The Bob dangled from five hundred feet overhead, its displacement showing just how badly askew the wooden chasm of the Forum was. Its carved onion shape, taller than two men, had been repainted in gaudy green and vermilion stripes by this year's Bobbing Day committee. Though it was only about twenty degrees out of plumb—Yggdrasil had prematurely swung the bough ten degrees toward the true before being checked—that was enough to hang it above the chalked line at a point that was nearly two hundred feet from the painted bull's-eye in the center of the floor.

“I don't like sitting this close to it,” Mim said, taking a sip of her All-Level Eve cocktail. “I always think it's going to fall and roll right over us."

“It would roll in a circle,” Orris said. “That's why it's that shape."

“It won't roll at all because it's not going to fall,” Marg said firmly. “I won't allow anything to spoil Bobbing Day."

Everybody at the table laughed. Marg was still the commanding presence she had always been. She was large, formidable, and matronly at this stage of her youthening, and poor Orris seemed a collection of sticks beside her.

“You'd better have a word with the acrobats, then,” Bram teased her. “If the one on top isn't careful, he's liable to get himself brained."

They all looked across to where the acrobats were forming a human pyramid, no more than thirty feet from their table. There were six of them: five brawny lads in loincloths and a little lightweight fellow in rainbow skin-tights at the apex. They had managed to hoist the little fellow high enough to reach the Bob and set it spinning.

“Who are they?” Trist asked. “I think I recognize the one at the bottom right."

“They all work together in the glucose-extraction plant. Nice boys. They've been practicing for months."

Next to Trist, who had been holding her hand as if they were still in their early bonding years together, Nen said, “We all have to congratulate you, Marg. This is one of the best All-Level Eves ever. The decorations, the food, the entertainment—everything!"

Marg flushed with pleasure, and Orris beamed proudly. “Everybody on the committee worked very hard,” Marg said.

Bram looked ‘around the Forum at Marg's handiwork. The immense arena was lit by torches in wall brackets that cast a resinous red glow around the perimeter, where almost the entire population of the tree, with the exception of the few hundred who remained on duty tonight, were seated at tables, each defined by a circle of light cast by a sputtering resin stick. Garlands of silver leaves crisscrossed the walls, making a pattern of reflections.

There was no way to decorate so vast an area as the main floor of the ellipse, so Marg had very wisely left it in darkness, except for a central blaze of illumination where colored spotlights mounted high on the walls picked out the bull's-eye where the Bob would come to rest. A few reddish glints here and there, where leaf arrangements had been strategically placed, gave an abstract geometric shape to the pool of darkness. More spotlights were aimed at the Bob itself and followed the entertainers.

The final touch, lending a sense of awe and mystery to the annual rite of rotation, was the beam of sullen, red-shifted light from the starbow, filtering down from a lenticel somewhere high above. On past All-Level Eves, the starlight had been jolly, multicolored, but now the surrounding vault of higher frequencies had contracted to a point forward of the direct line of vision from here, leaving only the bloody light that preceded darkness.

“Yes, here's to the committee,” Bram said, raising his glass. “What else are you going to have in the way of entertainment?"

“Oh, we'll have pattern dancing—three very talented couples from hydroponics—and some people who sing, and some very clever body puppets.” Marg turned to scold Mim. “And I'm very disappointed in you, Mim. I thought you were going to play the cello for us tonight."

“Oh, nobody wants to hear concert music tonight,” Mim said. “This is an evening to have fun in. Besides, some of these people have been waiting a whole year for a chance to be on stage."

The acrobats had given the little fellow a boost that allowed him to do a backflip past the onion bulge of the Bob, and now he hung by one knee from the suspending cord and swung the Bob in greater and greater arcs while the audience clapped and cheered.

Across the table from Bram, Ang dug her fingers into Jao's beefy arm. “He's going to swing right over us!” she squealed.

“Somebody pass the poor fellow a drink, then.” Jao belched. He took a mighty gulp from his glass and set it down. He squinted critically at the surface of his. drink. “Still an ellipse,” he said. “You know, it's an awesome thought. At the very moment the Bob becomes plumb, five thousand ellipses in five thousand glasses are going to become five thousand circles."

“And we'll stop being all tipsy,” Ang said.

“Ah, that's where you're wrong,” Jao said with a sly wink at the company. “The geometry of alcohol is not the same as the geometry of space. The object of All-Level Eve is for the people to become progressively more tipsy while the environment becomes progressively less so."

“In that case, you're doing very well,” Mim said with a laugh.

Out on the floor of the Forum, the rainbow-clad acrobat had dropped lightly to the shoulders of his fellows, to the applause of the surrounding ring of revelers. Noisemakers razzed and rattled. The human pyramid disassembled itself, acknowledged the applause with outflung arms and a curtsy, and tripped offstage. A singing duo took their place—a man and woman in clown costume: she with enormous quilted breasts, he with a braided rag phallus that trailed on the floor—and began singing bawdy songs to the rowdy encouragement of the onlookers.

“Who's the Momus?” Trist asked.

“Don't you recognize him?” Marg replied. “It's Willum-frth-willum."

“He's lost considerable dignity."

“He doesn't need it anymore. He's got his youth to look forward to."

“I didn't know he had such a good singing voice,” Mim said.

“He's kept it hidden all these years."

A lot of table hopping was going on as the time grew near for the swing of the Bob. People made their way across the tilting floor to drop off little Bobbing Day gifts, drink a toast with friends, embrace and kiss.

“It seems as if there's always been a Bobbing Day,” Mim said, leaning against Bram's shoulder. “I can hardly remember how it got started. What are we going to do when we leave the galaxy and stop accelerating and there's no more annual tree turning?"

“We'll think of something,” Bram said. “Human beings will always celebrate some sort of a year-festival."

“It's going to be a long, featureless ride between the galaxies,” Jao said. “Nothing to mark the years."

“Sounds dull,” Orris said. “But like Bram says, we'll think of something. We ought to appoint a revels committee to look into it.” He looked fondly at Marg. “We're putting you in charge of it."

“Almost time,” Bram said, feeling his waistwatch with his fingertips. Some of the younger generation had taken to wearing timepieces on their wrists—clever little holo displays that showed the ten hours of the day visually, in human numerals—but for Bram, old habits died hard.

