Subsequent to my October 2015 column (“Understanding Brightness”), I received the following email from Ross Warren of Statesville, North Carolina: “As an amateur astronomer, I’ve often wondered who had the bright (pun not intended!) idea of deciding faint fuzzies such as the Helix Nebula (NGC 7293) would be given a magnitude as if you took all their light and squeezed it into a fake star? Why didn’t they assign a more truthful ‘average surface brightness’ instead?”
Good question. Most observing guides assign the Helix a given magnitude of 7 — how bright it would appear if its light were concentrated into a point. Compare the Helix with another Aquarius planetary, the Saturn Nebula (NGC 7009). The latter is a full magnitude fainter, yet is far and away an easier telescopic target. How is this possible?
Size is the answer. The Helix Nebula covers an area half as wide as a Full Moon, while the much smaller Saturn Nebula is similar in apparent size to its namesake planet. Its light more condensed, the Saturn Nebula appears brighter because its surface brightness is greater. The surface brightness of the Helix is 20.8 magnitudes per square arcsecond, while that of the Saturn Nebula is 14.6 magnitudes per square arcsecond.
You can calculate a deep-sky object’s surface brightness using a simple formula I won’t list here. Fortunately, we can refer to sources that provide us with surface brightnesses.
Among the best is Roger N. Clark’s Visual Astronomy of the Deep Sky, a classic guide that addresses the concept of surface brightness in great detail. The book also contains an appendix that lists the surface brightnesses of more than 600 clusters, nebulae, and galaxies. Kepple and Sanner’s popular The Night Sky Observer’s Guide also supplies surface brightnesses — in this case for galaxies only and in magnitudes per square arcminute.
You can find a handy surface brightness calculator online (again, in magnitudes per square arcminute) at www.users.on.net/~dbenn/ECMAScript/surface_brightness.html. Plug in the magnitude and size, and voila! You have the surface brightness. In general, deep-sky objects with surface brightnesses below 22.0 magnitudes per square arcsecond (13.0 magnitudes per square arcminute) are considered faint.