“So we have this degenerate, dead core of helium, the burning shell around it. What next?”

“Now we start speculating. Uvarov, in a conventional giant, when the core mass is high enough — about half a Solar mass — the temperature becomes so high, a hundred million degrees or more, that a new fusion chain reaction starts up: the triple-alpha reaction, which — ”

“The fusion of the helium ash into carbon.”

“Yes. Suddenly the ‘dead’ core is flooded with helium fusion energy. Now remember what I told you, Uvarov: the core is degenerate. So it doesn’t expand, to compensate for all that heat…”

“You turn condescension into an art form,” Uvarov growled impatiently.

“Because it can’t expand, the core can’t cool off. There is a runaway fusion reaction — a helium flash — lasting no more than seconds. After that, the core starts to expand again, and eventually a new equilibrium is reached — ”

“All right. That’s the standard story; now let’s get back to the Sun. Sol isn’t a conventional giant, whatever it is.”

“No. But it’s approaching its helium flash point.”

“Won’t the action of the birds suppress this helium runaway — the helium flash just as they’ve suppressed hydrogen fusion, all this time?”

“No, Uvarov. They’re not taking out enough energy to stop the flash… Maybe they don’t intend to. And, of course, the fact that the core of Sol is so unusually hydrogen-rich is going to make a difference to the outcome. Perhaps there will be some hydrogen fusion in there as well, a complex multiple reaction.”

“Mark. You said a new equilibrium will be reached, after the helium flash.” Uvarov didn’t like the sound of that. He wondered if it would be healthy to be around, while an artificially induced red giant struggled to find a new stability after the explosion of its core… “What will happen, after the helium flash?”

“Well, the pulse of heat energy released by the flash will take time — some centuries — to work its way through the envelope. The envelope will expand further, seeking a new balance between gravity and radiation pressure. And the energy released in the flash will be immense, Uvarov.”

“Immense?”

“Uvarov, there will be a superwind.”

Superwind…

The helium flash would blow away half the mass of the Sun, into an expanding shell ballooning outwards at hundreds of miles a second.

The core — exposed, a shrunken thing of carbon-choked helium — would become a white dwarf star: cooling rapidly, with half the mass of Sol but just a few thousand miles across, no larger than old Earth. The flocks of photino birds, insubstantial star-killers, would continue to swoop around the heart of Sol’s diminished gravity well.

At present — before the flash — Sol was a red giant around two astronomical units across. After the superwind the envelope would be blown into a globe twenty thousand times that size, a billowing, cooling cloud three hundred light days across.

The furthest planet from the heart of old Sol was only forty astronomical units out — six light-hours. So the swelling envelope would, at last, smother all of Sol’s children.

Then, when the superwind was done, the dwarf remnant would emit a new wind of its own: a fizz of hot, fast particles which would blow at the expanding globe, pushing out the inner layers. The globe would become a planetary nebula — a huge, cooling, hollow shell of gas, fluorescing in the light of the dying dwarf at its heart.

Mark said, “At last, of course, the fusing helium in the core will be exhausted. Then the core will shrink once more, until the temperature of the regions around the core becomes high enough for helium fusion to start — in a shell outside the core, but within the hydrogen-burning shell. And the helium fusion will deposit carbon ash onto the core, growing in mass and heating it up — until the fusion of carbon begins…

“The cycle repeats, Uvarov. There will be carbon flashes — and, later, flashes of oxygen and silicon… At last, the giant might have a core of almost pure iron, with an onion-shell structure of fusing silicon, oxygen, carbon, helium and hydrogen around it. But iron is a dead end; it can only fuse by absorbing energy, not liberating it.”

“And all this will happen to the Sun?”

Mark hesitated. “Our standard models say that the reactions go all the way to iron only in stars a lot more massive than the Sun — say, twelve Solar masses or more.” He sighed, theatrically. “Will we get onion-shell fusion in the heart of the Sun? I don’t know, Uvarov. We may as well throw out our theoretical models, I guess. If the photino birds are as widespread as they seem to be, there may not be a single star in the Universe which has followed through a ‘standard’ lifecycle.”

“Superwind,” Uvarov breathed. “How soon is Sol’s helium flash?”

“Lieserl’s observations are sketchy on this. But, Uvarov, the conditions are right. The flash may even have happened by now. The superwind could already be working its way out…”

“How soon, damn you?”

“We have a few centuries. No more.”

Uvarov swept his blind face around the saloon. He pictured the ruined Jovian system beyond these walls, the bloated star dominating the sky outside.

“Then we can’t stay here,” he said.

By the time she’d climbed to the top of the giant kapok tree her hand-grips were slick with sweat, and her lungs were pumping rapidly. Spinner-of-Rope took off her spectacles and wiped the lenses on a corner of her loincloth. Zero-gee or not, it still took an effort to haul her bulk around this forest… an effort that seemed to be increasing with age, despite all the AS treatment in the world.

She was at the crown of the kapok. The great tree was a dense, tangled mass of branches beneath her. Seeds drifted everywhere, filling the rippling canopy with points of light — like roaming stars, she thought. Somewhere a group of howler monkeys shrieked out their presence. Their eerie ululations, rising and falling, reminded her of the klaxon which had once called the Undermen to their dreary work…

She put that thought out of her mind with determination. She pulled some dried meat from her belt and chewed on it, relishing the familiar, salty taste. She felt tired, damn it; she’d come here, alone, because she wanted — just for a few hours — to put all of the strangeness below the forest Deck, and beyond the skydome, out of her mind, to immerse herself once more in the simple world in which she’d grown up.

In the distance a bird flapped, shrieking, its colors gaudy against the bland afternoon blue of the skydome.

The bird was flying upside down.

“Spinner-of-Rope.”

The voice was close to her ear. Still chewing her meat, Spinner turned, slowly.

Louise Ye Armonk hovered a few feet away, standing on the squat, neat platform of a zero-gee scooter. Louise grinned. “Did I make you jump? I’m sorry for cheating with this scooter; I’m not sure I would have managed the climb.”

Spinner-of-Rope glared at her. “Louise. Never — never — sneak up on someone at the top of a tree.”

Louise didn’t look too concerned. “Why not? Because you might lose your grip, and drift off the branch a couple of feet? What a disaster.”

Spinner tried to maintain her anger, but she started to feel foolish. “Come on, Louise. I’m trying to make a point.”

Louise, skillfully, brought her scooter in closer to Spinner; without much grace she clambered off the scooter and onto the branch beside Spinner. “Actually,” she said gently, “so am I.” She breathed deeply of the moist forest air, and looked around the sky. “I saw you watching that bird.”

Spinner pushed her spectacles up the bridge of her nose. “So what?”