Unit Five: Investigating Stars
1. Observing Variable Stars
When we look up at the sky on a clear night, our visual impression is that the stars and galaxies that we can see are static and unchanging. For thousands of years, only a very few stars were recognized to vary in time—unusual stars with unusual names like Mira the Wonderful, or Algol the Demon star. Yet on closer examination, we find that most stars vary at least a little, even the Sun. All stars vary sometime in their life, and their variability can tell us about their size, temperature, and luminosity, and if they are just being born or are ready to die. Measuring changes in the brightness of variable stars is also a way in which high school students can contribute directly to professional astronomy.
Let's visit the data banks of the American Association of Variable Star Observers (AAVSO), download some variable star data and make some observations that can be added to the data bank.
- Go to AAVSO home page at: http://www.aavso.org/ There you will find lots of additional information about variable stars (click on Variable Stars in the Main sections of the Web menu) and how to observe them (click on Observing in the Main sections of the Web menu). But let's look at some data first. In the little box at upper left under Pick a Star, type in the name or ID number of a star. For example, type in 0214-03 (Mira), select Create a Light Curve, and click Go. The graph that appears shows the measurements of Mira's brightness (in magnitudes) that have been submitted to the data bank in the last 400 days.
- You can get longer or shorter sections of a star's data by clicking on the Light Curve Generator under Access Data, and fill out the form (the mystery of the Julian Date can be solved by selecting Observing, then Observing Aids, and finally JD Calendar and Calculator). You can even download the data in a form you can enter into Excel by selecting Access Data, then Download Data, and fill out the form.
- Select one of the following easy to observe variable stars:
0301+40 (Algol � an eclipsing binary star)
0122+88 (Polaris, the North Star - the brightest Cepheid variable in the sky), or
0214-03 (Mira � a long term semi-variable star).
- Download the graph or the data into Excel and create a graph for a period of time that includes at least two brightness cycles.
- Go out and measure the brightness of the star on three successive nights (weather permitting). Skipping a night or two due to clouds is permitted. You may observe naked eye, with binoculars, or with your own telescope (Or, you may want to try using the online MicroObservatory telescope at http://mo-www.harvard.edu/MicroObservatory/).
Detailed instructions on how to do the observations are found under Observing, then Observing Aids, then Visual Observing Manual or CCD Observing Manual.
- Add your observations to your graph and include your graph as part of a PowerPoint or hard-copy report describing how you made our observations, how your measured magnitudes compare with those in the graph done by other observers, and what the light curve tells you about your star.
- Submit your observations to the AAVSO by selecting Observing, and then Submit Observations. You will need to register with the AAVSO and follow their procedure.
For an additional challenge, join one of the ongoing Observing Programs or Observing Campaigns and contribute data to the project.