At this moment, in the heart of the tree, Jao's granddaughter would be checking and rechecking her meters, adjusting the voltages and gas pressures that would tickle Yggdrasil into one-twelfth of a revolution—or, rather, the portion of those thirty degrees it had not already anticipated. Once, early in the voyage, the humans had tried to accomplish the maneuver by brute force, using the plentiful hydrogen trapped by the drive section. But Yggdrasil had fought back and returned to starting position four times before the humans finally gave up. It was better to trick Yggdrasil into following its own imperatives.

“Too bad your granddaughter can't be with us tonight,” he said to Jao. “She missed the festivities last year, too; seems to me she ought to be able to alternate with other staffers in tree systems."

“Oh, Enyd? Don't waste your time feeling sorry for her. She could be here if she wanted to. No sense of fun, that girl, She's happier pushing her buttons. Sometimes I wonder if she's really our granddaughter.” He clapped a hairy hand on Ang's haunch. “What do you think, pet? Is she a case of mislabeled genes?"

“Oh, Jao!” Ang exclaimed. “She's just a little serious, that's all."

“Here comes Smeth,” Trist said. “Rounding up votes, no doubt."

Bram looked across the torch-lit perimeter. Smeth's gangling form could be discerned threading a route through the tables, lurching awkwardly across the tilted floor. A party of young constituents tried to detain him, but Smeth seemed distracted; he exchanged a few words, made a gesture declining an invitation to sit down, and kept coming.

“Something on his mind,” Nen said. “And it isn't votes."

“Now, Jao,” Ang said. “Remember you're not on duty tonight. You said yourself that your deputy can handle anything that comes up."

Jao patted her hand. “Wild forces couldn't drag me away."

Smeth stumbled the last few yards and loomed over the table.

“Sit down, Smeth,” Orris said. “Have a drink."

“Uh, thanks, but I just wanted to have a word with Jao,” Smeth said.

“I knew it!” Ang said.

“Nothing wrong with the drive?” Jao said. “Everything working all right?"

“The drive's fine ... uh ... at least it's coping with everything the core's throwing at us."

“Oh?” Jao's tufted eyebrows went up. He rose from his seat and steered Smeth by the elbow to a little distance away. Bram could see them talking earnestly, heads close together.

Jao came back to the table while Smeth waited. “Look, I'm just going down to the remote bridge for a few minutes—we've got the one in this bough hooked up now. Never fear—I'll be back in plenty of time for the turning. Marg, mix me up a libation."

“Do you want me to come with you?” Bram offered.

“No ... I'm just going to take some readings. Sit tight and enjoy the festivities. You too, Trist—no, don't get up."

He rejoined the fidgeting Smeth, and the two of them left.

Ang had begun a litany of complaint about Smeth. “...always dragging Jao off for some nonsense. Just because he lives for his work, he thinks everyone else does. I hope that when he gets young again, he'll find some woman who'll take him in hand.” Marg listened sympathetically.

Mim asked Bram unobtrusively, “Why does Smeth look so worried? I know he's a fusspot, tending his engines and guarding the sacred fusion flame like some kind of keeper of the mysteries, but he's got Jao worried, too."

Bram told her about Jun Davd's concern over the gas infall that had made the center of the galaxy a denser place than it ought to be. “Galactic cores are active places, but this one may be more active than most. More collisions between stars. More stars exploding or being ripped apart by tides and feeding the black hole. Smashed stars forming a soup that circles the hole at tremendous speeds, creating more turbulence, more friction, stronger magnetic fields."

Mim gave a shudder. “And we're heading toward that?"

“We're bending ourselves around it at a safe distance. Smeth may want Jao to alter our trajectory somewhat, based on what he can deduce from the junk falling into our scoop."

“Is that what he meant by coping with what the core's throwing at us?"

“Probably. We've run into the fringes of gas jets so far, and a couple of minor storms of relativistic electrons."

“Storms?"

“Caused by shock waves in the plasma. They accelerate the stripped electrons. Gives the ramscoop quite a diet."

“Oh, dear, I don't like the sound of that!"

“Don't worry. The more energy that's thrown at us, the stronger our fields are. The chief effect is the spurts of extra acceleration it's caused, before feedback can compensate. You've probably noticed times during these past weeks when you've felt heavier."

“I thought it was old age delaying its farewells."

He smiled. “No. There've been some episodes of minor accidents and breakage that no one paid attention to. Fortunately, outside travel isn't allowed without a tether, for vehicles or people. Otherwise..."

“Otherwise; what?"

“Somebody could've gotten left behind. Traveling at almost the speed of light in some heavy weather. Not that they'd know anything. The instant they left the shadow of our intake area—"

“Please, I don't want to know about it!"

“Sorry. But as I was saying, the chief effect is extra acceleration. And that may have put us a few days ahead of schedule on our black hole flyby.” He hesitated. “I've been thinking about sending everybody to the trunk when the time comes, to wait it out."

Mim looked alarmed. “Will that be necessary?"

“Oh, I doubt there's any real danger. If we ever ran into something we couldn't handle, the trunk wouldn't be any safer than an outer bough. But while I'm still year-captain, everybody's safety is my responsibility, and moving to the trunk would put us in toward the center of our umbrella, where the field is strongest, just while we're swinging around."

He caught Trist looking at him from across the table. Trist compressed his mouth as a signal for Bram to shut up.

“That wouldn't make you very popular,” Mim said. “Everybody's getting settled into their new quarters, unpacking and sweeping out rooms they haven't seen for twelve years, and tomorrow morning the floors will finally be level."

Bram left it there. Across the table, Trist said loudly, “Who wants another drink? I think we've got time for one more before Leveltide."

“Look,” Orris said. “Here come the clowns!"

Jao still hadn't returned when the Bob began to swing.

“Twenty ... nineteen and a half ... nineteen...” the crowd chanted in unison, counting the degrees as the bulbous painted shape followed the chalk line toward the bull's-eye in the center of the Forum. Globular membrances lit from within by a coating of biolights drifted down, released from somewhere high above. Hitherto invisible sparklers were touched off, making a star pattern on the floor. To one side, the clowns were still gamely performing their skit, though nobody was watching: Two of them wearing twelve-foot body puppets were vying for possession of a papier-mâché imitation of the Bob, while three more, making a Nar with too many legs, danced around them, trying to make peace.

