The Sun Stares Back

It's a tapeworm! I swear to Christ it's a fucking cosmic tapeworm! Listen! It's 27,003.8 km long, it lives in the sun, it returns every 76 years 4 months 2 weeks after feeding, it
[DATA EXPUNGED]
You'll just have to wait, doc.

— SCP-1682

Starcages are enigmatic megastructures found in several systems. They're alien in origin, and at least 3 million years old.

True to their name, they are found around stars. The star is always a G-class yellow sun - some stars surrounded by starcages have evidence of massive starlifting operations to make them G-class. The bygone alien culture which built these vast constructions (tentatively dubbed the Starcage-Building Culture Group by xenoarchaeologists) evidently took great pains to make starcages happen.

The cage has a swarm architecture. Tens of thousands of flat, pentagonal statites, each the size of a small continent and arrayed with respect to the others in a fractalic dodecahedral pattern, hover suspended in the star's chromosphere, pushed upwards by the pressure of solar wind. Electromagnetic spectrum imaging reveals an intricate network of magnetic flux lines bound into tight arcs, directed through the statites, making up a colossal polyhedral lattice of energy. Electromagnetic force forms a constricting cage around the star, confining its vast eruptions.

Starcages aren't energy collectors. They aren't even protective structures, set up against solar flares. They are animal pens.

coronal mass ejection

It's not entirely clear what animal lives inside these stars. Based on what little is visible of them, they're probably shaped like worms. Sometimes, their bodies arc out of the star's nuclear atmosphere, a Loch Ness monster of blinding white plasma. The very smallest of them are the size of the continental United States. The largest could wind their body at least one and a half times around the Earth.

When uncaged, they tend to migrate. They launch themselves out of their home star, achieve solar escape velocity, and spin a thin, hard cocoon from space dust they gather via their innate magnetic field. They travel slower than light, coasting for decades or centuries in complete dormancy, until they reach another G-type star to colonize.

coronal rain

Each star surrounded by a starcage contains 5d100 sunswimmers. Don't stat them. They are a narrative problem. Getting directly attacked by a sunswimmer is like slamming into a goodly-sized planetoid, and no spaceship-sized weaponry can even concievably harm one. An encounter with a sunswimmer is an open-ended scenario unto itself. The players will have to figure out how to deal with it themselves.

If you encounter them in space, odds are they're cocooned and either in transit or dormant. They might be woken. 

If you encounter them in their natural habitat, your spaceship is inside a star, and mechanical questions are moot anyway.

sunspot

4 adventure hooks

1. A scientific institute contracts the players to drop a disposable probe into a nearby star so they can figure out what's inside the starcage. The star is a popular pilgrimage site of the Solarian Church. When they learn of the players' mission, the devout Solarians accompanying them on their trip desperately try to stop the heresy, first by impassioned pleading, then by violence.
2. A derelict starcage statite is found drifting in a close orbit around an uncaged star in a newly-colonized system. This is a megadungeon; statites are huge. Play it either as cosmic horror, the exploration of the ruins of near-incomprehensible precursors, or as an alien death trap, with reckless treasure hunters being massacred by three million year old defense systems. Keep in mind that this colossal monolith is the Starcager equivalent of a fence post.
3. The Company's latest job for the players is to tow an entire asteroid to a "processing station" for disassembly. The asteroid is actually the cocoon of a dormant sunswimmer, and the processing station is a corporate research facility. This information is above the players' pay grade.
4. An uncaged star is undergoing extreme starquakes and stellar eruptions as a sunswimmer inside prepares for its migration launch. A solar research station calls for help as its heat shields begin to fail. Can you reach and rescue them before the vast creature emerges into space?

oh yeah the sun just occasionally does shit like this, don't worry about it lol