DY Pegasi in the constellation Pegasus is an SX Phoenicis type variable star, and SX Phoenicis type variable stars are a subset of Delta Scuti type variable stars. This should instantly be confusing to almost everyone (except for those people who study variable stars) because variable star observers like to name classes of stars after one of the stars in that class.
So let's start with what Delta Scuti variable stars are. If people are interested, I discussed one of these stars way back in episode 6, so if you want, you can go listen to that episode. Delta Scuti stars are similar in most ways to the Sun. They are slightly brighter and more massive than the Sun, but they are still powered by the fusion of hydrogen into helium in their cores. However, the outer atmospheres of Delta Scuti variable stars are unstable and go through pulsation cycles [1]. These cycles go on continuously, and it's a bit tricky to say that they have any specific start or end point, so let's start with when a Delta Scuti star is at its faintest. This would correspond to when the star's outer atmosphere is the most opaque, and the outer layer at this point would get hotter and also expand. However, once the star has expanded too much and gotten too hot, the outer layer actually turns transparent, and then the star cools down, and its outer gas layer contracts until it reaches the stage where it becomes opaque again, and then the outer layer heats up and expands again. As a warning, I am going to now mention two other classes of variable stars. The Cepheid class of variable stars and the RR Lyrae variable stars also vary in brightness in the same way as Delta Scuti variable stars, but Cepheids are similar to red giants where their cores have filled up with helium, while RR Lyrae stars are beyond the red giant stage and are at the point where helium is fusing into carbon in their cores.
So that covers Delta Scuti variables. SX Phoenicis variable stars are just a subset of Delta Scuti stars that contain very few elements other than hydrogen or helium in their outer atmospheres, but otherwise, SX Phoenicis and Delta Scuti variable stars are the same [1]. Just to clarify, most stars that fuse hydrogen into helium in their cores like the Sun and like Delta Scuti and SX Phoenicis have outer atmospheres mostly consisting of hydrogen with some helium. However, stars like the Sun would have formed out of nebulae produced when other stars exploded as supernovae or transformed into planetary nebulae, and these types of events would have eject all sorts of elements heavier than hydrogen or helium into the instellar gas that the Sun formed out of. SX Phoenicis stars just seem to have either formed in locations that contain fewer of these heavier elements or formed at a time further in the past when stars had not yet produced so many of these heavy elements.
And now back to the star we are discussing in this episode. DY Pegasi was identified as a variable star in 1934 by Otto Morgenroth [2], but it literally took decades before people could figure out that it fits into the SX Phoenicis class of variable stars, in part because it also took decades for people to define that class of stars itself. Before that, DY Pegasi was just described as some sort of pulsating star. The star is located at a distance of 1327 light years (407 pc) [3, 4] and has a pusation period of about 1 hour and 45 minutes [5]. Given how long ago the star was identified as a variable star, it has been extremely well studied, and almost a century of data has been accumulated on the object. This in and of itself makes the star truly important in the study of variable stars generally and understanding the details of how these pulsation mehcanisms actually work.
In fact, such a large heap of data has been accumulated on DY Pegasi that some people have noticed slight variations in the timing of the star's pulsations, which indicates that DY Pegasi may be orbiting another star [5]. The other star would have gone through the red giant phase and then formed a planetary nebula a while ago, leaving behind just a white dwarf. If the white dwarf exists, then it and the variable star would orbit each other about once every 42 years [5]. However, the people who found this star indicated that the existence of this white dwarf needs to be confirmed [5], and I hope to see a science paper at some point providing proof that DY Pegasi is indeed a binary star system.