“Where is he?” Ang fretted. “He's going to miss it."

“Never mind,” Trist said gallantly. “I've saved an extra kiss for you. Jao'll have to kiss Smeth. It'll serve him right."

“Five...” the crowd chanted. “Four..."

Bram could feel the faint trembling in the floor as Jao's granddaughter, a hundred and fifty miles overhead in the trunk, began to cancel inertia in order to bring the Bob precisely level. He had to admit that she was an artist at it. In some previous years, before she had become tree systems officer, the Bob had been as much as three or four degrees off. Everybody had had to make the best of it—the clowns would rush out with a big, round target-painted rug and wrestle it into place under the Bob while people egged them on, and Yggdrasil would gradually be corrected over the next few days. But Jao's granddaughter—he must remember that her name was Enyd—never missed.

“Here's to all you lovely people and another safe year,” Marg said, raising her glass.

The Bob settled into place, swinging in a small diminishing arc that finally came to rest. More sparklers went off, and noisemakers raised a din. People were shaking hands, kissing, embracing.

Bram felt the shudder.

Others must have felt it, too. Around the arena there was a sudden dip in the noise level, then, as people decided they had been mistaken, things warmed up again.

Trist was staring at the Bob, his eyebrows knit together. Bram followed his lead. The Bob had started swinging again, making a small ellipse that finally settled precisely over the center of the bull's-eye once again and hovered there, trembling, only a few feet above the floor.

“Your granddaughter's losing her touch,” Orris teased Ang. “She usually gets it on the first try."

Bram and Trist exchanged glances. Orris had missed the point, and so had most of the others at the table and in the festivities beyond. The babble of happy ringside voices continued undiminished.

It was not some small adjustment in the angle of radius that had set the Bob swaying again. If that had been the case, the Bob would not have returned to the same spot.

No, something had bumped Yggdrasil here in the interstellar night. Something violent enough to buffet a planetoid-size object stubborn with relativistic mass.

Bram rose to his feet. “I think I'd better—"

And then the thing struck again, knocking him off his feet.

Nobody could miss it this time. People went sprawling, tables overturned, and drinks went flying. Some reflex screaming was going on. The Bob swung in great pendulum arcs over the heads of the crowd. Some wall torches fell to the floor, and a few quick-witted people moved to stamp out the flames. The electric lights flickered, dimmed, then grew bright again.

And from above, where the red-shifted light had been filtering through the lenticule, there was a sudden hideous flare as great snakes of fire flashed by and dopplered through the spectrum.

Orris, white-faced, said, “What's happening?"

“Everybody better stay put,” Bram said. “There's a lot of broken glass around.” People were milling around, but the situation seemed to be coming under control again. “Orris, you look after things here. Trist, I'll need your help.” Trist nodded and rose.

And then, suddenly, Jao was at Bram's elbow, his forehead bleeding from a gash where he must have fallen against something.

“Better come to the bridge,” Jao said. “Jun Davd's trapped in the trunk, but I've got him on the fiber-optic link. And Smeth's in touch with his black gang in the engine section."

“What's wrong?” Bram asked.

“The galaxy is exploding."

Bram stared straight ahead into a representation of hell.

The viewscreen that showed the spectrum-corrected forward view was a smear of red-hot coals punctuated by glaring white intersections and eerie violet blobs that throbbed at the headachy limits of vision.

At the center of the screen, a multicolored vortex of fire swirled around a tiny central blaze of eye-hurting brightness. Time was speeded up enough so that Bram could see the crushed stars breaking up, lengthening, feeding their substance into the rushing swirl of light.

Another flattened whirlpool flamed at the edge of the screen, tilted just enough to reveal a similar blinding center. The second vortex seemed even bigger, more violent, than the first.

The whole screen pulsed. At regular intervals of a few seconds, brightness swelled, the field of coals seemed to ripple, and a dazzling shower of sparks danced in front of the view. Each time this happened, Bram felt the floor beneath him shudder, heard the vast creak and groan of the wooden worldlet around him.

“It's not a literal view, of course,” Jun Davd's calm voice came over the communications link. “It's the entire electromagnetic spectrum done in visible light. But I've done it in a logarithmic progression, so you can more or less trust your eyes between blue-green and yellow-orange. Then it really starts to stretch out. In the blues, you're seeing by x-rays. In the far violet, between four thousand and forty-five hundred angstroms, you're seeing by gamma radiation. And those dull reds are very long radio waves. I had to do it that way so you could make some sense of the view. The dust obscures everything. But infrared gets passed from particle to particle, and some of the energetic gamma punches through."

“Thank you, Jun Davd,” Bram said. “You must have stayed up all night to do that."

Jun Davd chuckled. “I don't imagine there was much sleep for anybody."

That was true enough. Bram rubbed at his grainy eyes with a fist. They were red, burning. All his joints were stiff.

The others in the long, sweeping loggia that served as this bough's bridge had suffered equally from lack of sleep. Smeth looked bedraggled, his salt-and-pepper hair sticking up in tufts. Jao and Trist moved as if they had weights attached to their feet, and Bram could see the weary, drawn faces of the people hunched over the monitors.

Mim gave him a wan smile. She was still in her party dress. She had stayed here through the long night, making herself useful. Marg was here, too. She had put Orris to work cleaning up the shambles of the festivities, then she had gotten busy organizing hot food and drink for those on duty.

The bridge itself was fully functional, with everything plugged in, though a lot of unopened cases were still shoved against the rear wall, and some of the equipment was dispersed helter-skelter wherever convenient.

A great gout of incandescence leaped out of the screen. Bram flinched. It reached toward him, a violet serpent with a beady red and orange gut showing through, and writhed offscreen. Bram turned his head to look out the observation wall and saw a cross section of fire flash by, flaring from yellow to red in seconds. Yggdrasil gave a lurch.

“What was that?” Bram said.

“Jet,” Jun Davd said. “I'd estimate it at about twenty-five light-years long and still growing. It's moving at about three-fourths of the speed of light, but of course it's emitting a lot of relativistic electrons that are traveling faster."

