Credit: Tony Hallas
Key Takeaways:
- IC 1396, a large emission nebula in the constellation Cepheus, spans 2.8° by 2.3° and contains various celestial features including dark nebulae and a bright star cluster.
- Within IC 1396, the Elephant Trunk Nebula is a notable region of sinuous bright and dark nebulosity, identified as a "cometary globule" and signifying an active star-forming area.
- Mu Cephei, also known as Herschel’s Garnet Star, is a nearby, intensely red, cool supergiant with a magnitude variation from 3.4 to 5.1 over a 730-day period, caused by carbon compounds on its surface.
- Observing IC 1396 effectively requires at least an 8-inch telescope from a dark site, revealing a circular mist with dark lanes and featuring the open cluster Trumpler 37, which includes a central triple star system.
Scanning the constellation Cepheus the King leads us to one of the sky’s largest emission nebulae — IC 1396. It measures a whopping 2.8° by 2.3°. But this region also contains dark nebulae and a bright star cluster. Plus, there’s a famous colorful star nearby. Get ready to spend lots of time observing this wonderful region.
In images of IC 1396, the most noticeable part is the Elephant Trunk Nebula. This area of sinuous bright and dark nebulosity conceals a new star-forming region. Because such a region resembles a comet, it’s earned the name “cometary globule.” In addition to the gas the globules contain, they have dusty heads and elongated tails.
Don’t miss Mu
The brightest star near IC 1396 is Mu Cephei, also known as Herschel’s Garnet Star. This intensely red luminary varies in magnitude from 3.4 to 5.1 over a 730-day period. Mu is a cool, red supergiant that dredges up carbon compounds from its interior. The carbon temporarily settles on the surface, dimming and reddening the star’s light. Eventually, the dark material absorbs enough energy to escape into space. Because of its proximity to Mu, some early 19th-century astronomers referred to IC 1396 as the Garnet Star Nebula. That name is now obsolete.
If you can see Mu Cephei with your naked eyes, then it’s near its maximum brightness. Note the color, and, because of its period, note it in your observing log, because it may be a couple of years before you notice a change. If, on the other hand, the star is barely visible to you (or you can only see it through binoculars), then it’s near minimum brightness. Look at it then, and you’ll quickly see why it was called Herschel’s Garnet Star.
Observe
If you choose to observe IC 1396 without a nebula filter, use at least an 8-inch telescope from a dark site. Although you’ll detect it through smaller instruments, you won’t see much detail. If your field of view measures at least 2°, you’ll see a circular mist crossed by many dark lanes. The most prominent lane points from the nebula’s northwestern edge to the center.
At the nebula’s heart, you’ll find the naked-eye open cluster Trumpler 37, a 1°-wide collection of more than 50 stars between magnitudes 10 and 12. Increase the magnification past 150x to view the triple star at the cluster’s center — a magnitude 6 sun flanked by two 8th-magnitude companions.
This isn’t the easiest celestial object to coax details out of, but it’s well worth your time. Good luck!
