium software (not all) include such a surface brightness magnitude. The difference between the native surface brightness of the sky and the surface brightness of a galaxy is going to tell you how much contrast there is between the galaxy and the background.

A classic example of a relatively high surface brightness galaxy is M104 (The Sombrero Galaxy). It has (overall, averaged) a surface brightness magnitude of 11.6 -- ie every square arc-minute has the light from a magnitude +11.6 star smeared over that arc-minute. A classic low surface brightness galaxy is NGC 45 in Cetus. It has a relatively high integrated magnitude -- just below 10, but, it is a quite large (nearby) galaxy with a large surface area so the surface-brightness magnitude is +14.7

Conventional wisdom says a S.B magnitude of about 14.5 per arc-minute, given very good conditions is about the limit for a 30cm telescope, 25cm -14, 20cm about 13.5. The larger the difference between the native sky brightness and the surface brightness the better it will look. Telescopes always show more galaxies and more detail within them under a truly dark sky.

Surface brightnesses in observing literature are usually quoted in magnitudes per square arc minute. Because the magnitude scale is logarithmic, calculating surface brightness is not performed by simple division of magnitude by area. Instead, for a source with a total or integrated magnitude m extending over a visual area of A square arcseconds, the surface brightness S is given by

S = m + 2.5 ⋅ log (sub)10 ⁡ A .

Seeing detail:

So far I've been talking about averaged surface brightness, but some of the brighter galaxies show a great deal of variance between this bit and that bit with their haloes. Many of you will have seen the dark lane in M104 before. The reason it is visible in such small telescopes is there is a very high surface brightness contrast between the bright bit and the dark bit. Most of the galaxies where you can (in the right sized telescope) see some spiral structure is where there are large, or a better adjective is abrupt differences in surface brightness between "this bit" and "that bit". M51 (The Whirlpool Galaxy) is a classic example. There is a very big (and abrupt) (ie gradient) difference between the arm and the bit in between. Same goes for NGC 1365.

Larger telescopes allows you to detect (more easily) these differences in surface brightness between "this bit" and "that bit" -- you will more frequently and with greater ease and confidence be able to see spiral structure (in those objects with spiral structure) and other details in galaxies and other "non-point" (not stars) objects.

Another brief word on the subject of integrated magnitude. Not all supplied magnitudes you see in guide books, planetarium software etc are measured at visual wavelengths. If you see it prefixed with a (V) -- this is a visual magnitude. Our eyes have different sensitivities to photographic plates, CCDs and photo-electric photometers (both with and without coloured filters). If you see something like Magnitude (B)+12.5 this is magnitude measure from a blue sensitive plate, There are other prefixes like (p) photographic (ir) infra-red (z) another photographic system are sometimes useful as a loose guide but are a long way from being reliable so far as your eye is concerned.

Also, the fainter you go a great many of the supplied magnitudes (fluxes) in the literature become increasingly unreliable for the visual observer. Above (integrated) magnitude 12, nearly all of them are very accurate. Magnitude 13 many, probably most, magnitude 14 some might be a useful guide, magnitude 15 -- pirates code. You might grumble "why" -- there is a good reason that has to do with the professional study of galaxies and galaxy formation. It doesn't help much to have an integrated visual magnitude for most purposes in the professional world few bother.

Best,

L.

P.S I add this addendum to highlight to the reader that some of the measures I have used are arc-seconds and in other cases arc minutes -- allowances should be made accordingly on the figures.

P.P.S See further addendum below.