Smeth looked around, strain showing on his face. “We swallowed some of the fringes. That was the bump you felt."

“What caused it?” Bram asked.

Jun Davd's composure was undisturbed despite the backhand swipe the cosmos had just taken at him. “It's that black hole we're heading toward. If you'll keep your eye on it, you can see the process as it gets ready to toss the next one at us. The hole must be spinning very fast. It must have a very strange geometry—sliced off flat at the poles, but with the curvature of its circumference undisturbed. The gas and dismantled stars flowing into it would have a very strong magnetic field. You can't anchor a magnetic field in a black hole, but some of the field lines would penetrate the accretion disk and attach themselves very close to the event horizon. Then it's crack the whip—a million stars at a time."

“Why are we heading directly toward it?” Bram asked Jao. “I thought we were on a course that gave it a wide berth."

“We were,” Jao said, his face grim.

“We still are,” Jun Davd's voice said. “That object you see is not the black hole at the center of the galaxy."

“What is it, then?” Bram said, but he was afraid he already knew.

“It's another black hole—in orbit around the galaxy's central hypermass. The black hole at the center of the galaxy has a second black hole as its satellite."

Except for those who were not able to leave their monitor boards, everyone on the bridge had gathered around the viewscreen showing Jun Davd's display. Nobody was doing much talking.

Bram stared, fascinated, at the flat, double-ended funnel of fire that was sucking in the stars. You couldn't see the black hole itself, of course. You couldn't even see the accretion disk. But you could see those whirlpools of superheated gas by the inferno of radiation they gave off as they fell down that cosmic drain. And that intense, tiny blaze at the center was where the condensed matter crossed the static limit and doomed itself to leave the universe forever.

Jun Davd's model of binary black holes explained a lot of things. It explained the rolling yawn of the satellite hole: that was caused by the precession of its spin axis. And it explained those fingers of fire across the bed of coals: the satellite hole was sweeping out the rotating gas cloud of its primary. The geometry of space-time must be very complicated in there. Eventually the orbit of the satellite hole would decay, and it would fall into its primary.

And that would make quite a splash! If anyone in the nearby universe was trying to prove the existence of gravity waves, it would make his day.

“How did it happen?” Bram said.

“The black hole may have been snatched from the Bonfire when the two galaxies met,” Jun Davd said. “That might help to explain why the Bonfire lost its shape."

Jao spoke wonderingly. “That would have been quite a meal for our galaxy to digest. First it nibbles around the edges. Then it reaches in and pulls out a plum."

Jun Davd cackled appreciatively. “The stolen hole would have fallen to the center, sweeping up stars and gas,” he went on. “By the time it took up residence as part of a binary pair, it would have been quite massive. We can assume, from the present remnants of the Bonfire, that its central black hole could not have been much more than a hundred million solar masses. However, the satellite hole appears to be three or four times that mass. In fact, I'd put it at a fourth to a third the size of its primary, which I now estimate at well over a billion solar masses—much bigger than I expected. Or...”

“Or?” Bram prompted.

“Alternatively, the orbiting black hole might have been born right here in the galactic nucleus—maybe with the help of turbulence caused by the passing of the Bonfire."

“You don't sound very convinced."

“The dust is certainly thick enough and stellar collisions frequent enough to aggregate a second black hole of a few thousand solar masses. A single collision would be enough to start the process if the stars were massive enough to begin with."

“But we're dealing with a hole of three or four hundred million solar masses."

Jao burst in: “Why stop at one orbiting black hole? Why not a whole planetary system of them?"

“Exactly,” Jun Davd said. “Of course, a number of additional black holes may exist. In fact, I'd be surprised if they didn't. And two or more of them may have consolidated to form a larger hole. But nothing big enough to explain what we're facing here."

“What, then?"

“What we've got here is a binary system, not a planetary system. Both of them have a very rapid spin, they're very close—only a few diameters apart—and one is at least a fourth the size of the other. In fact, they fit almost perfectly the picture of contact binaries."

Bram struggled to remember his rusty astronomy. “Contact binaries. When the primordial cloud gains too much angular momentum as it condenses and relieves itself by forming a disk. And the disk condenses into a companion, not a planetary system, because it contains too much material and the lines of magnetic force aren't strong enough to cause spin-down and the spiraling outward of the disk.” He frowned as if Jun Davd could see him. “But you're describing the formation of stars."

Jun Davd's disembodied voice said, “What, essentially, are black holes?"

Outside the observation wall, the flickering tongue of ionized gas continued to rattle through the spectrum, a slice at a time. How long had Jun Davd said it was? Twenty-five light-years. It had grown another light-year while they were talking.

The viewscreen gave him a better impression of what the tongue was doing. It appeared as a mottled serpent swinging laterally away from them on its whiplash path, swallowing stars as it went.

Bram saw something that must have been a supernova—a wink of brilliant light, that was gone in an instant, on Yggdrasil's speeded-up time scale. Then another flash and another, each of them bathing space in a cosmic instant of inconceivable radiation, each blowing most of its substance off to add more fuel to whatever was happening here in the heart of the galaxy, leaving its core behind as a neutron star.

Chain reaction!

In the galaxy outside, perhaps forty or fifty days had gone by in the last few minutes. Supernovae weren't supposed to occur that close together. They were supposed to occur once every century or two.

Now he could see another cosmic jet forming at the hub of whirling gases that marked the central black hole. It whipped around, growing by the light-year, ready to slap Yggdrasil when it was long enough.

“Jun Davd,” Bram said, feeling sick, “how long can these jets grow?"

Jun Davd's voice was grave. “Millions of light-years in the most extreme cases we know. More often, a few tens of thousands of light-years."

“And in this case? From what you've been able to observe?"

“Long enough to reach the edge of the galaxy. That's what you wanted to know, isn't it?"

“And they're growing at seventy-five percent of the speed of light?"

“They'll slow down a bit as they proceed outward. Some of them will fall back."

Bram grasped at a slender hope. “Then there would be time to warn the Father World, wouldn't there? We could stop broadcasting the Message and use our radio beacon to beam a warning. We know they're listening to the center of the galaxy. They'd have at least twelve thousand years to get ready. Maybe ... maybe build a fleet of ships to migrate. Or ... or set up ramjet screens for whole planets."

