HD 210121 is a relatively ordinary B-type main sequence star located at a distance of 1090 light years (334 pc) [1. 2] in the constellation Aquarius. B-type main sequence stars are blue hydrogen-fusing stars that are hotter, brighter, and more massive than the Sun, and the spectra of these stars have dark lines that appear at specific wavelengths that are related to hydrogen and neutral helium (but not ionized helium). The B classification is divided into several subclassifications ranging from B0, which is the hottest, brightest, and most massive subclass, to B9, which is the coolest, faintest, and least massive subclass.
While people generally agree that HD 210121 is a B-type star, people disagree about which subclassification applies to the star. Various studies have assigned it to the subclasses B3, B3.5, B5, B6, or B9 [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. This translates to the star's total energy output ranging somewhere between 62 and 900 times the Sun's, and the mass of the star lying somehwere between 2.8 and 5.5.
The reason why people disagree about the classification of HD 210121 (and most importantly, the brightness of the star) is because the star, as seen from Earth, lies behind a relatively thick cloud of interstellar gas and dust, and the dust specifically heavily obscures the light from the star. It's actually this cloud of gas and dust that makes the star interesting. The star itself is fairly boring, but since the starlight passes through this thick cloud of interstellar material, it's a great way for astronomers to study that material and in particular to study how the interstellar dust either absorbs or scatters the light from HD 210121 as a function of wavelength [7, 12, 13, 14, 15, 16].
I've used a lot of real-life analogies for related astronomical situations in past episodes, but the analogy I would suggest for HD 210121 is that it is like holding up a letter inside a closed envelope up to a light. When doing this, not much of the light gets through the envelope, but it might be possible to see what's written on the letter inside the envelope without opening it. However, the dust cloud in front of HD 210121 is particularly thick compared to most other stars behind dust clouds that people use for this type of analysis, so looking at HD 210121 might be more analogous to shining a light through an envelope containing a lengthy set of complicated tax forms rather than shining a light through an envelope comtaining a one-page coupon for a new set of eyeglasses.
Anyway, measuring the broadbad spectrum of light is very important for a couple of reasons. First and foremost, the light from many stars in our galaxy as well as from the stars in many other galaxies is at least partly affected by the obscuring effects of interstellar dust, so it is important to understand how light is obscured as a function of wavelength to correct for these obscuration effects. This not only includes correcting light in the visible part of the electromagnetic spectrum but also in the ultraviolet and near-infrared parts of the spectrum. In fact, HD 210121 is so heavily obscured that it can be used to fine-tune models of dust extinction effects for extreme situations.
However, the second important reason why these measurements from stars like HD 210121 are so important is that it helps astronomers to build theoretical models of the interstellar dust grains themselves. This includes being able to model the size of the dust grains (which are typically around 0.1 micrometers), the chemical composition of the dust grains, and even the structure of the dust grains. Overall, observations of how dust obscures stars like HD 210121 gives us better insights into one of the most important parts of the interstellar medium. On the other hand, I myself study interstellar dust, so I might be biased.