Surface brightness

Surface brightness is the measure of an object's average brightness per square arcsecond of its area. Absolute and apparent magnitudes are calculated by integrating the magnitudes over the whole area of the object, and thus give a value for brightness that the object would have, were it compressed to a point. For extended objects, this does not give sufficient information about the properties of the object. Large nebulae, such as NGC 7000, can have an apparent magnitude of 4, which would suggest that it is easily visible, but because of its large area, the light is diffused over a sizable area, which makes it much more difficult to observe than a more compact object of equivalent magnitude.

In visual observation, the surface brightness of an object is at its maximum with unaided dark-adapted observation. Magnifying the object will never make it appear brighter, and the value is a constant if the exit pupil of the observational device matches or exceeds the pupil diameter of the eye. Magnification past that point will make the surface brightnesses of objects drop, but it will also amplify the area of the objects. Often it is useful to sacrifice brightness for size, as the eye has an easier time seeing larger objects of slightly lower brightness. Excessive magnification, however, will diffuse the light too much, which is why on large objects, it's often recommended to use very low magnifications.

Surface brightness measurements do not generally take into account irregular shapes and brightnesses of the objects, which means that it is often just a guideline value. Objects with rapidly diminishing brightness gradients, such as globular clusters, and galaxies with bright cores can often have low surface brightnesses, but be very simple to detect, because the surface brightnesses of the brighter areas are much higher than the averaged value for the whole object. For example, the reflection nebula M78 has a surface brightnesss of 12.00, which is the same as that of globular cluster M13, but the latter is far easier to observe. The value is generally best used for extended objects of uniform brightness, such as various galaxies that don't have very pronounced cores.