The other answers focus on purely natural chemical processes, but why not a cause with a sentient origin?

#Chlorophyll

... When she looks into the sky she sees only a handful of stars: those bright enough to shine through the moonlight and the sparkling river of the ring. Of the green star that the butterflies have revealed there is no longer any sign. But she knows it is still there, just too faint to be seen. Once revealed, it is not something that can ever be forgotten.

 

She knows that there is nothing actually wrong with the star. Its fusion processes have not been unbalanced; its atmospheric chemistry has not been perturbed. It shines as hot as it did a century ago, and the neutrinos spilling from its core attest to normal conditions of pressure, temperature and nucleotide abundance. But something very wrong has happened to the system that once orbited the star. Its worlds have been unmade, stripped back to raw atoms, then reassembled into a cloud of glassy bubbles: air-and-water-filled habitats, countless numbers of them. Vast mirrors—forged in the same orgy of demolition and reconstruction—trap every outgoing photon of starlight and pump it into the swarm of habitats. Nothing is wasted; nothing is squandered. In the bubbles, the sunlight feeds complex, teetering webs of closed-cycle biochemistry. Plants and animals thrive in the swarm, machines tending to their every need. People are welcome: indeed, it was people for whom the swarm was made in the first place.

From the epilogue of Absolution Gap by Alastair Reynolds.

The other answers focus on purely natural chemical processes, but why not a cause with a sentient origin?

Chlorophyll

... When she looks into the sky she sees only a handful of stars: those bright enough to shine through the moonlight and the sparkling river of the ring. Of the green star that the butterflies have revealed there is no longer any sign. But she knows it is still there, just too faint to be seen. Once revealed, it is not something that can ever be forgotten.

She knows that there is nothing actually wrong with the star. Its fusion processes have not been unbalanced; its atmospheric chemistry has not been perturbed. It shines as hot as it did a century ago, and the neutrinos spilling from its core attest to normal conditions of pressure, temperature and nucleotide abundance. But something very wrong has happened to the system that once orbited the star. Its worlds have been unmade, stripped back to raw atoms, then reassembled into a cloud of glassy bubbles: air-and-water-filled habitats, countless numbers of them. Vast mirrors—forged in the same orgy of demolition and reconstruction—trap every outgoing photon of starlight and pump it into the swarm of habitats. Nothing is wasted; nothing is squandered. In the bubbles, the sunlight feeds complex, teetering webs of closed-cycle biochemistry. Plants and animals thrive in the swarm, machines tending to their every need. People are welcome: indeed, it was people for whom the swarm was made in the first place.

From the epilogue of Absolution Gap by Alastair Reynolds.