“The primary threat isn't the jets themselves,” Jun Davd said gently. “It's the wave front of radiation coming from the events that have already begun here in the central region—and that travels at the speed of light.” He went on as if he were giving one of his lectures. “The jets themselves contribute to that wave front—relativistic aberration makes them radiate most intensely in the direction of their motion. But they're only a part of the story. The supernovae are adding their increment of radiation. But, chiefly, it's coming from that whirligig of black holes and their accretion disks. The soup of matter that engine is feeding on is getting progressively thicker—it's now a self-sustaining process—and friction is causing the smaller, four-hundred-million-sun hole to spiral inward at an increasing rate."

How can he be so calm, Bram wondered.

Jun Davd's voice remained steady. “The radiation is pulsing outward in a series of shells that at this point are only light-days apart. You've noticed the flickering of the viewscreen and the rhythmic surges of acceleration as our engine harvests H-II pushed by that broom of photons. I've been taking measurements over the last shipboard hour, and those pulses are coming closer together."

Bram snatched at another straw. “The inverse-square law! By the time the shells of radiation reach the outskirts of the galaxy—"

“They'll still be fatal,” Jun Davd said flatly. “In any case, no life anywhere in the galaxy will be able to survive the final event, when the two black holes merge."

A stunned silence pervaded the bridge. Everybody had been listening, of course—trying not to let it all sink in, taking refuge in Jun Davd's lucid exposition, clinging to some last atom of hope that out of his calm parade of revelations would come the one final fact that was a reprieve for the Father World and its two races.

Now people avoided looking at one another, as if in some obscure shame.

Out of the silence came a strange sound—one that became a shocking sound a moment later as people on the bridge realized that it was the sound of Jun Davd sobbing.

Bram shocked them further by putting the question that sooner or later would have to be asked.

“Will Yggdrasil survive?"

Jun Davd needed a moment to regain control of his voice, “We are now disposing of more energy than is available to the entire Nar civilization. The fields are still holding. We are protected for the time being. But I don't know if we can handle something on the scale of a climactic merger of the primary hole and its satellite."

There was a delayed moan of grief from someone on the bridge. Grief for the loss of the Nar and all their works. These were superb people, Bram's racing mind said. Personal fear would come later.

Jun Davd took refuge in more pedantry. “When two black holes become one, the resulting event horizon has a greater area than the sum of the areas of the event horizons of the original holes. It does not attain its final shape immediately. During the fraction of a second when the collapse takes place, there is a shifting and complicated topography. The geometry of space-time around it is ... irregular. The mathematics to describe it does not exist. And the ... distorted ... event horizon vibrates. The gravity waves generated by two vibrating masses equivalent to a billion and a half suns will be tremendous. What that will do to the surrounding plasma, you can well imagine. Next, angular momentum will increase abruptly at the same time that queer event horizon expands—and because the conjugate hole is spinning rapidly, the accretion disk will be embedded well below the static limit, transferring mechanical energy and magnetic force. A tremendous explosion of gamma radiation will travel outward at the speed of light, followed by a sphere of stripped matter traveling at relativistic speeds. We can hope to outrun the matter. But our ability to survive the shell of radiation may depend on our distance from the core when it overtakes us."

There was one last question for Bram to ask.

“How soon before the holes merge?"

“We'll be lucky to make it around the core."

“We're making a run for it, anyway,” Jao said.

“That we are,” Bram agreed. “All we can do is to pour on the gravities."

He lay propped on the floor pad, feeling the oppressive weight on his chest, his shoulders, his neck as he tried to hold his head up. All down the length of the observation loggia, dozens of people lay similarly sprawled on pads, working prone, their instruments on the floor beside them. Bram had ordered everybody to crawl, not walk, if they absolutely had to move about. The bulk of the population of the tree were in their quarters, lying down. It was still half a day till periastron, and they would remain there until then.

Jao said, “No, that's not all we can do. Bram, I've got to ask you about something."

“Ask away."

“There's a decision to make about the core maneuver."

“Why ask me? I'm only year-captain. Discuss it with Jun Davd."

“I've already discussed it with Jun Davd. But it's not strictly a technical decision. It involves the lives of everyone aboard. Jun Davd says you're the only one who can make the decision, and I agree."

With a sinking feeling, Bram said, “Go ahead."

“We've got to get in and out of the galactic core fast. There's no way we can simply back up, the laws of physics being what they are. We've got to use the mass at the center to bend our path in a hyperbola and swing around and out. Now our patron mass turns out to be two masses, very close together—right—but for the purposes of our hyperbolic orbit, we're treating it as a single mass."

“Yes,” Bram said, wondering what Jao was driving at.

“Even if there were some magical new law of physics that would let us dump all our inertia at once and come to a dead stop without rattling the glassware, it wouldn't do us a bit of good. Because we'd have to back out from a standing start and build up all our lovely gamma factor all over again, and that would take us fifty thousand years longer than whipping around the focus of the hyperbola. So we're committed."

“Yes, certainly. Everyone understands that."

Jao tried to wave a leaden hand, gave up the effort. “We also have to add a vector to angle the outward path somewhat above the plane of the galaxy if we want to aim at the Milky Way. That makes it even more tricky, but I won't go into that now."

“Yes, yes,” Bram said, wishing Jao would get to the point.

“So, as you said, we decided to pile on all the acceleration that Yggdrasil will bear in the hope of beating the merger of the two black holes and the final explosion by the widest margin possible. But the extra speed means we need an even tighter hyperbola—we've got to brush that doomsday engine ever closer, and that has its own dangers."

“We all decided to take the risk."

“Otherwise,” Jao said, ignoring the interruption, “we'd escape the galaxy, all right, and save our own skins. But we'd sail on out into intergalactic space and miss our target.” He looked around and lowered his voice. “Of course, there may be some who are frightened enough of the core maneuver to want to do that on purpose."

“If you're asking—"

“Bram, there's a way we can pick up some extra velocity without adding more g's and taking the risk of cracking Yggdrasil's branches—to say nothing of our own sacroiliacs."

“Go on."

Jao hitched himself closer on his elbows. He took another look around the bridge to make sure that he would not be overheard.

