Date: February 7, 2009

Title: Observing Variable Stars

Podcaster: Travis Searle, Rebecca Turner and Mike Simonsen

Organization: American Association of Variable Star Observers (AAVSO) http://www.aavso.org

Description: There are tens of thousands of known variable stars. Professional astronomers don’t have the time or telescopes to keep track of the changes to thousands of stars, so amateurs play an important role in variable star research. How do astronomers observe variable stars? We’ll discuss the methods and instruments used to observe variable stars. How to locate them in the sky, how to measure their brightness over time and how to collect and submit your data so you can contribute to real science!

Bio: The AAVSO was founded in 1911 at Harvard College Observatory to coordinate variable star observations made largely by amateur astronomers. In 1954, the AAVSO became an independent, private research organization headquartered in Cambridge, Massachusetts. Today, with members in 45 countries and over 15.5 million observations, the AAVSO is the world’s largest association of variable star observers.  Membership in the AAVSO is open to anyone — professionals, amateurs, and educators alike — interested in variable stars and in contributing to the support of valuable research.

Today’s Sponsor:  The National Radio Astronomy Observatory (NRAO), celebrating five decades of training young scientists through summer programs.  Explore the hidden Universe at in radio www.nrao.edu

Transcript:

Introduction:
Hello and welcome to ‘Restless Universe’, the podcast of the American Association of Variable Star Observers. You can find us on the web at www.aavso.org.

I’m your host Travis Searle along side Rebecca Turner and Mike Simonsen.

In Episode number 2, we’re going to discuss how and why astronomers observe variable stars, and how amateur astronomers contribute to science by making variable star observations.

Astronomers are interested in variable stars because they tell the story of how stars are born, live, and die. Variable star astronomy is all about stellar evolution. Young stars vary because they haven’t quite settled down yet to a quiet life on the main sequence. Other stars vary because they’ve been around a long time and have used up much of the fuel in their core, and this causes them to become unstable. Some massive stars end their lives in a blaze of glory called a supernova.

There are tens of thousands of known variable stars. Professional astronomers don’t have the time or telescopes to keep track of the nightly changes in thousands of stars, so amateurs play an important role in variable star research. Amateur astronomers have one big advantage over professional astronomers. They own their equipment and can observe whatever they want, whenever they want.

Choosing the target:
So if I want to go outside and observe variable stars tonight, how do I know where they are? There are millions of stars. How do I know which ones are the variable stars?

Just like any other type of celestial objects you choose to observe, there are catalogs containing the names, positions, and characteristics of these stars. For variable stars the oldest catalog is the General Catalog of Variable Stars, or GCVS as its popularly known. There is also a newer, more inclusive database of variable stars called the International Variable Star Index, or VSX, which is published and maintained by the AAVSO. Both of these databases are available online, and anyone can use them.

So how does an observer decide which stars out of the thousands of variables out there he or she should observe?

Part of the answer is it depends on the time of year. You need to know whether or not the star you want to observe is going to be visible tonight when you are out there observing.

Another factor is- how often do you observe? If you live in a place where it is cloudy a lot you may want to observe stars that change slowly. One observation per month is enough to keep track of your favorite long period variables.

Stars that can change suddenly, like cataclysmic variables, need much more frequent observations to record their changes. You’ll want to observe them every clear night. Some stars actually require observations every minute or so for several hours when they are active, like eclipsing binaries.

Some of your decision depends on the equipment you plan to use to observe the stars, right?

Absolutely. There are some variable stars that are bright enough to observe with the naked eye or a pair of binoculars. There are dozens of stars visible on any given night that never get fainter than 9th magnitude, the typical limit for binoculars.

Some variables may be so faint at minimum you’ll need a telescope to observe them. With a 6 or 8-inch telescope there are thousands of variable stars you can observe.
Other stars never get brighter than 12th or 13th magnitude and require a 12-inch or bigger telescope.

