Ceeta: That's the galactic core. We don't want to go there.
The galactic core, also known as galactic bulge, a general term referring to the innermost region of the galaxy that bulges outward (as opposed to the rest of the galaxy, the "disc" or "arms" depending on who's writing, which lie much flatter on the galactic plane), is not a place where people go. At the very least, it's a place where POV characters do not travel, for one reason or another. Maybe it's an Eldritch Location, or is simply inaccessible and remote despite the rest of the galaxy being traversible, or is full of dangerous and hostile aliens — the point is, we may go wherever we want to go in the galactic disc, but we do not travel into the core — at least not lightly and without a lot of preparation and gravitas.
On the Real Life side, the area around the galactic core of a galaxynote is in fact thought to be a rather strange and hostile place.
Just for starters, the closely packed stars, and their proximity to the supermassive black hole which forms the center of the core, make life in the core constantly interesting thanks to chaotic gravitational interactions. Stars can be flung off into deep space or into the core, directly collide with one another to merge (or explode), or even be ripped apart and cannibalized by larger neighborsnote . And that's after you consider the fact that all of those stars and the core itself are constantly pouring out vast amounts of radiation across the entire spectrum. If there are planets out there, they must have truly weird skies, full of the "fuzz" created by the light of billions of old, but still ridiculously bright, stars in close proximity to each other.
Additionally, heavy elements like gold, iridium and bismuth are only created by massive stars in the final instants of their lives, right before they create enough exotic unstable elements that we haven't named yet to go supernova. Here in the suburbs of the galaxy where we live, we have only had a few generations of stars. This means that lighter elements like silicon, carbon, oxygen, and nitrogen make up most of our world, and most of our solar system in general.
In the core, however, the extreme density of material, and the fact that this is the oldest neighborhood of the galaxy, means that there have been many generations of huge, fast-burning stars, and thus there is a much higher density of heavy elements. Its entirely plausible that there are stars in the galactic core with planets comprised primarily of metallic ore, with dense atmospheres of boiling iodine and mercury. If life could exist at all on these worlds, it would be radically different from anything found here on Earth. Silicon-Based Life and Starfish Aliens would be the least bizarre lifeforms one could expect to encounter.
To top it off (at least in our galaxy the Milky Way), while most of the stars there are old as the galaxy itself and there is little star-forming material, one can find at its very center a lot of young stars that include some of the most luminous known, often crammed in very dense star clusters, and enough interstellar matter to form similar ones in the future. It's in fact expected that some hundreds of millions of years in the future enough gas will accumulate in the galactic center to transform said very center in a starburst region, with both copious amount of star formation and supernovae, stellar winds, and radiations.
And that's just in the Milky Way! Some other galaxies have active galactic nuclei, in which the accretion of matter spiraling into a supermassive black hole accelerates its rotation faster and faster until it's outputting energy far beyond anything else in the universe - sometimes strong enough to drown out the entire rest of the galaxy they're part of, becoming a quasar. It's probably for the best that Earth isn't in one of those galaxies.
- The Space Monsters attacking humanity in GunBuster originate from the supermassive black hole at the center of the galaxy, so what does humanity do? Why, turn Jupiter into a massive bomb and blow it up, of course!
- Star Trek V: The Final Frontier: The galactic core is an area of intense radiation that makes life unlikely. The planet of Sha Ka Ree located there is also the prison of a powerful alien that lured Sybok and the Enterprise there in an effort to escape.
- Aeon 14: Ascended Artificial Intelligences from the Sentience Wars early in the setting's history, are revealed to have taken refuge in the galactic core... in the form of pure information residing within the Sagittarius A* black hole at its center. One character, Sera's mother, once sought them out and had her mind ripped from her body and turned into Airtha, one of the Orion War series' major villains.
- In Larry Niven's Known Space books, the stars in the galactic core have all exploded long ago, but the light — and more importantly the deadly, deadly radiation generated by several million stars exploding together — won't get to settled space for another twenty thousand years, so most spacefaring cultures are not particularly fussed about it.
- The final novel of Sergey Snegov's Men Like Gods trilogy (not to be confused with H. G. Wells' novel by the same title) sees humanity mount an expedition into the deep core, only to be turned into subjects for Time Travel experiments by a race so advanced, they cannot even begin to comprehend them or their motives.
- In Greg Egan's short story "Riding the Crocodile", the galaxy has been fully explored and is populated by a multicultural group of civilizations known, appropriately, as the Amalgam. The only region of the galaxy left unexplored is the galactic core (the "bulge"). No one has ever attempted to enter the region, and all nonsentient probes sent in have come back out several hundred years later (despite how it sounds, that's an extremely short timeframe in context) with no record of anything between when they disappeared and returned. It's obvious that the cause is artificial, but beyond that, absolutely nothing is known about whoever is sending the probes back. The story chronicles a couples' attempt to enter the bulge and meet with the elusive race or culture that the Amalgam has dubbed "the Aloof" due to their total unwillingness to engage or communicate with anyone outside the bulge.
- Star Wars Legends: The Deep Core is an enormous and extremely dangerous black hole cluster, even deadlier than the Maw outside the Kessel system. Even outside of that, the density of stars and stellar remnants, and consequently of the mass shadows their gravity wells project into hyperspace, make faster-than-light travel effectively suicidal. In the New Jedi Order series the Jedi build a base there after the Yuuzhan Vong capture Yavin IV, on grounds that it's next to impossible to reach the location alive without a Force-sensitive at the helm.
