When I get asked this question in an interview, on a plane, talking to a potential donor, heck, even when I'm doing a presentation and can take as much time as I want, I rarely get to answer the question to my satisfaction. There is always the lingering feeling that I left something important out, or that I could have worded it better for that particular audience or listener.
It happened again last Friday on SLOOH Radio. We were kinda jumping around from topic to topic when the question finally came up. I had to back track a little and then try to answer the question as succinctly as possible, relating to what we had already discussed and hinting at the interesting topics to come.
I suppose it all turned out alright in the end. The hosts, Marty and Mike, asked me to come again and took me to their favorite pub after for dinner and drinks, so it must not have sucked.
So, what are variable stars?
Here is what I wish I'd said.
Variable stars are stars that vary in brightness on timescales ranging from seconds to centuries. Astronomers find them interesting because many of the changes that are occurring give us information about the distance, mass, size, age and composition of these stars. They have and continue to provide us with important clues about the life cycle of stars. Some vary because they are young and unsettled. Some vary because they are middle aged and going through changes in their interiors. Some represent the end stages, or old age of stars, and some are the brilliant cataclysmic destruction of a star that spews its guts out into space seeding the next generation of stars and planets with material from which to build.
Some variable stars vary so minutely that it takes specialized, highly calibrated instruments to detect the changes. Some vary so greatly that occasionally a star only visible in a telescope becomes the brightest star in the sky, visible to the unaided eye for weeks or months. The light from some stars varies because it is being eclipsed by a planet transiting in front of it from our point of view.
Even more bizarre, some things that look like stars and vary like variable stars are not stars at all! They are the light being given off as super-massive black holes at the heart of distant galaxies devour the gas and stars around them.
In spite of all our technological advancement, the study of variable stars is still one area of science where amateurs can and do make valuable contributions to science.
And that is where I will begin to explain what the AAVSO is all about.
I hope I can remember all this, for the next person who asks me on a plane...