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Surface brightness

How easy is it to see a deep-sky object?
ChapleGlenn
Let’s see how magnitude versus surface brightness works by comparing a pair of faint autumn galaxies with one that should be faint but isn’t. The first is the 6th-magnitude Pinwheel Galaxy (M33) in Triangulum. Two Full Moons in apparent diameter, M33 has a paltry surface brightness of 22.8. Another hard-to-see faceon spiral is M74 in Pisces, listed at 9th magnitude. Its light spreads across a circle 9' in diameter, giving a surface brightness of 22.4. Compare M33 and M74 to the magnitude 10 galaxy M77 in nearby Cetus. It may be fainter, but its light packs into an area 3.5' by 1.7'. The resulting surface brightness is 20.2.

Surface brightness is more telling than magnitude, but it’s not perfect. You can view the Andromeda Galaxy (M31) telescopically from light-polluted urban areas, yet its surface brightness is just 22.3. One thing to remember is that these values represent the average surface brightnesses of objects. M31 has a bright nucleus that rapidly gives way to faint spiral arms, which extend outward for several degrees. It might help more to publish a surface brightness for the entire galaxy and another for the bright nuclear region.

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