Download Winter Variable Star Observing Activity in the

Winter Variable Star Observing Activity in the Constellation Gemini
Zeta Geminorum in the Constellation Gemini
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
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.
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.
Stellar Magnitude Key:
1
2
3
4
5
6
Brighter
Dimmer
North
South
The boxed chart above at right 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)
The two brightest stars in Gemini are Castor and Pollux, the
twin half-brothers in Greek mythology who give the constellation
its name. Castor and Pollux were born to Leda, the queen of Sparta
in Greece; however, the twins had different fathers. Castor was
the son of Leda’s husband, the king of Sparta, and Pollux was the
son of the Greek god Zeus.
Castor and Pollux accompanied Jason and the Argonauts in
their search for the Golden Fleece, and had many great adventures.
Afterwards, Castor was killed during a cattle raid. Pollux, who
was immortal, mourned for his brother and wanted to follow him
into Hades (the underworld). Zeus took pity on his son Pollux and
allowed him and his brother to visit on alternate days Hades and
Mount Olympus (where the gods lived).