The name "S254-S258 Complex" doesn't quite sound like the name of an astronomical object. It sounds like the name for a set of interconnected business units in an industrial park. Nonetheless, it is the name of an extended structure in the constellation Orion (very close to the constellation Gemini) formed by five astronomical objects named S254, S255, S256, S257, and, surprisingly, S258, although S255 is also called IC 2162. Each of these objects is a bubble-like cloud of ionized hydrogen gas referred to as an HII region, which is the stupid term that astronomers use to refer to ionized hydrogen instead of H+.
S255 was discovered at the end of the nineteenth century [1], but the other four regions were discovered by Stewart Sharpless in a photographic survey of ionized gas regions that he published in 1959 [2]. The ionized gas produces emission at very specific wavelengths in the visible part of the electromagnetic spectrum, which is what makes these ionized gas regions so easy to identify. Except for S256, they are all part of a relatively straight structure running from southeast to northwest as seen in Earth's night sky [3]. S255 and S257 are the brightest bubble-like regions and sit next to each other near the center of the complex. S254 is larger but fainter, and it sits to the west of the center of the complex just next to S257. S256 is a very small bubble of ionized gas located below the point where S254 and S257 meet, and S258 is another very small bubble located to the east of everything. The complex is located at a distance of roughly 8150 light years (2.5 kpc) [4,5], and the distance from the outer edge of S254 to the outer edge of S258 is approximately 57 light years. The complex covers a region that's roughly 0.4 degrees wide on the sky, which is just a little smaller than the diameter of the Moon [3].
Each of these spherical ionized gas clouds have hot blue stars at their centers that produce huge amounts of ultraviolet light, and this ultraviolet light is what ionizes the gas. The presence of these hot blue stars is an indication that more stars are forming within the S254-S258 Complex. I'll explain how. When stars form, they aren't all the same. Some stars are very massive, very hot, and very blue like the ones that we see at the centers of the ionized gas clouds in this complex, some stars are like the Sun, and some stars are very small, relatively cold (by star standards), and very red. Initially, all of these stars are powered by the fusion of hydrogen into helium in their cores, but they will eventually run out of hydrogen in their cores and transform into red giants or supergiants before (in the case of the very massive stars) exploding as supernovae or (in the case of Sun-like stars and smaller stars) just expelling their outer atmospheres as planetary nebulae. Stars like the Sun last for about 10 billion years, and even smaller red dwarf stars seem like they could exist forever, but the very hot blue stars will only last for a few million years. Hence, the presence of these short-lived ionizing blue stars in the S254-S258 Complex indicates that at least these big stars but also more than likely other smaller stars recently formed here, and it's a good place to search for even more stars that are still in the process of forming.
Most people who have read about how stars form might picture one really big poofy cloud of gas collapsing under its own gravitational forces to form a cluster of hundreds of stars. However, astronomers have found that stars do not form out of poofy clouds but instead form out of thin, elongated filamentary-like structures of cold, non-ionized interstellar gas and dust. These filaments are impossible to see in the visible part of the electromagnetic spectrum, but they are very easy to see in far-infrared and millimeter-wavelength radiation, which are forms of electromagnetic radiation that have wavelengths much longer than what we can see with our eyes. Like I said earlier, four of the photoionized gas clouds lie along a line in the sky, and in infrared light, you can see a filament of interstellar dust and gas connecting these regions and extending further to the southeast [3].
Aside from the hot young blue stars, you can also see more individual young stars and protostellar objects within the filamentary structure when you look at the complex in near-infrared light [3,5,6,7,8], which is electromagnetic radiation with wavelengths just slightly larger than the wavelengths of the light that we can see with our eyes. These things are stars that are just beginning to form, and fusion has not yet started in their cores. However, because they are basically balls of gravitationally-compressed gas and because gas gets warm when it's compressed, the protostellar objects produce lots of near-infrared light, which is why astronomers use infrared telescopes to go look for them.
What's extra freaky about the S254-S258 Complex is that a couple of the hot blue stars, by ionizing the surrounding gas, may have caused even more stars to form out of the colder gas in the filaments [7,9,10]. Stars can't form out of ionized gas because it's too hot to collapse under its own gravitational forces, so when hot blue stars ionize the gas around them, any star formation would initially stop. However, the ionization also heats up the gas. This causes the gas to expand as the bubble-like features we see in the S254-S258 Complex, and when this gas expands, it pushed through the surrounding colder gas and compresses it, causing the cold gas to collapse and form more stars.
Overall, the S254-S258 Complex is, well, complicated. I could have said it's complex, but that joke's too obvious.