“We use the satellite hole as a gravity machine."

“What?"

“We're already going to pick up the rotational energy of the primary hole. It's got a rotation parameter of point nine nine eight, and it's doing weird things to the space around it, and the field lines from the accretion disk are going to reach out with magnetic fingers and fling us along by our own magnetic field. Now, what I'm saying is that we can refine our orbit and loop around the satellite hole, and pick up its orbital energy. It's whizzing around its primary at a ferocious rate at this point. We could pick up fifty thousand g's of acceleration without it costing us a thing. We wouldn't even feel it!"

“We'd have to fly between the two holes to make that loop."

“Uh huh."

“Jun Davd says they're only a few diameters apart at this stage. And they're spiraling closer. The final dive could happen at any moment."

“He thinks we have enough time to squeak through."

Bram's heart was pounding. “He said that at the present high rate of spin, the static limit is well above the edge of the accretion disk. The region between them must be very weird. We could get sucked into the hole with no warning. Into either one of the holes."

“Yah. That's why he says it's your decision."

Bram was going to ask for an estimate of their chances. But that would be begging the question—asking Jao and Jun Davd to make the decision.

“Do it,” he said.

Jao nodded and crawled off on his elbows and knees to confer with Smeth.

The ghost of a star drifted by, a ball of red so dull as to be at the limits of visibility. The universe outside the long observation wall was no longer blind; it was filled with a meaningless red fog that showed a suggestion of vast billows, specks, twisting sheets. The phantom star cleared a tunnel ahead of it, but that was an illusion; actually, the star was going the other way, and the tunnel, of course, was its wake.

“Can you see it?” Jun Davd's voice said over the loudspeaker.

“Yes,” Bram called to the directional pickup.

“Interesting,” Jun Davd said. “I wasn't sure it would be visible. That shows you how chaotic the galaxy's final act has become. Imagine the odds against encountering a fellow traveler at anything like a fraction of our relativistic speed! I wish I had a window here."

“There isn't that much to see, Jun Davd. Just red mist."

“Yes, a pale reflection of events behind us. Supernova explosions proceeding outward, perhaps. Possibly local turbulence in those expanding jets, sending shock waves in the opposite direction. Puffs of stripped matter, relativistic electrons, moving inward to add to the mischief. The death throes of a galaxy."

Mim's hand sought Bram's. Her floor mat was pulled up close to his. He had tried to make her nap, but she was unable to. The party dress was bedraggled. It had been a long time since she or anyone else had eaten; Marg and her helpers had had to give up when acceleration increased.

“Easy,” Bram said. “Remember, the Father World is still alive at this moment. Nothing will happen to them for almost fifty thousand years."

“Oh, Bram, it's so awful! Isn't there anything we can do?"

He shrugged. “You heard Jun Davd. The first shells of radiation are already on their way. The big one will be right behind us. We can use the Message beacon to send a warning as soon as we swing around the core, but our warning would reach them only a few years before the radiation front did. It would almost be cruel. Then what? Suppose they managed to fit out a few space arks with ramjet drives to save a few thousand Nar and humans. They have no time to build up their gamma to the point we have, and they can't outrun the expanding shell. They can't reach another galaxy, anyway, not in a million years of subjective time, because they can't use the H-II in the inner galaxy to accelerate the way we did."

“It's all so ironic,” she said bitterly. “We were supposed to ensure the survival of the Nar species. We came all this way. And now it's all gone for nothing. Instead of broadcasting their Message, all we can do is to give them their death sentence."

He squeezed her hand. There was absolutely nothing he could say.

The people around him, fanned out on mats in front of the forward viewscreen, didn't have much to say, either. Where two people lay together, they might murmur at each other at intervals, but mostly they watched the view painted by Jun Davd's computer.

Other screens around the bridge, and the screens in the living quarters where the bulk of the people lay waiting it out, had now been plugged into Jun Davd's program. Some of the people watching with Bram and Mim had access to their own screens, but there seemed to be a compulsion to watch in groups.

“Hold on, everybody,” Jao's voice said over the loudspeakers. “We just have to give it one more nudge."

The tree lurched. Tortured wood fibers groaned. Something somewhere gave with a snap. Interior lights flickered.

“How did Yggdrasil come through that?” Bram asked the tree systems center through the portable console on the floor next to his head.

The voice of Jao's granddaughter, Enyd, answered. “We're all right. We lost a few minor branches—all deadwood, anyway. Yggdrasil's reacting to the increased g-forces by acting to strengthen its compartments, but it won't last long enough to cause any serious imbalance."

“How are you people doing in there?” he asked.

Her voice softened, lost its formality. “We're all right, Captain,” she said.

He switched off. The view in the forward screen was astonishing. A maelstrom of star-stuff churned around and around, fast enough to see. The private whirlpool of the satellite hole could be seen at this flattened angle of approach as an eddy in the main vortex—an eddy that itself circled around the main swirl of gases, dragging streamers with it. The tilt of its precession gave a peek into its heart of brightness every time it came around. Rags of plasma glittering with embedded stars enclosed the double whirlpool like a disintegrating bird's nest made of fire.

The background stars were squashed, stretched around the immediate vicinity of the holes, their light bent. A circle of splashed stars followed the smaller hole around, snapping back when it had passed.

Nowhere could Bram see a path through the torrent of radiance. The concentric swirls of gas seemed to make one bean-shaped disk whose irregular contour, twisting with the secondary hole, smoothed out almost to an oval with distance from the dual center.

The sky around that cosmic gullet—if you could call it a sky—was crowded with stars of every possible color, jammed so closely together that in places the blackness of space seemed to be nothing more than pavement showing through. Gigantic irregular blobs of glowing gas wandered among the stars, grazing on them. Every once in a while, the flash of a supernova explosion, speeded up twenty thousand times, could be seen through an en-gorged blob. The blobs had an average drift toward the black holes; the ones closest to the double maelstrom bulged yearningly toward it.

A cloud rushed toward them. “Hold on again,” Jao's voice said. “We're going through."

The tree shook as the fields compensated. Weight fluctuated. Bram hoped nobody was moving about; otherwise, there might be broken bones that couldn't be attended to for some hours. Starfog enveloped them, dimming the fire ahead but not hiding it. Violet stars bobbed by, bloating themselves on the feast of gas; their violet color was the computer program's translation of the x-rays kindled by that rain of starmist.

