Some Suburban Messiers

Last night was the first of this lunation to have decent transparency and no Moon in the evening sky. It's not unusual to get just once such night per month here in the eastern U.S., so I couldn't waste the opportunity.

Ideally, I would drive far away to a semi-dark site on every moonless night, but there's a limit to how long I can stay awake after a hard day's work. And I had little desire to waste an hour or two of that precious evening sitting behind the wheel of a car.

This is exactly the kind of situation where it's really handy to have a suburban stargazing project in my pocket. I can with a clean conscience "waste" the moonless night in the suburbs because I have to complete my project. (Actually, my observing career is littered with projects that never got completed, but I can overlook that fact when it's convenient.)

So off I drove to Arlington, MA, the next town out from Cambridge, where I'm viewing as many Messier objects as possible through my 70-mm refractor. I've done that already near my home in Cambridge, but I think that Arlington is more representative of typical stargazer habitat. Most people in Arlington live in detached single-family homes, though the houses are packed together quite tightly. It has about 8,000 people per square mile -- roughly the same as Los Angeles, though just half the density of Cambridge or San Francisco. The skies in Arlington average about ½ mag darker than near my home. That's enough to make 7 stars visible in the Little Dipper with considerable effort, and to show just a hint of the Cygnus Starcloud when it's directly overhead.

I ended up viewing just 5 Messier objects in 1{1/2} hours: M4, M80, M7, M6, and M23. Under dark skies, the last three of these are readily visible naked-eye. Many people can also see M4 without optical aid, though I can't due to the glare of nearby Antares. But my 10x30 binoculars show all five of these star clusters with ease. And when I push the binoculars' image-stabilization (IS) button, I can resolve many stars in M7 and M6, and a few in M23.

In Arlington, I was doubly handicapped, both by the light pollution and by the fact that these objects are always very low in the sky when seen from latitude 42° N. According to my measurements, the sky 3 is times brighter 20° above the southern horizon than at the zenith at this site, and 6 times brighter 10° above the horizon.

I scanned and star-hopped for a long time with my 10x30 binoculars, but the only one of the five clusters that I could see was M7. It did show pretty well, though, resolving about 10 individual stars when I pressed the IS button.

Through a telescope, M4 was soberingly difficult to see. I've had this experience before, in Cambridge, but hoped things would be easier in Arlington. All I could make out was an extremely vague, though large, blob, and it only appeared with averted vision when I was consciously moving my eye. If it hadn't been in exactly the right spot, I wouldn't have been 100% sure I'd seen. No wonder this globular cluster causes trouble for neophyte backyard astronomers. It's all light pollution's fault!

M80 is much tougher than M4 at a dark site, but vastly easier in city or suburbs. That's because it's a concentrated cluster with lots of stars packed into a tiny chunk of sky. That makes it stand out much better against the skyglow, as long as you use enough magnification to see that it's not just a single star.

The most interesting case was M23. At low magnification, I couldn't see a thing; the diffuse glow of unresolved stars was swamped by the background skyglow. But when I boosted the magnification to 60X, 20 individual stars suddenly appeared out of nowhere. They'd been just below the threshold of visibility at low power.

A passing bank of clouds gave me an excuse to go home. I was still having fun, but I didn't want to loose too much sleep in advance of the big new-Moon/Perseid/Stellafane weekend. I felt a little rueful thinking how much more beautiful all the things I'd been viewing would have been under a dark sky — to say nothing of being easier to locate! But better to see them in adverse conditions than not at all.