Winter Variable Star Observing Activity in the Constellation Gemini

Zeta Geminorum in the Constellation Gemini

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Most of us are familiar with the winter constellation Orion the Hunter. The constellation Gemini, above and to the northeast of Orion, is also easily visible in the winter nighttime sky.

One interesting object located in Gemini is Zeta Geminorum (Zeta Gem). Zeta Gem is the most distant star in the constellation, located about 1200 light-years from Earth. It is a Cepheid variable star. Cepheid variable stars pulsate with a regular period, alternately expanding and contracting due to instabilities within the star. The ancient Arabic name for Zeta Gem is "Mekbuda," which means "pulled-in paw." Perhaps the ancient Arabs noticed the variability of this star and associated it with claws that can be extended or retracted. In 1844, Julius Schmidt, a teenage amateur astronomer and student at Bonn University, suspected that Zeta Gem was a variable star, and confirmed its variability in 1847.

Zeta Gem pulsates from its dimmest visual magnitude of 4.2 to its brightest magnitude of 3.6, and then back to 4.2 again, every 10 days. The magnitude scale is used to rate the brightness of a star. When the system was devised, the brightest stars were classified as "first magnitude," and the faintest as "sixth magnitude," which is why magnitude 3.6 is brighter than magnitude 4.2. The cycle of going from brightest magnitude to dimmest magnitude and back to brightest magnitude again is called the period of a variable star. With a period of 10 days, you can observe Zeta Gem through its entire period of brightness variation approximately three times every month.

Locating Zeta Gem

Use a sky chart like the one at left to help you locate Zeta Gem in the sky. Face south and hold the chart in front of you so that the southern horizon is on the bottom. First find Orion the Hunter, then look above and to the left of Orion to locate Gemini. Castor and Pollux are the most conspicuous stars in the constellation. Gemini will appear about the same size as your hand held at arm's length.

This sky chart shows the night sky as seen from middle northern latitudes for the following times: Late December, 12 midnight; early January, 11 p.m., early February, 9 p.m., late February, 8 p.m., early March, 7 p.m.

The chart above shows the location of Zeta Gem (circled) and some of the bright stars in Gemini. The magnitudes are given for several non-variable stars. These stars are the comparison stars that you will use to estimate the magnitude of Zeta Gem. The most convenient stars to use as comparison stars are Lambda Gem (below and to the left of Zeta) at visual magnitude 3.6, and Nu (to the right of Zeta) at magnitude 4.2 (both are underlined). Zeta Gem varies approximately between these two magnitudes. For example, if the brightness of Zeta appeared to be halfway between the brightness of Lambda and Nu, you would estimate its magnitude as 3.9. If Zeta appears only slightly fainter than Lambda, you would estimate the magnitude as 3.7.

Once you have located Gemini, Zeta Gem, and the comparison stars Lambda and Nu in the night sky, you are ready to start observing the variation in brightness of Zeta Gem and recording your observations.

Observing Procedure

1. Find a location with a sky dark enough to see Zeta Gem and its comparison stars. You should be accompanied by a relative, teacher, or friend, and make sure that your location is safe. Maybe you know a local amateur astonomer who would be willing to assist you.
2. Check weather conditions and the phase of the Moon to make sure your view of Gemini is not restricted by moonlight or clouds. Wear warm clothes!
3. Bring your sky chart, paper and pencil, and a small red flashlight. An ordinary flashlight with red cellophane taped over the light will work. The red light will allow you to look at your sky chart and record your data without interfering with the dark adaptation of your eyes.
4. Locate Zeta Gem and its comparison stars. Quickly look back and forth from Zeta Gem to the comparison stars and estimate the magnitude of Zeta Gem. Record your estimate. Observe Zeta Gem two more times and record your estimates. Take the average of the three estimates. This will give a more accurate estimate. Also, record the date and time of your observation. Try to observe Zeta Gem at the same time every clear evening for a 30-day period.
5. After you have recorded a few observations, you may want to plot them on graph paper so you can determine the period. You will plot the average of the three magnitude estimates which you recorded versus the date and time* for each averaged observation. This plot is called a light curve. A sample light curve for a different variable star, Delta Cephei, is shown above. A smooth curve is drawn through the points which represents the changing brightness of Delta Cephei.
6. Enter your data in the table below; tomorrow you can compare your magnitude estimates with those of other student observers from around the country (and possibly from around the world!). Even if you had some cloudy nights and were unable to observe Zeta Gem, in other parts of the country students had a clear sky and made observations that will fill in the gaps in your data. So don't feel bad if you don't have many observations, because they are still very important. The more observations plotted, the more accurate the light curve will be.

* For extra credit, record your observations using the Julian Date. For information about the Julian Date, and instructions on converting "regular" dates and times to the Julian Date format, visit the AAVSO website and click on "Julian Date Conversion." Be sure to check out the other information and exercises available on the HOA and AAVSO websites!

Gemini (the Twins)