Outside the clear elastic windows, the cloud manifested itself only as a dull red flicker—random flashes caused by encounters of gas molecules with relativistic electrons and orphaned protons that happened to be traveling in the same direction as Yggdrasil.

They broke through the cloud a minute later, into the awful radiance of the galaxy's inner heart. The whirlpool was just ahead, a roaring cataract of flame.

“Here we go!” Jao's voice said.

An enormous force seized the tree. Abruptly, weight was gone. Yggdrasil's acceleration was insignificant within that tremendous grip; it was like rowing upstream against a waterfall.

“Stay down!” Bram shouted.

His voice cut through the sudden babble, and people who had started to move clung to their mats. Bram thumbed the intercom switch and repeated his warning for those in their quarters.

Tidal forces changed their orientation as Yggdrasil, now in orbit around the outside hole, swung loose on its tether. The probe, uselessly spitting hadronic photons, must similarly have been trying to align its axis with its strange new parent body.

Bram fervently and irrationally hoped the probe's drive was pointed down, not up. Not that it would significantly matter in these few minutes and in view of the greater forces acting on the vehicle. In any case, he reminded himself, breaking free of orbit now would be every bit as deadly as diving into the black hole itself.

Equipment went sliding and crashing. People gasped as they felt the tidal pseudogravity change its direction. The hole didn't care which axis it struck through the wider diameter of the dumbbell-shaped body it had captured, but Yggdrasil, bless it, did, and the tree's struggle to “remember” its Bobbing Day vertical saved the humans from injury.

“Hold on to something if you can!” Bram shouted. “Try to stay in contact with the floor!"

Yggdrasil held together. Jun Davd had done his calculations well. The smaller black hole, with a solar-system-size circumference of over a billion and a half miles, was big enough, and their orbital distance from it great enough that tidal forces could not pull the tree apart. And the size of an individual human being was too insignificant to matter. Bram could feel the differential in the tug between his head and his feet, but it was no worse than the second-rate artificial gravity to be found in a spinning space vehicle that happened to have a diameter of only forty or fifty feet.

“Here comes the fun part,” Jao said over the loudspeaker.

Yggdrasil swooped around the focus of its orbit. It was now attaining the inside loop of its curve. It was no longer speeding around the black hole in a direction contrary to the hole's own path around the parent hole, but was picking up fifty thousand g's worth of acceleration from the hole's orbital motion.

All for free, as Jao had pointed out. It was fifty thousand gravities that would have crushed them to paste if they had not been in free fall around the body that was providing it.

The only scary part of it was the fact that the inside loop was taking the treeful of humans between the two black holes.

“Oh, Bram,” Mim whispered, squeezing his hand in a death grip. “Can such things be?"

The screen was blinding, even though it showed a bowdlerized version of the radiance outside. They passed between cascades of fire that sheeted to infinity. The eye followed those ravening walls along a path of bent light that made nonsense of perspective—on and on past the point where they should have recurved and eclipsed themselves.

At the center of each infinity, the fires poured into an iris: nothingness surrounded by violet inferno.

“I wasn't sure we'd be able to see the holes themselves,” Jun Davd's awed voice said. “I thought the accretion disks and the surrounding infall would block them from sight. But geometry doesn't mean anything here. We're within a few billion miles of each of them—less than a light-day away. If you're willing to accept an arbitrary definition of distance in the vicinity of gravitational fields like these, that is."

Bram's fear that there would be no path between the two accretion disks proved to be unfounded. What had appeared from a distance to be a merging of the twin vortices was a less substantial barrier up close. The accretion disks traded material, it was true, but the gaseous zone between them was tenuous enough to pass through. And Yggdrasil wasn't going to be here long enough to worry about orbital decay. As they plunged between the fountaining sheets of flame, a curtain seemed to open up continually before them.

There was a moment when a giant eye looked in at them through the observation wall. People cried aloud in wonder. In the yawning eternity that the hole's gravity had made of time, light was downshifted even more than Yggdrasil's tremendous speeds had done thus far. Theoretically, there should have been nothing visible.

And nothing was what they were seeing.

Then the hole flung them away. The tree whipped around the loop, its path bent back on itself, and headed out of the galaxy. The transfer of orbital energy had been tremendous, and the hole's precession had given the tree the additional vector that would angle its path above the ecliptic.

The weight of acceleration returned. People picked themselves up and started to move about. The g-forces were back to normal now. There was no need to force the pace. Whatever the danger from the front of radiation that would inevitably overtake them at some undetermined time in the years ahead, they had successfully navigated between those terrible whirlpools without being caught in the ultimate collapse.

Bram helped Mim to her feet. Together they went to the view wall. Other people were lined up along the safety rail, looking out.

There was something to see now, for the first time since the starbow had moved out of sight.

The universe was filled with light. Light of all colors, in wisps and ribbons and streamers.

An elongated blob of mottled blues and greens stretched, writhed, broke up into smaller blobs of violet and orange. A long pennant of pale blue flapped and disintegrated into a thousand filaments. Iridescent tendrils reached after the craft and fell behind. The striations of color played against a background of white radiance that cast long multiple shadows behind the people at the rail.

Yggdrasil was riding the shock waves out of the galactic center. In the nearby volume of space, collimated beams of accelerated matter were traveling in the same relativistic frame as Yggdrasil and shedding some of their radiation in the visible part of the spectrum.

The grateful tree dwellers drank in the spectacle, chattering happily after the tension of the past hours. Nobody was stopping to think about what it meant.

“It's beautiful,” Mim said.

Bram said nothing. It was the beauty of death.

“It's hard to believe two hundred thousand years have gone by since we left the galaxy,” Marg said. The tiny curved smile on her lips indicated an attempt at whimsy. “You don't look a day older, Mim. In fact, quite the reverse."

Mim, holding the new baby, beamed. “I'm going to name her Lydis,” she said. “It comes from the tonality that Beethoven chose for a quartet movement he intended as a song of thanksgiving."

“Very appropriate,” Orris said, “seeing that she was born on Safepassage Day."

