Located in the constellation of Camelopardalis (or the giraffe), G191-B2B is a very nearby white dwarf, although at a distance of 171.2 light years (52.5 pc) [1, 2], it's not close enough that I can use my Star Wars sound effects. Still, its relative proximity to Earth makes it a good target for scientific study. It only has an apparent magnitude of 11.7 [3], so it's not going to be visible without a telescope, but what do you expect for a white dwarf? It also has a mass of about 0.52 times the mass of the Sun and a diameter about 2.2 times the diameter of the Earth [4], so the white dwarf is very dense, and gravity is very strong on the white dwarf's surface. This last point will be important later.
White dwarfs are the leftover cores of Sun-like stars when they die. Currently, the Sun is at the stage where it is fusing hydrogen into helium in its core for energy, but when the core fills up with helium, it will expand to become a red giant, the fusion of hydrogen into helium will continue in a shell around the core, and then, a little later, the Sun will reach the stage where it will fuse helium into carbon and oxygen in its core for energy. However, Sun-sized stars do not have the mass to trigger the fusion of carbon and oxygen into heavier elements on large enough scales to power the star, so when the core of such a star fills up with carbon and oxygen, it will shed its outer gas layers to form a planetary nebula, leaving behind the inert core of carbon and oxygen.
At this point, it's worthwhile talking about the detailed composition of white dwarfs. Basic astronomy textbooks will describe them as though they contain just carbon because that can form straightforwardly from the fusion of helium [5], but that isn't quite accurate. Advanced astronomy textbooks will also mention that white dwarfs contain oxygen as well as carbon because it's also possible through fusing carbon with helium to make oxygen in the cores of stars just before they reach the planetary nebula stage [6]. People who work on stellar evolution and on white dwarfs themselves will tell you that the white dwarfs are not always pure balls of carbon and oxygen but may contain some sort of helium layer or some sort of hydrogen layer. What makes G191-B2B really interesting is that it is a hydrogen-rich white dwarf [7], which means that it has a layer of hydrogen on top.
Also, this outer gas layer on the surface of G191-B2B contains small amounts of various other elements that might have been mixed in with the gas that the star originally formed out of before it became a red giant or that might have formed in exotic but slow, low level fusion processes that took place outside the star's core when the star was beginning to shed its outer gas layers. Note that these fusion processes would not have produced large quantities of elements heavier than oxygen and that they also would not have produced any notable amount of energy. So now I get to list a whole bunch of elements, which is always fun to do in one of my podcast episodes. Astronomers have so far detected hydrogen, carbon, nitrogen, oxygen, aluminum, silicon, phosphorus, sulfur, iron, nickel, zinc, gallium, germanium, selenium, strontium, zirconium, molybdenum, tin, tellurium, iodine, xenon, and barium in the outer layers of G191-B2B [4, 8, 9, 10, 11, 12, 13, 14, 15, 16]. That's more than a fifth of the non-radioactive elements in the periodic table. It's also worth emphasizing again that, while all of these elements can be found in ther outer layers of G191-B2B, that outer gas layer still predominantly contains hydrogen.
These weird extra elements in the stellar atmosphere of G191-B2B allow astronomers to make all sorts of unusual physics measurements that they would not be able to do in laboratories on Earth or in most other places in the universe. A lot of this involves looking at the transitions of electrons between lower and higher energy orbital levels within these various elements [9, 10, 11, 12, 13, 14, 15], but one particular scientific highlight was a study led by J. Hu (and I could not figure out what the J stands for) that measured the fine structure constant in G191-B2B [16]. This constant is one of several values that describes the differences in electrons' energy levels in an atom, and it's typically measured to be approximately 1/137, but Hu and their collaborators indicate that the value of this constant in G191-B2B might be slightly higher than normal, and they indicate that this change (if it is real) could have been caused by the extreme gravitational fields in the outer gas layers of the white dwarf [16]. This result would indicate that at least one fundamental constant involved in quantum mechanics may actually be variable, which would be quite revolutionary and may even be worthy of a Nobel prize (if the result can be verified of course).
Aside from these studies of atomic physics in the atmosphere of G191-B2B, the white dwarf also serves another important purpose in astronomy. Apparently, it's relatively easy to model the light emitted from white dwarfs with outer layers composed almost entirely of hydrogen, and the model used to describe the emission from G191-B2B describes it as an object with a temperature of 60000 K (or 59727 degrees C or 107540 degrees F if you really need Fahrenheit measurements) [17, 18]. Anyhow, this makes it possible to use the light from G191-B2B to calibrated the detectors on various telescopes. In the visible and near-infrared parts of the electromagnetic spectrum, these detectors basically count the number of photons falling on the individual pixels within the detectors, but this isn't a 100% efficient process, so it's necessary to cross calibrate the number of counted photons with the expected amount of light from an object like G191-B2B that has been very accurately modelled. Most notably, G191-B2B has been used to calibrate the Hubble Space Telescope [17], which really surprised me. This shows just how useful this one white dwarf is in the field of astronomy.