By Rich Roberts
I have an admission to make; I really want a star in the Milky Way to blow up. Seriously blow up. The last naked eye supernova occurred in 1604; we’ve had two others since then, but they were on the other side of the galaxy and obscured by dust. It’s been long enough and we’re due for something close (but hopefully not too close) and bright. Speculation swirled as Betelgeuse dimmed in late 2019 to early 2020 that we were about to see a beautiful supernova dominate our night sky. Well, Betelgeuse is still there and likely will continue to be there for a while. I still look at it on winter nights with the hope that it blows up while I’m staring at it, but for now it appears I must continue to wait. So why not think about some similar stars which may blow up anytime? Wikipedia has a list of possible supernova progenitors in our galaxy. Besides Betelgeuse, the list includes four other evolved red hypergiants which fit into the realm of the LPV section: Antares, VY CMa, V1489 Cyg, and miu Cep. Since miu Cep is a LPV program star, let’s take a deeper dive into this beautiful garnet-colored star.
When most people think about LPVs, Mira variables come first to mind; however, the LPV section covers a broader spectrum of red (and some yellow) pulsating stars. More details on the LPV classifications can be found in AAVSO's Long Period Variables Observing Section. Miu Cep is classified as a SRc type variable, which is a group that significantly differs from other red variables. From Percy (2007), “They [SRc variables] are massive, young, extreme Population I stars, whereas most red variables are less massive, older, disc Population I or Population II stars.” Percy also notes the rarity of these stars and that only a few, most notably Betelgeuse and miu Cep, are well studied. The two stars are similar in many ways, such as their size, convective cells, and magnetic fields.
Miu Cep varies between 3.43 and 5.1 V, so is naked-eye visible to those who are blessed with dark, clear skies. The star is easy to locate between a (2.4 V) and z Cep (3.6V). The star is also circumpolar for most northern observers with a +58.8o declination. Although technically circumpolar from my location in Virgina, it dips to only six degrees above the horizon, making late spring/early summer observations essentially impossible. I hope that miu Cep is considerate of my viewing requirements and picks the fall/early winter when it decides to blow up. Due to these generally favorable conditions, the star is very well observed by AAVSO observers. 82,430 observations, 92% of which are visual, have been logged in the AID dating back to 1845. The constellation of Cepheus is also home to the prototype for Cepheid Variables, delta Cep, and 11 total LPV program stars, so Cepheus is a good place to spend some observing time.
Below are the last 25 years of Johnson V data in the AID. The light curve is somewhat erratic and does show evidence of deep minima similar to what we experienced recently with Betelgeuse.
The following light curve zooms in to the last 10 years (July 2011–July 2021) and shows Johnson V data (green) as well as a seven-day binned means line from the visual data (blue). The visual binned mean tracks the photometric data well, but is consistently dimmer. This is expected due to the red color and bright apparent magnitude of this star.
VSX lists the period for this star at 835 days. DCFT analysis on all the visual data in the AID produced a similar result of 850 days; however, the phase plot using this result was a garbled mess. Several other periods in the 650- to 800-day range showed good strength. AOV analysis showed strong strength at 710 and 663.4 days. Of all these periods, 663.4 days produced the best phase plot on the Johnson V data, but I did not find it totally satisfactory.
No workable phase plot came out of the visual data. This could indicate that the star’s period has evolved over time and/or multiple periods are in play. According to Kiss et al. (2006), these types of stars generally do have two periods: a short one of a few hundred days and a long one of a few thousand days, as well as photometric irregularities due to convective cells. This again is similar to what we see with Betelgeuse.
Miu Cep is a huge star with a radius estimated to be about 1,000 times larger than our Sun’s. This means if miu Cep was placed at the center of our solar system, it would extend out to the orbit of Jupiter. Due to their large size, red supergiants have relatively low surface gravity, and thus high mass loss rates, although the mechanisms of their mass loss is not well understood. Their extended molecular structures are also similar to those found in Mira stars, although the pulsation modes in the two classes of stars are different. Shenoy et al. (2016) did find that the mass loss rate of miu Cep has decreased by a factor of five over the past 13,000 years.
So, I hope everyone will join me in looking to the sky with the hopes of cosmic fireworks. Betelgeuse isn’t our only option! Plus, although Cepheus may not be a great constellation for star gazing due to its dim naked eye stars, it certainly contains a treasure trove of interesting stars waiting for us to fix our telescopes and binoculars upon.
Arroyo-Torres, B., Wittkowski, M., Chiavassa, A., et al. (2015) A&A, 575, A50
Kiss, L., Szabo, G., Bedding, T. (2006) Mon. Not. R. Astron. Soc. 372, 1721–1734
Kravchenko, K., Chiavassa, A., Van Eck, S., Jorissen, A., Merle, T., Freytag, B., Plez, B. (2019) A&A 632, A28
Percy, J. (2007). Understanding Variable Stars. New York, NY: Cambridge University Press.
Shenoy et. al. The Astronomical Journal, 151:51 (14pp), 2016 March
Tessore, B., Lèbre, A., Morin, J., Mathias, P., Josselin, E., Aurière, M. (2017) A&A 603, A129