“I was going to wait a few more years before getting pregnant,” Mim said. “But then I decided I was young enough."

“You're not going to get much younger,” Marg said. “None of us are."

Marg's own baby was two years old now. She and Orris had finally taken the plunge when Jun Davd had announced that they had officially left the exploding galaxy behind. There had been a rash of births that year.

“Oh, I don't know.” Jun Davd laughed, trying to extricate a gnarled finger from the baby's grip. “Some of us elders still have quite a few years to go before we lose our wrinkles."

Bram, hovering proudly over Mim and the baby and trying not to look too smug, said, “I don't like to think of it as two hundred thousand years. Let the outside universe take care of itself. I prefer to count it in Safepassage Days—three more of them. We're all still alive, and we're even ahead of schedule."

The first Safepassage Day had been celebrated the year after the black hole maneuver. Nobody had thought it up—it had just seemed to happen by itself. It came the day after Bobbing Day, and the two holidays had naturally merged into one extended festivity. A new set of rituals had quickly sprung up around Safepassage Day, with gift giving, overeating, and much hilarity. Bobbing Day was still observed, though the reason for it was now gone, with Yggdrasil under spin. It was mostly for the children, who celebrated with miniature trees, candy, and little gaily painted plumb bobs.

“Yes,” Trist said. “We picked up a tremendous boost not only from Jao's gravity machine, but from all that dense stuff the ramscoop swallowed on the way out.” He grinned at Jao. “Jao, my hairy friend, what's your refined estimate of our terminal velocity, and how long before we reach the galaxy of Original Man?"

Jao fidgeted on a seating puff next to the maternal nest. Mim had passed the baby to a cooing, clucking Ang, and he was afraid that he was going to be asked to hold it.

“Well,” he said, “there's still a lot of plus and minus in the observations, but Jun Davd's latest figures show us coasting at within a hundred millionth of one percent of the speed of light. That means we should cover thirty-seven million light-years in about five hundred and twenty-eight years, our time."

There was a harrumph from the fringe of the little group, where Smeth had parked himself at a safe distance from the baby.

“Not precisely,” he said. “The ramscoop is still on standby. We're bound to pick up a stray hydrogen cloud or two, even between galaxies. We still may shave a few years from that estimate."

Smeth had finally been elected year-captain. He took it very seriously, even though there wouldn't be much to do during the next five hundred years. He was driving everyone crazy, poking his nose into the cellulose plant, the glucose-processing operation, housing expansion, and the kitchens.

“How about it, Jao?” Bram asked.

Jao scowled. “I hope not,” he said. “We've got too much velocity to shed as it is. The Milky Way was supposed to be a pretty fair match for our galaxy in mass and configuration. Nobody figured on binary black holes and core explosions."

“How are you going to brake?"

Jao shrugged. “We'll spiral in, spiral out. We've got thirty-seven million years to figure out an approach. By that time the Milky Way's own hypermass may have grown some."

There was an uneasy shifting in the circle of people around Mim's nest. Jun Davd said quickly, “A larger black hole at the center of a galaxy should in itself present no dangers. In any case, with a normal accretion rate, no black hole could swallow its galaxy within the probable lifetime of the universe."

“Hey, hear that, little Lydis?” Trist said with theatrical heartiness. “Your new home is going to be around for what passes for forever!"

People made themselves laugh, but a small pall had invaded the maternity chamber. Bram knew that everyone was thinking about the Nar and their vanished civilization.

His eyes strayed to the window wall. There was nothing to see out there anymore except Yggdrasil itself—mile after mile of great twisting subbranches and carpets of leaves, lit up by the banks of spotlights that were trained on them from the shaft of the probe: not only to give Yggdrasil a sense of its own rotation, but to provide the human passengers with a sense of place in a universe that otherwise had gone blank.

Somewhere beyond Yggdrasil's horizons was an exploding galaxy, its light blotted out by red shift. It didn't bear thinking about. But it was impossible to shut it out of the mind.

Nen cleared her throat. “What heights must they have reached in the fifty thousand years they had left to them?” she asked. “They spread so fast through their arm of the galaxy in that little time. Expanding their domain. As if they knew they were racing against the end of everything."

On the way out of the galaxy, Trist had used the antenna array to monitor the radio emissions of the spreading Nar civilization until they stopped—replaced by the random radio noise of frying suns. Bram had watched Trist change, become progressively haunted during those last years, and he knew that Trist must have brought some of his despair home to Nen in their quarters at the end of each day.

“In another million years they might have started up Skybridge, toward the Bonfire,” Trist said softly.

“That wonderful web of life—it's all gone now!” Jao's granddaughter said with unexpected fervor. “The Nar, the humans and other organisms they brought into being, the forests of space poplars multiplying through the cometary halos from star to star!"

Bram wished the conversation hadn't taken this turn. All this was upsetting Mim on what should have been a happy day for her.

But it was Mim herself who wouldn't let it go. “Trist,” she said. “Do you think they ever received our warning?"

“We can't ever know,” he said. “It couldn't have made any difference anyway."

Mim had retrieved the baby from Ang. Bram thought that it made a pretty picture—Mim's dark hair spilled down around her shoulders, the baby nuzzling at her breast. Life, he thought. Life out of the ashes of universal death.

“They gave life to humankind twice, you know,” Mim said almost in a whisper. “And there's no way we can pay the debt."

A slow dawning overtook Bram. He smiled at Mim. “Yes,” he said. “Yes, there is."

Halfway across the night, Yggdrasil encountered a star. It was a fading red dwarf, so dim that they were almost upon it before the sensors reacted. In minutes of treeboard time, it was light-years behind them—too quick to call the passengers for a real-time look—but Jun Davd made a record for later replay by anyone who was interested.

“It must have broken free from its parent galaxy a very long time ago,” Bram said to Mim as they watched the replay in their quarters. “Any star much higher on the main sequence would have burnt out long ago."

“Oh, Bram,” Mim said, moved by the sight. “Could a star like that have planets? With living creatures on them?"

He touched her arm. “If so, they live in a very lonely universe. Under a sky with nothing in it except a fuzzy patch or two."

“Like us,” Mim whispered.

“No,” Bram said, “not like us. Someday we'll have stars in our sky again."