If you plan to observe visually, at the eyepiece of the telescope, you want to select stars that vary enough that you’ll be able to detect the changes with just your eyes. So stars with larger amplitudes of variation work best for visual observers.

Some stars fluctuations are much more subtle, and require observers to use higher precision instruments such as photoelectric photometers or charged coupled devises, better known as CCDs.

It’s possible to study fainter stars with a CCD than visually with the same telescope .

Mike, you’ve worked on variable star charts for the AAVSO for years. What are variable star charts?

Variable star observers use what are called comparison sequence charts. These are special star charts you can download and print for free from the AAVSO website for any variable you want to observe. Not only do these charts help you locate the variables, they are labeled with a special set of comparison stars called a sequence. These are constant stars for which we have accurate measurements of their magnitudes. Observers use these comparison stars to measure the brightness of the variable.

It’s important to note here that the data collected is a lot more useful if everyone uses the same comparison star sequences for measuring variables. There is a lot of planetarium software out there that will state magnitudes from the Guide Star Catalog and other sources, but these are not really accurate enough to be used as comparison stars.

So- once we’ve decided which star we want to observe, we’ve printed out our comparison sequence chart from AAVSO, and we’ve gone outside and actually located the star with our binoculars or telescope, what happens next?

Well, finding the variable is not always that simple. In fact, that’s probably one of the hardest things for beginners to learn to do. It took me several tries on a couple of nights to find my first variable star. Of course, I was totally geeked when I finally did find it. I’ll never forget that night.

Anyway, once you’re certain you’re looking at the right star field, you identify the variable star on your chart and then you find a comparison star that is a little brighter than the variable and another one that is slightly fainter.

To make your estimate you need to interpolate between the two comparison stars. If one is magnitude 12.4 and the other is 13.0 and the variable appears to be exactly half way between the brightness of the two comparisons, your estimate will be 12.7. If it is almost as faint as the 13.0 your estimate might be 12.9. Sometimes the variable is exactly the same brightness as one of the comparison stars. After examining the field carefully, you may decide the variable in this case is exactly the same brightness as the 12.4. Then that would be your estimate.

It’s actually easier than it sounds once you’ve done it. It’s a lot like reading the gas gauge in your car. If the needle is half way between the full and half full, you know you have three quarters of a tank of gas. Experienced observers can make estimates accurate to within a tenth of a magnitude.

The process is very much the same for CCD observers. The main difference is the measurements are made using software to compare the variable to the comparison stars on the images they produce. With practice, CCD observers can measure the brightness of stars to within a hundredth of a magnitude.

Of course making the observation isn’t the last step. Your observations won’t benefit science if you keep them to yourself. You need to report your results to the AAVSO, where they will become a part of the International Database, along with the millions of other observations reported over the years by thousands of amateur and professional astronomers.

Your observations can be submitted on paper, by FAX or email, but the most common method these days is to submit your data online, using AAVSO software developed specifically for this purpose. When making your observations you need to record the name of the star, the time you made the observation, the magnitude of the variable, the comparison stars you used to make the measurement and the chart identification number of the chart you used.

Many observers begin observing variable stars because they want to contribute to science, but they keep doing it year after year because frankly, it’s a lot of fun. In fact, it can be quite addicting! Leslie Peltier, one of the most famous amateur astronomers of all time said it best.

“I feel it my duty to warn any others who may show signs of star susceptibility that they approach the observing of variable stars with the utmost caution. It is easy to become an addict and, as usual, the longer the indulgence is continued the more difficult it becomes to make a clean break and go back to a normal life.”

After having made over 50,000 variable star observations myself, I can tell you that is absolutely the truth.

That’s all for this episode. Join us next time when we’ll be discussing what happens to your observations once they arrive at the AAVSO. We’ll also tell you some of the history of the AAVSO and describe the science and education goals of the organization.

End of podcast:

365 Days of Astronomy
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