Editing Surface brightness (section)

==General description==
The total magnitude is a measure of the brightness of an extended object such as a nebula, cluster, galaxy or comet. It can be obtained by summing up the luminosity over the area of the object. Alternatively, a [[photometer]] can be used by applying apertures or slits of different sizes of diameter.<ref name=daintith_gould2005>{{cite book
 | author=Daintith, John
 | author2=Gould, William | date=2006
 | title=The Facts on File dictionary of astronomy
 | page=489 | series=Facts on File science library
 | edition=5th | publisher=[[Infobase Publishing]]
 | isbn=0-8160-5998-5 }}</ref> The background light is then subtracted from the measurement to obtain the total brightness.<ref name=sa12_164>{{cite journal
 | last=Palei | first=A. B.
 | title=Integrating Photometers
 | journal=Soviet Astronomy | volume=12 | page=164
 |date=August 1968
 | bibcode=1968SvA....12..164P
 }}</ref> The resulting magnitude value is the same as a point-like source that is emitting the same amount of energy.<ref name=sherrod_koed2003>{{cite book
 | author=Sherrod, P. Clay
 | author2=Koed, Thomas L. | date=2003
 | title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations
 | page=266 | series=Astronomy Series
 | publisher=Courier Dover Publications
 | isbn=0-486-42820-6 }}</ref> The total magnitude of a [[comet]] is the combined magnitude of the [[coma (cometary)|coma]] and [[comet nucleus|nucleus]].

The [[apparent magnitude]] of an astronomical object is generally given as an integrated value—if a [[galaxy]] is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, a [[star]] is so small it is effectively a [[wikt:point source|point source]] in most observations (the largest [[angular diameter]], that of [[R Doradus]], is 0.057&nbsp;± 0.005 [[Arcsecond|arcsec]]), whereas a galaxy may extend over several [[arcsecond]]s or [[arcminute]]s. Therefore, the galaxy will be harder to see than the star against the [[airglow]] background light. Apparent magnitude is a good indication of visibility if the object is point-like or small, whereas surface brightness is a better indicator if the object is large. What counts as small or large depends on the specific viewing conditions and follows from [[Ricco's law]].<ref>{{cite journal | doi= 10.1093/mnras/stu992  | author=Crumey, Andrew | title= Human contrast threshold and astronomical visibility | journal=Monthly Notices of the Royal Astronomical Society | date=2014 | volume=442 | issue=3 | pages=2600–2619 | doi-access=free | bibcode=2014MNRAS.442.2600C | arxiv=1405.4209}}</ref> In general, in order to adequately assess an object's visibility one needs to know both parameters.

This is the reason the extreme [[naked eye]] limit for viewing a star is [[apparent magnitude#extremestar|apparent magnitude 8]],<ref name="Bortle">{{cite web |date=February 2001 |title=The Bortle Dark-Sky Scale |publisher=Sky & Telescope |author=John E. Bortle |url=https://skyandtelescope.org/astronomy-resources/light-pollution-and-astronomy-the-bortle-dark-sky-scale/ |access-date=2009-11-18 |archive-url=https://web.archive.org/web/20090323232806/http://www.skyandtelescope.com/resources/darksky/3304011.html |archive-date=23 March 2009 |url-status=live }}</ref> but only [[apparent magnitude#extremegalaxy|apparent magnitude 6.9]] for galaxies.<ref name="SEDS">{{cite web|url=http://messier.seds.org/m/m081.html|title=Messier 81|date=2007-09-02|publisher=SEDS (Students for the Exploration and Development of Space)|access-date=2009-11-28|archive-url=https://web.archive.org/web/20170714113244/http://messier.seds.org/m/m081.html|archive-date=2017-07-14|url-status=live}}</ref>

{| class="wikitable" style="text-align: center; font-size: 0.9em;"
|+Diffuse objects visible to the naked eye
! Object
! [[Apparent Magnitude|apmag]]
|-
| [[Andromeda Galaxy]] (M31) || 3.4
|-
| [[Orion Nebula]] (M42) || 4
|-
| [[Triangulum Galaxy]] (M33) || 5.7
|-
| [[Messier 81|Bode's Galaxy]] (M81) || 6.9
|}