May 2010

Urban Galaxies

M31, M32, & M110 by Richard ArendtFor urban astronomers, galaxies are consistently disappointing. Galaxies that are stunning from a dark-site can be stubbornly invisible in even mildly washed-out skies. A very slight loss of contrast is enough to hide spiral arms, dark lanes and any other structural details. What makes galaxies even more frustrating is that none of the usual tricks for enhancing contrast work on them. Because they are extended objects with low surface brightness, increasing magnification just makes their already faint and diffuse light even more diffuse and fainter. Because galaxies radiate primarily stellar light, narrowband filters which can be very effective on emission and planetary nebulae don’t help at all with galaxies. So what’s an urban astronomer to do? Just give up on galaxies? Well, luckily there are a few galaxies that can be reliably seen under washed-out skies...

(Photo of M31 by Richard Arendt.)