If you love a cosmic mystery – and which one of us doesn’t – then you’re going to really enjoy what’s about to occur in the night sky. It all has to do with an easily located variable star in the constellation of Cepheus and an unseen companion which crosses its path every 5.6 years…The star’s name is EE Cephei (RA 22 09 22.76 Dec +55 45 24.2) and and 10.8 magnitude it’s well within range of large binoculars and small telescopes. You’ll find it located about a degree and a half southwest of 4.2-magnitude Epsilon Cephei (about a finger width held at arm’s length). This will get you in the correct approximate field. For smaller optics you’ll see far fewer stars than what are depicted on the photographic chart, but the brighter ones will lead the way. However, in larger telescopes you’ll easy pick out the star patterns – so use the inset to help guide you to the right star! Now, here’s why it’s so important…
According to Mike Simonsen’s excellent blog: “This story starts in the 1950’s with the discovery of the variable nature of the star EE Cephei (Cep). Astronomers noticed it fainter than normal in 1947 and again in 1952. At first it was suspected of being an R Coronae Borealis type star. These are giant Carbon-rich, Hydrogen-poor stars that exhibit unpredictable fading episodes, believed to be caused by dust forming episodes in the outer layers of these stars’ atmospheres. The dust blocks the visible light, so we see the star fade, sometimes dramatically, by up to 9 magnitudes. It can take a year or more for them to return to maximum light, where they will shine contentedly for another undetermined period before coughing up dust and fading again.
When EE Cep faded again in 1958, Italian astronomers Romano and Perissinnotto suggested it might actually be an eclipsing binary with a very long period. Eclipsing binaries are stars that orbit around a common center of mass, and due to a line of sight effect we see them fade at regular intervals as one star passes in front of the other from our point of view. Sometimes, the alignment is so nearly edge on that we see a secondary eclipse as the smaller star of the binary pair disappears behind the primary. Because the orbits of these binaries are usually quite stable and the eclipses occur at regular intervals, observing eclipsing binaries is extremely helpful to astronomers in determining stellar masses, sizes, temperatures, luminosities and orbital parameters. Most have periods measured in hours, days or weeks because they are compact systems, with the stars in close proximity to each other, if not actually in contact.”
Exciting? Maybe not to some, but to those of us who not only enjoy astronomy as a passtime, but as a vocation – any event is welcomed and thoroughly studied. The EE Cephei event was confirmed after eclipses were observed again in 1964 and 1969 by L. Meinunger published the first ephemeris and established a period of 2049 days. All of this was well and good – but no secondary eclipse has ever been observed.
Says Mike: “The mysteries about this star were far from being unraveled though. One of the striking characteristics of EE Cep is the different eclipse depths and durations. Unlike many eclipses, whose periods can be measured to 8 significant digits, and whose range in magnitudes is very predictable, all of the observed eclipses of EE Cep have been different from each other in depth and duration.”
What’s happening is something strange is occurring with the light curve – it’s bottoming out and there may be a very good reason. As a highly respected member of the American Association of Variable Star Observers (AAVSO), Mike Simonsen has an answer to that mystery, too. “The most popular model to explain the secondary is that of a dark, opaque, relatively thick disk around a low-mass single star or a close binary. Differences in the shape of the particular eclipses could be explained by changes in both the inclination of the disc to the line of sight, and the tilt of its cross-section to the direction of motion.
The majority of the eclipses exhibit five repeatable phases that can be explained if the secondary is a disk shaped object with a gap in the center, like a giant cosmic donut. First, atmospheric and real ingress, where the dusty disk begins to obscure the light from the primary star, and then obscures it more fully as thicker, more opaque material blocks the light from the primary. Then a sloped-bottom transit, as the primary shines through the hole in the donut as it passes in front of the star. Then finally, real and atmospheric egress, as the disc moves away from in front of the primary star. The unique, flat-bottomed eclipse observed in 1969, can be explained by a nearly edge-on, non-tilted eclipse of the primary by the disc.
The color filter observations from the last eclipse show two increases in blue light (blue maxima) about 9 days before and after mid-eclipse. These subtle increases can be explained by the primary being a rapidly rotating Be star. These stars are darker around the equator and bluer at the poles. The reason there are two blue maxima can be explained if the disc is divided into two parts by a transparent gap. Spectroscopic observations show that the eclipsed component is a rapidly rotating Be star.”
Does this answer all the questions about EE Cephei? No. That’s the purpose of this article… More observations are needed and so is the help of all amateur astronomers ready and willing to take on the task. According to Mike, “The issue is far from settled. The light and color variations may have more to do with the different opacities in different parts of the disk. And here is where you can help write the story of this mysterious object. The next eclipse of EE Cephei starts right now. Mid-eclipse is predicted for January 14-15, 2009. The critical time to catch the blue maximums will fall between January 2nd and 27th. The longest eclipse lasted 60 days, so early December is the time to start taking data on this star, and observations should continue through the end of February.
If you have a CCD equipped with one or more science filters (UBVRI), astronomers at AAVSO will be very anxious to have you submit your data. If you are a visual observer, you can submit data on this eclipse also. EE Cep is normally a 10.8 magnitude star, and fades to anywhere from 11.5 to 12.5V. Thus it is easily observed with a telescope of 4” or more. Comparison charts for this star can be downloaded from the AAVSO’s Variable Star Plotter (VSP). There is a handy one page instruction for using VSP linked right from the top of that page.”
So, what are you waiting for? Here’s your chance to practice some serious astronomy!