The story of a variable star observation
Astronomy magazine columnist Glenn Chaple explains how to observe variable stars and submit brightness data to the American Association of Variable Star Observers' web site.
January 22, 2010
|January 22, 2010|
The American Association of Variable Star Observers (AAVSO) is the world's largest organization dedicated to observing and studying variable stars. Headquartered in Cambridge, Massachusetts, the AAVSO boasts more than 2,500 members and contributing observers from more than 55 countries. These dedicated individuals monitor the brightness of variable stars visually or with charge-coupled devices and forward the information to the AAVSO database, where it is made available to astronomers around the world. Since its formation in 1911, the AAVSO has received 17 million variable star estimates. Here's the story of just one.
Selecting a star
I begin by logging on to the AAVSO website at www.aavso.org. Scrolling down the menu on the left side of the homepage, I arrive at "Variable Stars," which, in turn, leads me to a section titled "Stars Easy-to-observe."
From the list of easily observable stars, I select the Mira-type variable T Cassiopeiae. It's currently visible in the evening sky, and its magnitude range (6.9-13.0 over a 445-day cycle) is within the grasp of my 10-inch reflector.
Making a chart
To estimate T Cas' brightness, I'll need a chart showing the variable and nearby comparison stars — field stars whose magnitudes are noted. A few decades ago, this meant filling out a chart request form and mailing it, along with the appropriate amount of money, to AAVSO Headquarters in Cambridge. After about a week, the chart would arrive in the mail.
How times have changed! Returning to the menu on the left side of the AAVSO home page, I scroll down to "Make a Chart." This brings me to a form (AAVSO Variable Star Plotter — VSP, for short) that allows me to tailor-make charts. Following the prompts, I generate and print out three charts: a 15°-wide "A" scale chart that includes enough of Cassiopeia's main stars to help me locate T Cas; a "B" scale chart whose 3° field and 11th-magnitude limit allows me to study T Cas when it is at its brightest; and a 0.5° "E" scale chart set for a limiting magnitude of 14 for times when T Cas is near minimum. What used to take a week now requires less than 10 minutes — and it's free!
Making and recording a brightness estimate
Here's where the fun begins! Using the "A" chart, I aim my telescope toward a bright star in Cassiopeia (Zeta [ζ] Cas). Switching to the "B" chart, I star-hop from Zeta to T Cas. T Cas lies beside an 8.1 magnitude comparison star (labeled "81" on the chart — decimals are omitted so they won't be mistaken for field stars). Because T Cas is noticeably fainter than its magnitude 8.1 neighbor, I scan the field for a fainter comp star. One labeled "88" appears to be equal in magnitude to T Cas, and a magnitude 9.2 field star is slightly fainter. After careful scrutiny, I decide that T Cas and the 8.8 mag star are essentially equal in brightness. On a data sheet, I record the date and time of my observation (in Julian date form), the star name, its magnitude, and the magnitudes of the comparison stars.
Sending the data to AAVSO Headquarters
Three decades ago, I would save all my data sheets until month's end when I'd transpose all variable-star estimates (often numbering in the hundreds) onto AAVSO observing forms. The process often took hours to complete. The Internet makes things quicker, easier, and (most importantly) timelier.
After observing T Cas, I return home, turn on the computer, and log on to the AAVSO home page. Scrolling down the left side menu, I select "Web Obs." After entering my AAVSO username and password, I see a page that allows me to submit my T Cas information to the AAVSO database. The entire process takes less than a minute. The AAVSO gets the data immediately, not at the end of the month.
A final step — instant gratification!
Mission accomplished! Now, I'd like some idea of the accuracy of my observation. Years ago, I knew a few AAVSO members who encouraged me to mail them my variable star estimates, after which they'd write back and let me know how mine compared with theirs. Not exactly the best way to get timely verification! Fortunately, the Internet has changed that.
Seconds after submitting my data on T Cas, I return to the AAVSO home page and select "Quick Look" on the menu. The Quick Look request form appears on the screen, and I type in the star name and click on the "Search" prompt. This results in a chronological listing of magnitude estimates of T Cas submitted by AAVSO observers. Previous estimates of T Cas agree closely with mine. My observation is validated!
I'm not done! Once more, I return to the AAVSO homepage, this time selecting "Light Curve Generator." This produces a graph (the light curve) that combines all of the observations submitted for a particular variable star over a specific time period — a dramatic visual display of its recent behavior. Within seconds of typing in the star name and selecting a time increment (I choose the past 500 days — a little longer than T Cas' period), its light curve appears on the screen. A sine-like wave, composed of hundreds of dots, shows the rise and fall of T Cas through a complete cycle. Each of those dots represents another AAVSO observer's data. Mine, being the most recent, lies at the graph's far right. Like a solitary note from a musician, that one single observation, combined with those of dozens of other AAVSO members, has formed this beautiful symphony of a light curve!
And that, my friend, is the story of a single variable star estimate.
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