μ Cep

What’s in a name? That which we call a rose
By any other name would smell as sweet...  

Shakespeare reminds us in "Romeo and Juliet" (Act 2 Scene 2) that names don't matter--it's the reality behind the label that matters. So just as Juliet Capulet wasn't concerned with the name Montague, I'm not particularly concerned with whether this magnificent star is called Mu Cephei, Mu Cep, μ Cep, Herschel's Garnet Star, Erakis, HIP 107259, HD 206936 or one of this star's many other names, so let's get down to the realities of this beast. Mu Cep is classed as either an M2 Ia or M2e Ia. 

M = Type M star (Red and cool)
2 = Rather hot for a cool Type M star
Ia = Luminous supergiant/hypergiant
e = Emission lines presents in the stellar spectrum.

But why did I call the star a beast? It has to do with the other characteristics of the star. Let's look at Mu Cep compared to our sun:

Mass 19.2 M☉
Radius 972±228 R☉
Luminosity 269,000 L☉ (latest research values)
Temperature 3,551 K
Age 10.0 Myr
AbsMag -7.63

Mu Cep is one of the biggest and brightest around! 19 times heavier, 972 times bigger, 269,000 times brighter than our old Sol and a baby at a mere 10 million years old.

Mu Cephei is a member of Open Cluster IC 1396 (Near the Elephant Trunk nebula), but is moving away from the cluster at about 21 km/s and probably has been since the early days of the cluster. Two theories account for 'runaway' stars like this: (1) a binary (partner) star supernovas and throws the survivor off course at its original orbital velocity (usually > 40 km/s), or (2) the star is gravitationally ejected (at a slower velocity) during the initial expansion of the cluster. 

I vote for explanation (2), and here's why. First, there is no apparent trace of a SNR (although if you use telescopius.com you'll see that there are references to IC 1396A and IC 1396B as SNR. Simbad, however, lists these as molecular clouds). Second, according to the latest research, the radial velocity of 21 km/s is too low for a SN rejection (see Bhat A., Irrgang A. and Heber U., 2022). Stars that fall into this second category are no longer called runaway stars, but instead are "walkaway" stars. (I really want to laugh.)

Regardless, Mu Cep is still regarded as a member of the cluster, but on Revision A you can see its location in relation to the main cluster, located near the Elephant Trunk. It's luminosity still dominates the entire area! For much of its journey from the center of the cluster to its present location it was still a Blue Supergiant, ionizing the molecular cloud as it journeyed along. Looking at the Revision you can almost see the path that it has carved along the way.

Perhaps one of the most interesting and important things to me about Erakis, is that it is very similar to Betelgeuse. The latter has appeared much in the news in the last ten years, with stories about Betelgeuse's impending death-by-supernova. Although we can't predict the future, we do have a pretty good idea about the evolutionary trajectory of both of these incredible stellar masterpieces.

They both formed Type O Supergiants and have evolved to Red Supergiants. They are both burning Helium at their core and their metallicity is increasing. They are throwing off mass rather quickly, and are surrounded by shells of molecular gases and well as a bow shockwave of gas and dust ahead of them. They have comparable Mass, Radius, Luminosity and Temperature (Mu Cep edges out Betelgeuse in most of these categories). They are both runaways--uh, walkaways.

Finally, they are both nearing the end of their lives. At some point the cores of these two beautiful stars will collapse and we'll be treated to supernovae. Which one will go first? Your guess is as good as mine, but we don't need CNN and other news organizations getting our hopes up. Seriously now.

*****

John Muir, American Naturalist in his Meditations wrote: "...All that is necessary to make any landscape visible and therefore impressive is to regard it from a new point of view, or from the old one with our heads upside down. Then we behold a new heaven and earth and are born again..." Sometimes photographing a single star is just what our stellar landscape needs to gain a new understanding or appreciation of heaven and earth. What I particularly like about these single stars is that we can all do them: from novice to expert. How many of us, though, go back to the simplicity of this task to remind us of our roots and to learn the lessons of the single star?

Thank you for taking the time to look and read and comment on a simple star field!

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A few sources to consider for further study:

"The origin of early-type runaway stars from open clusters."
Bhat A., Irrgang A. and Heber U. 
Astronomy and Astrophysics, volume 663A, 39-39 (2022/7-1).

"What's behind the Elephant's trunk? Identifying young stellar objects on the outskirts of IC 1396."
Silverberg S.M., Gunther H.M., Kim J.S., Principe D.A. and Wolk S.J. 
Astron. J., 162, 279-279 (2021/December-0).

“The structure of the IC1396 region.”
Weikard, H., Wouterloot, J. G. A., Castets, A., Winnewisser, G., and Sugitani, K.
Astronomy and Astrophysics, vol. 309, pp. 581–611, 1996.

"The circumstellar dust of MU Cephei."
Rogers, C., Martin, P. G., and Crabtree, D. R.
The Astrophysical Journal, vol. 272, pp. 175–181, 1983. doi:10.1086/161275.

“Red giants in open clusters. XIV. Mean radial velocities for 1309 stars and 166 open clusters.”
Mermilliod, J. C., Mayor, M., and Udry, S.
Astronomy and Astrophysics, vol. 485, no. 1, pp. 303–314, 2008.