Snow, drifting down from the huge atmosphere, began to lace across her faceplate; it was white and stringy, and — when she tried to wipe it off with her glove — it left clinging remnants. It was a snow of complex organic polymers, drifting down from the hundred-mile-thick chemical soup above her head.
“Louise? Can you still hear me?”
“I hear you, Spinner.”
She took a few steps forward, away from the gleaming pod; soon, its lights were almost lost in the polymer sleet.
“You know, we terraformed Titan,” Louise told Spinner. “There were ships to extract food and air from the seas. You could walk about on the surface in nothing more than a heated suit. We got the atmosphere clear, Spinner-of-Rope. You could see Saturn, and the rings. And the Sun. You knew you weren’t alone down here — that you were part of the System…”
Now, the terraforming had collapsed. Titan had reverted. It was as if humans had never walked Titan’s surface.
“There used to be a city here, Spinner. Port Cassini. Huge, glittering caverns in the ice; igloos on the surface… A hundred thousand people, at least.
“Mark was born here. Did you know that?” She looked around, dimly. “And as far as I can remember this was the site of his parents’ home…”
She tried to imagine how it must have been to stand here as the final defense around Titan fell, and the Xeelee onslaught began. The starbreaker beams — cherry-red, geometrical abstractions — burned down, through the hydrocarbon smog, from the invisible nightfighters far above the surface. Methane seas flash-evaporated in moments — and the ancient water-ice of the mantle flowed liquid for the first time in billions of years…
“Louise? Are you ready to go home, now?”
“Home?” Louise raised her face to the hidden sky and allowed the primeval, polymeric snow to build up over her faceplate; for a moment, tears, ancient and salty, blinded her. “Yes. Let’s go home, Spinner-of-Rope.”
“Helium flash,” Mark said.
Uvarov had been dozing; his dreams, as usual, were filled with birds: ugly carrion-eaters, with immense black wings, diving into a yellow Sun. When Mark spoke the dreams imploded, leaving him blind and trapped in his chair once more. He felt a thin, cold sensation in his right arm: another input of concentrated foodstuffs, provided by his chair.
Yum, he thought. Breakfast.
“Mark,” he whispered.
“Are you all right?”
“All the better for your cheery questioning, you — construct.” He spoke with a huge effort, fighting off his all-encompassing tiredness. “If you’re so concerned about my health, plug yourself into my chair’s diagnostics and find out for yourself. Now. Tell me again what you said. And what in Lethe it means…”
“Helium flash,” Mark repeated.
Uvarov felt old and stupid; he tried to assemble his scattered thoughts.
“We’ve heard from Lieserl. Uvarov, the birds are continuing to accelerate the evolution of the Sun.” Mark hesitated; his intonation had gone flat, a sign to Uvarov of his distraction. “I’ve put together Lieserl’s observations with a little extrapolation of my own. I think we can tell what’s going to come next… Uvarov, I wish I could show you. In pictures — a Virtual simulation — it would be easy.”
“Well, you can’t,” Uvarov said sourly, twisting his face from side to side. “Sorry to be so inconvenient. You’re just going to have to hook up a few more processor banks to enhance your imagination and tell me, aren’t you?”
“…Uvarov, the Sun is dying.”
For millions of years, the photino birds had fed off the Sun’s hydrogen-fusing core. Each sip of energy, by each of Lieserl’s birds, had lowered the temperature of the core, minutely.
In time, after billions of interactions, the core temperature had dropped so far that hydrogen fusion was no longer possible. The core had become a ball of helium, dead, contracting. Meanwhile, a shell of fusing hydrogen burned its way out of the Sun, dropping a rain of helium ash onto the core.
“The inert core has steadily got more massive — contracting, and heating up. Eventually the helium in the collapsing core became degenerate — it stopped behaving as a gas, because — ”
“I know what degenerate matter is.”
“All right. But you have to be clear about why that’s important, for what comes next. Uvarov, if you heat up degenerate matter, it doesn’t expand, as a gas would… Degenerate matter is not a gas; it doesn’t obey anything like the gas laws.”
“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.