- Zones of Thought: The laws of physics vary based on distance from the center of the galaxy, and can be divided into Zones of Thought due to the fact that the farther out you go, the more technology is possible. The innermost Zone—the galactic core—is known as the Unthinking Depths, because the laws of physics there are so restrictive that conscious thought isn't even possible—upon entering the Depths, most sentient life forms would simply die immediately due to their brains shutting down.
- The "Empress Wave" in Traveller is a mysterious wave emanating from the galactic core, which (indirectly) led to the Rebellion. Avery's mission in the New Era was to find out what it was: a telepathic warning and a plea for aid from a civilization that died as a supergiant star was torn apart by a black hole.
- Many of the traits of the Squats/Leagues of Votann in Warhammer 40,000 are because they're from the galactic core. They were originally humans who were sent to mine the minerals in the core worlds, but they were genetically modified to be hardier and live in heavily protected fortresses.
- Galaxy Angel II sees the Will attempt to use the Central Globe to trigger a Big Crunch; They are eventually defeated by the Rune Angel Wing, with the Hyper Weapon of player-controlled Kazuya Shiranami's chosen Angel dealing the final blow.
- In Mass Effect 2, the galactic core is revealed to be the location of the Collectors' home base. They use it because they have the technology to make it a viable location for a permanent installation, whereas most species would be unable to cope with the high concentration of black holes and stars, which makes it an excellent secret hideout. The final main story mission is pulling off a commando raid on said base.
Jacob Taylor: The core is just black holes and exploding suns. There are no habitable planets there.
- In the game Space Pirates and Zombies, the galactic core is home to millions and millions of zombies, and its discovery causes them all to begin invading the rest of the galaxy.
- Spore: The galactic core is surrounded by the civilization of the biomechanical Abusive Precursors the Grox, and reaching the core itself reveals a wormhole allowing you to make contact with a civilization of more Benevolent Precursors, making this a Subverted Trope. Even ignoring the Grox, the effect of the core's heavy gravity means that your ship's travel radius is shorter, so navigating becomes much more tricky.
- In the Precursor backstory about the Zroni, their overuse of the Shroud, another dimension and the source of all Psychic Powers, was expanding the blackhole at the center of the galaxy. The Zroni fought a Civil War over whether the physical world was worth preserving, culminating in the Heroic Suicide of the ones that cared against the ones that didn't.
- Largely subverted in actual gameplay for vanilla Stellaris, where the galactic core is devoid of hyperlanes and largely inaccessible. The Astral Planes DLC introduced an event that places one system in the galactic core, which your scientists note should be impossible, but the strange physics-bending workings of the astral rifts seem to be making it survivable.
- One very popular mod for the game, the Gigastructural Engineering And More mod, fills in the galactic core.
- One possibility for what it fills in includes a civilization based in the center of the galaxy known as the Aeternum, which serve as a massive end-game challenge when they come out and start attacking everyone.
- Any sufficiently advanced empire that takes control of the Galaxy's supermassive quasar can build a superweapon called the Quasi-Stellar Obliterator, allowing them to take out entire regions. Doing so can unite most of the galaxy against them.
- Drive (Dave Kellett): The Continuum of Makers, one of the most powerful civilizations in existence and a bitter and deadly enemy of humanity, is centered over the Milky Way's galactic core.
- Schlock Mercenary: The galactic core is mostly exploding suns and black holes, which makes it perfect ground for the experiments of the Gatekeepers. They eventually start up a zero-point energy generator using the core itself, which was actually designed as a bomb by dark matter entities from Andromeda who wanted to destroy the Milky Way. Petey takes control of it and declares himself a God. Well, god-ish.
- The real life galactic core of the Milky Way is everything but peaceful. As noted above it's surrounded by huge dust clouds, many star remnants surround the central zone, and a good part of them consists of neutron stars and black holes (objects you should never get close to). A few light years from the centernote we have a system formed by at least a dozen stars, which orbit at a mindblowing speed (the closest star to it reached an estimated speed of over 5000 km/s at its perihelion) Sagittarius A*, a body as heavy as 4 million suns that is considered the actual center of the galaxy. That is, a black hole large enough to dwarf our Sun —in mass, in size it's way larger than the Daystar but not as much as one would think, the same way our Sun dwarfs Earth. And we've been lucky to not live near an active galactic core.
- Active galactic nuclei (AGNs), as mentioned above, are the single most luminous class of object in the universe. They come in several types:
- Seyfert galaxies are the least luminous type, but that's very relative; they're defined as galaxies with AGNs where the rest of the galaxy is still detectable - the nucleus is still emitting energy on the level of entire galaxies. Some of these are close enough to be the strongest radio sources in Earth's sky.
- Quasars are luminous enough that their emissions drown out those of the rest of their galaxy. These ones can have relativistic jets of material shooting out millions of light-years from their poles. No one knows what makes these jets.
- Blazars are what happens when a quasar's jet is pointing at Earth. As such, they're extremely bright, and relativistic lensing sometimes makes it look like the material in the jet is moving faster than light at first glance.