AAVSO at 20 Million

The American Association of Variable Star Observers hit the 20 million observations mark last month.


As an active observer between 1980 and 2000, I am immensely proud of this organization.

In the early years of the 20th Century, the idea of a citizen scientist was fading. Over the course of three hundred years, science had gone from individual enthusiasts making observations about the world — David Fabricius and many others were churchmen, Robert Boyle an independently wealthy landowner, and Darwin was a lapsed medical student, — to formally educated specialists. In astronomy, it was difficult to enter the profession without an advanced degree, unless you were an instrument maker, or a woman. If you were a woman, the only jobs you could get were as a ‘computer’ (what we call computers today were initially called electronic computers, to differentiate them from real computers, who were women), or a plate scanner. Typical women of the era were Annie Cannon (who helped found the Harvard spectrographic classification system), or Henrietta Levitt (who discovered the Cepheid class of pulsating variables), and even they had formal astronomical training.

But in 1911, the year before Leavitt published her detailed studies on variables of the Small Magellanic Cloud in the Harvard College Observatory Circular, amateur astronomer William Tyler Olcott, inspired by Edward C. Pickering, Leavitt’s boss, started the group of correspondents that ultimately became the AAVSO.

So, what does AAVSO do? Well, a friend of mine once described it as “The Star Trek equivalent of the Civil Air Patrol.” If you replace the trekkiness with Astronomical, you’d have it about right.

A variable star is a star whose observed brightness varies over time. It might be because it is pulsating, like Delta Cephei, or because it is eclipsed by a darker companion, like Algol (Beta Persei), or for any one of a number of other reasons. If you want to learn about such stars, you have to measure their changes in brightness regularly and consistently, over a long period of time. Such measurements (depending on the kind of variable) will let you catch changes in their chemical composition, in the size and placement of starspots on their surface, or in the dynamics of their eclipsing orbits. But the measurements must be regular, consistent, and prolonged. There are hundreds of such stars visible in binoculars, and thousands visible in modest amateur telescopes. And therein lies the problem. There are far too many stars, and far too few professional astronomers, with telescopes that are far too big for the purpose.

Someone has to do the drudge work. Someone has to be willing to take on the cold, unending, essentially thankless task of observing these stars, night after night, year after year. The observers set up right after sunset, to catch those stars that are close to the western horizon as soon as it gets dark enough to see. Later in the year they stay up past midnight, or crawl out of bed in the cold hours of the morning, because the turning of the celestial sphere has moved the observing window for those same stars to the predawn hours. Their telescopes are small, by professional standards — many wouldn’t qualify as finder scopes for the big optics. The human eyeball is their primary (for decades their only) sensor tool. They do exactly what the Civil Air Patrol does, filling in for the professionals until the time comes for professional equipment to be called in.

So, how does one observe a variable star, and how is it that simple human eyes can contribute to a task normally assigned to equipment capable of counting single photons? It’s really quite simple. You don’t have to measure brightness, you don’t even have to measure relative brightness. All you have to do be able to look at two stars and say which one is brighter. One star is the target variable. The other is a comparison star, chosen by the AAVSO because its brightness is accurately known. If your variable is dimmer than that comparison star, then you look for the next comparison on your chart, until you find one that is dimmer than the variable. Now you have the variable bracketed, and all you have to do is judge which star the variable is closer to in brightness. With the charts provided by AAVSO you can easily come within 0.2 magnitudes, which is close enough for this task.

You see, you are not alone. On any given night, for a reasonably bright variable, there may be a dozen observations made, by observers around the globe, under all sorts of conditions. Since most variables (except for recurrent novae and eclipsing binaries) change relatively slowly, it’s possible to combine two nights of observations, or a week’s worth, and get a sample big enough to do statistics on. Thousands of observers contribute to this, to the tune of 20 million observations since 1911.

For twenty years, I was one of those observers. I remember cold, clear nights in the snow in Korea, turning my head so my breath wouldn’t fog the eyepiece. I remember being eaten by mosquitos in the summer in northern Virginia, and tracking a far-southern variable right down on the horizon from a beach in Florida. I remember hunching over the eyepiece, waiting for that hole in the clouds to pass over my star and trying to get my comparisons done before it drifted away. In one case it wasn’t clouds, it was the house down the street burning down and producing irritatingly dense clouds of smoke. My observations were not particularly copious — Leslie Peltier regularly made more observations in one night that I did in my best year — but they are there, they contribute. Like a fine wine, they will get more valuable as time goes on. There may be twenty million observations in the repository, but some of them are mine.


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