The coordinates for this episode point to the star Z Ursae Minoris. Quite honestly, it's about time we got an object in Ursae Minor. The star is a red supergiant, but because the star sits at a distance of roughly 15 thousand light years (4.6 kpc) [1, 2], the star appears sufficiently faint that it's impossible to see with the naked eye, and it would look completely indistinct from anything else when viewed through a telescope.
In the early 1990s, Priscilla J. Benson at Wellesley College, as part of her pioneering development of undergraduate research projects using CCD imagers and the campus's 24 inch diameter telescope, discovered that Z Ursae Minoris varied in brightness [3]. Benson's work on these undergraduate variable star research projects quite impressed me. The detection of new variable stars is now done by automated sky surveys, which can trivially identify hundreds or thousands of variabile stars and even detect exoplanets by how they eclipse their host stars, but it's still really interesting to read about someone monitoring variable stars in the 1990s using just the telescope at their campus observatory and some undergraduate assistants [4, 5].
Anyway, Benson first observed that Z Ursae Minoris gradually increase in brightness from about magnitude 12 to about magnitude 11 between April 1992 and July 1993 [3]. By October 1993, however, the star's brightness dropped by 6 magnitudes [3]. In January the following year, the star began to brighten again [3]. Rather than wait for the star to reach its original brightness, however, Benson and a couple of collaborators published a paper indicating not only that Z Ursae Minoris was a variable star but that it belonged to a class of stars called R Coronae Borealis variables [3].
This class of stars is named after R Coronae Borealis because that star is the first star ever discovered to vary in brightness in this specific way. Like Z Ursae Minoris, all R Coronae Borealis stars are red supergiants. Most of the time, the brightness of these stars is relatively steady, but at random intervals, the stars will decrease in brightness by a few magnitudes for a couple of months [6]. The general explanation for why these stars vary in brightness is that they occasionally produce small puffs of dust that then dissipate [7, 8, 9]. However, people still don't quite understand exactly how these stars produce the dust grains that go into these dust puffs.
The even weirder thing about R Coronae Borealis stars like Z Ursae Minoris is that current theories suggest that they formed from the merging of two white dwarf stars [10, 11]. Most ordinary red supergiants would have formed from the evolution of some sort of hot blue star multiple times the mass of the Sun that would have initially fused hydrogen into helium in its core. These hot blue stars transform into red supergiants when their cores fill up with helium and they start fusing hydrogen into helium in shells around those cores and also fuse the helium into carbon and oxygen in their cores (and potentially fuse those things into even heavier elements). However, the outer atmospheres of normal red supergiants still contains a lot of hydrogen, while in comparison, the outer atmospheres of R Coronae Borealis stars appear to lack hydrogen, although they do contain a lot of carbon [10, 11]. White dwarfs, which are the leftover cores from when Sun-sized stars die and expel their outer atmospheres to form planetary nebulae, also have virtually no hydrogen, but they do contain a lot of carbon and also helium and oxygen. A single white dwarf by itself is not going to suddenly transform into a red supergiant, but if two white dwarfs in a binary star system were to merge, one of them could get tidally disrupted in a way that it forms a layer of gas on top of the other, making something that looks like a weird red supergiant [10, 11].
While the details of how white dwarf stars form R Coronae Borealis stars is still being investigated, people are still very interested in monitoring Z Ursae Minoris specifically because it is one of the brighter objects within this class of stars in the Earth's sky and because it has been more carefully observed than most other stars in this class. In fact, Bradley Schaefer published a paper in 2024 that delved into historical observational data to produce a timeline of Z Ursae Minoris's light variations extending back to 1892, and he identified 27 times when Z Ursae Minoris dipped in brightness from some sort of dust puff [6]. Hopefully, this type of data will be useful for unlocking the secrets of how R Coronae Borealis stars formed and how they work.