Eclipsing Stars

Observing an eclipse of RW Geminorum

This is the final example of my not widely shared observations made at Winfree Observatory at Randolph Macon Woman's College (now called Randolph College).

A total solar eclipse, when our Moon passes in front of the Sun, is a spectacular and somewhat rare event! People travel all over the globe to watch. So far I've been fortunate to see only one total solar eclipse in my life, on July 10, 1972 on Prince Edward Island, Canada.

With C at the 1972 eclipse site on Prince Edward Island

A total solar eclipse looks roughly like the image below when viewed with ordinary binoculars during totality. Notice the pink flame-like prominences which become visible on the Sun's rim!

A view similar to what I saw from Prince Edward Island with unfiltered binoculars

As wonderful as a solar eclipse is, it must be tame compared to one star eclipsing another! Wouldn't it be amazing to watch one star pass in front of another? Actually, this happens frequently in the heavens when two stars orbit each other. These are called binary stars. If a distant planet happened to orbit a binary star, it might experience double sunsets like Luke Skywalker observed from the planet Tatooine in this memorable scene from the first Star Wars film:

There are a great number of binary stars with orbital planes oriented edge-on to our line of sight. As these stars orbit they regularly pass in front of one another and produce eclipses from our point of view. One such eclipsing binary star is called RW Geminorum (meaning the variable star system RW in the constellation Gemini). The red X locates the position of RW Geminorum in the constellation Gemini below:

(click to enlarge)

What if, from our point of view, two orbiting stars completely overlap during the orbit? The light from the system should then vary like this:

A more detailed, but somewhat different, illustration is:

Notice how the brightness curve of the binary system is flat and horizontal during the deeper primary (1st minimum) and shallower secondary (2nd minimum) eclipses. These flat and horizontal portions of the light curve happen when a smaller star fits completely in front of or behind a larger star. This is the situation for the two stars, A and B, in RW Geminorum. Brighter star A has a mass of 4.97 Solar Masses, a radius of 3.69 Solar Radii, a surface temperature of 12,390K, and a bolometric luminosity of 285.25 Solar Luminosities. Dimmer star B has a mass of 2.24 Solar Masses, a radius of 4.67 Solar Radii, a surface temperature of 6,200K, and a bolometric luminosity of 28.47 Solar Luminosities. (Bolometric means including all wavelengths of light.) During the primary eclipse of RW Geminorum the smaller diameter, brighter star A is entirely behind the larger diameter, dimmer star B. This primary eclipse totality time lasts for more than an hour.

During the fall of 2003 senior physics major, Sarah Priester (class of 2004), and I decided to observe the primary eclipse of RW Geminorum. Since the complete orbital period of RW Geminorum is 2.8654972 days, we decided there would not be enough observing time in the fall semester to capture the light curve for the entire orbit. Therefore, we set out to observe only the primary eclipse. This required about 15 hours of observing.

During October and November of 2003 we observed RW Geminorum on 3 different nights. We made 189 V-filter CCD images, one every 5 minutes during each session. The exposure times varied between 60 seconds, and 240 seconds. After many hours carefully measuring and analyzing all 189 images we obtained the light curve shown below which clearly shows the primary eclipse.

This is one of the most beautiful light curves I ever achieved at Winfree Observatory!

The vertical scale is an astronomer's (logarithmic) scale of brightness measured through a photometric green filter (also known as a visual, or V, filter). The smaller the magnitude, the brighter the star. The horizontal scale, orbital phase, is a portion of the time for a complete orbit. The time for a complete orbit is 2.8654972 days. This is 1.00 cycle. The horizontal scale runs from 0.400 (40 percent) of the orbital cycle to 0.650 (65 percent) of the orbital cycle with the center of the primary eclipse located halfway through at 0.500 (50 percent) of the orbital cycle.

The duration of the flat total eclipse portion is one hour, 22 minutes with an uncertainty of about 5 minutes. The two unfortunate gaps in the otherwise beautiful curve are caused by the annoying inability of our German equatorial telescope mount to continuously follow celestial objects as they cross the meridian. At two meridian crossings no images could be taken while the telescope slewed from one side of the pier to the other.

We didn't observe the entire eclipse continuously, rather we watched like we would watch a long movie with several intermissions! There's a good chance we were the only people on Earth observing this particular eclipse at this particular time!