Object 84: HD 214362
Podcast release date: 17 October 2022
Right ascension: 22:37:58.7
Corresponding Earth location: About 2300 km east of Rio de Janiero in the Atlantic Ocean
HD 214362 is a large red star that many people would be tempted to initially call a red giant. However, astronomers techncially refer to it as a horizontal branch star, which is similar to but not quite the same as a red giant. Stars like the Sun fuse hydrogen into helium in their cores, but when their cores fill up with helium, they expand and turn red to become red giants. Red giants fuse hydrogen into helium in shells around their helium cores, but at some point, the pressures and temperatures within the cores of red giant stars will get high enough that they can trigger the fusion of helium into carbon and oxygen. At this point, the stars will change color but will not change brightness. In a plot with brightness on the y-axis and color (or surface temperature) on the x-axis, these stars lie on a horizontal line, which is why they are called horizontal branch stars. It's not the best name when you think about it, unless you're really into graphs or something.
Anyhow, HD 214362 is one of these horizontal branch stars. It's actually a fairly ordinary-looking horizontal branch star, too. It's large and red, but so are most other horizontal branch stars. At a distance of 1628 light years (499 pc) [1, 2], it isn't particularly close to Earth, so it doesn't look particularly bright in the sky. It isn't even visible without a halfway-decent amateur telescope, and I don't think a somewhat experienced amateur astronomer would be able to identify it without a lot of assistance. It also isn't in a binary star system, at least as far as I know, and no one has found any exoplanets or anything else orbiting it. It also doesn't vary in brightness or produce weird radio or X-ray flares or anything exciting like that.
So, HD 214362 is just an ordinary-looking red horizontal branch star, and in fact, based on it's appearance, it's rather boring, and you would have thought that I should have run the random number generator again to try to find the coordinates of a more interesting object. However, HD 214362 is moving in a completely random direction , which makes it much more interesting than your average everyday horizontal branch star.
The Sun and most stars near the Sun are part of the disk of the Milky Way, and they travel more-or-less in roughly circular orbits around the center of our galaxy. Additionally, most of the interstellar gas in the Milky Way is also located in the galaxy's disk and also travels in roughly circular orbits around the center of the Milky Way, and this includes all of the cold nebulae that stars form from. When stars do form from these nebulae, the stars end up retaining the nebulae's momentum and travel in the same direction.
HD 214362, however, has a really elongated orbit. Astronomers use a quantity called eccentricity to describe the elongation of the orbits of various astronomical objects, although the word "eccentricity" might as well be used in its everyday sense (i.e., peculiarity or strangeness) to describe the orbit of this star. The eccentricity for a circle is 0, while the eccentricity for an extremely elongated ellipse is close to 1. For reference, the Earth's orbit has an eccentricity of 0.0167, which means that it's very close to circular . The eccentricity for the orbit of HD 214362 around the center of the Milky Way is about 0.95, which is really stretched out . This is actually more extreme than the eccentricities of the orbits of a lot of comets in our Solar System, although it is close to the eccentricity for the orbit of Halley's Comet .
Another way to think about HD 214362's orbit is to look at how it varies in distance from our galaxy's center. HD 214362 gets as close as 6500 light years to the center of our galaxy before travelling further than 200000 light years away from it . In comparison, the Sun more or less orbits the center of the Milky Way at around its current distance of 26700 light years . The eccentric orbit of HD 214362 (where "eccentric" could still mean "elongated" or "generally weird") also takes the star outside of the disk of the Milky Way into its halo. In fact, HD 214362 is just barely gravitationally bound to our galaxy.
So, the big question is, what happened to cause HD 214362 to orbit in such an eccentric way? That is not immediately clear. One possibility is that the star originates from a much smaller galaxy that was gravitationally pulled apart when it passed too close to ours . Another possibility was that the star was ejected from a globular cluster orbiting in our galaxy's halo after the star had gravitationally interacted with some of the other stars in that cluster . A third possibility is that the star was ejected from the center of our galaxy after gravitationally interacting with the other stars in that region .
In any case, HD 214362 and other stars like it are some of the very few individual evolved red stars that astronomers can identify as very likely originating from somewhere other than our galaxy's disk. Astronomers like being able to find stars like these because they provide new insights into the nature of stars from the other parts of our galaxy or from other galaxies altoghether, and this is why astronomer will continue to focus their research efforts on HD 214362.
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Data Release 3: Summary of the content and survey properties, 2022, arXiv e-prints, arXiv:2208.00211
 Pereira, C. B. et al., A study of two high-velocity red horizontal branch stars, 2013, Astronomy & Astrophysics, 559, A12
 GRAVITY Collaboration et al., A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty, 2019, Astronomy & Astrophysics, 625, L10