"Here we're looking through the Observatory roof and out of the Nebula. Normally the atmosphere shields this view from us, but the Telescope can unscramble the atmospheric scattering and show us what we'd see if the air were stripped away."

Nead peered closer at the picture. "What are those blobs? Are they stars?"

Rees shook his head. "They're other nebulae: some larger than ours, some smaller, some younger — the blue ones — and some older. As far as we can see with this Telescope — and that's hundreds of millons of miles — space is filled with them.

"All right; let's move inwards." With a single keystroke the picture changed to reveal a blue-purple sky; stars glittered, white as diamonds.

"That's beautiful," Nead breathed. "But it can't be in our Nebula—"

"But it is." Rees smiled sadly. "You're looking at the topmost layer, where the lightest gases — hydrogen and helium — separate out. That is where stars form. Turbulence causes clumps of higher density; the clumps implode and new stars burst to life." The stars, balls of fusion fire, formed dense bow waves in the thin atmosphere as they began their long, slow fall into the Nebula. Rees went on, "The stars shine for about a thousand shifts before burning out and dropping, as a cool ball of iron, into the Core… Most of them anyway; one or two of the kernels end up in stable orbits around the Core. That's where the star mines come from."

Nead frowned. "And if the path of a falling star intersects the orbit of the Raft—"

"Then we're in trouble, and we must use the trees to change the Raft's orbit. Fortunately star and Raft converge slowly enough for us to track the star on its way toward us…"

"If new stars are being formed, why do people say the Nebula is dying?"

"Because there are far fewer than before. When the Nebula was formed it was almost pure hydrogen. The stars have turned a lot of the hydrogen into helium, carbon and other heavy elements. That's how the complex substances which support life here were formed.

"Or rather, it's life for us. But it's a slow, choking death for the Nebula. From its point of view oxygen, carbon and the rest are waste products. Heavier than hydrogen, they settle slowly around the Core; the residual hydrogen gets less and less until — as today — it's reduced to a thin crust around the Nebula."

Nead stared at the sparse young stars. "What will happen in the end?"

Rees shrugged. "Well, we've observed other nebulae. The last stars will fail and die. Deprived of energy the airborne life of the Nebula — the whales, the sky wolves, the trees, and the lesser creatures they feed on — will cease to exist.»

"Are there truly such things as whales? I thought they were just stories—"

Rees shrugged. "We never see them out here, but we have plenty of evidence from travellers who've entered the depths of the Nebula."

"What, as far as the Belt mine, you mean?"

Rees suppressed a smile. "No, even further than that. The Nebula is a big place, lad; there is room to hide a lot of mystery, Perhaps there are even lost human colonies; perhaps the Boneys really exist, and all those legends are true… of the subhuman whale-singers lost in the sky."

The boy shuddered.

"Of course," Rees mused, "there are puzzles about the native life of the Nebula. For example, how can it exist at all? Our records show that life in the home universe took thousands of billions of shifts to evolve. The Nebula isn't anything like that old — and will be far younger when it dies. So how did life arise?"

"You were telling me what will happen after the stars go out…"

"Yes. The atmosphere, darkened, will steadily lose heat, and — less able to resist the gravity of the Core — will collapse. Finally the Nebula will be reduced to a layer a few inches thick around the Core, slowly falling inwards…"

The young man, his face pale, nodded slowly.

"All right," Rees said briskly. "Let's look inwards now — past the Raft's level, which is a thousand miles from the edge of the Nebula — and in to the center."

Now the monitor filled up with a familiar ruddy sky. Stars were scattered sparsely through the air. Rees punched a key—

— and stars exploded out of the picture. The focus plummeted into the Nebula and it was as if they were falling.

Finally the star cloud began to thin and a darker knot of matter emerged at its center.

"What you're seeing here is a layer of detritus in close orbit around the Core," Rees said quietly. "At the heart of this Nebula is a black hole. If you're not sure what that is right now, don't worry… The black hole is about a hundredth of an inch wide; the large object we call the Core is a dense mass of material surrounding the hole. We can't see through this cloud of rubble to the Core itself, but we believe it's an ellipsoid about fifty miles across. And somewhere inside the Core will be the black hole itself and an accretion disc around it, a region perhaps a hundred feet wide in which matter is crushed out of existence as it is dragged into the hole…

"At the surface of the Core the hole's gravity is down to a mere several hundred gee. At the outer edge of the Nebula — where we are — it's down to about one per cent of a gee; but even though it's so small here the hole's gravity is what binds this Nebula together.

"And if we could travel into the Core itself we would find gravity climbing to thousands, millons of gee. Hollerbach has some theories about what happens near and within the Core, a realm of what he calls 'gravitic chemistry'—"

Nead frowned. "I don't understand."

"I bet you don't," Rees laughed. "But I'll tell you anyway, so you'll know the questions to ask…

"You see, in the day-to-day turmoil of things we — even we Scientists — tend to forget the central, astonishing fact of this cosmos — that the gravitational constant is a billon times larger than in the universe from which man arose. Oh, we see the macroscopic effects — for instance, a human body exerts a respectable gravitational field! — but what about the small, the subtle, the microscopic effects?"

In man's original universe, Rees went on, gravity was the only significant force over the interstellar scale. But over short ranges — on the scale of an individual atom — gravity was so tiny as to be negligible. "It is utterly dominated by even the electromagnetic force," Rees said. "And that is why our bodies are shambling cages of electromagnetism; and attractive electrical forces between molecules drive the chemistry that sustains our being.

"But here…" He rubbed his nose thoughtfully. "Here, things are different. Here, in certain circumstances, gravity can be as significant on the atomic scale as other forces — even dominant.

"Hollerbach talks of a new kind of 'atom.' Its fundamental particles would be massive — perhaps they would be tiny black holes — and the atom would be bonded by gravity in novel, complex structures. A new type of chemistry — a gravitic chemistry — would be possible; a new realm of nature about whose form even Hollerbach can scarcely begin to speculate."

Nead frowned. "But why haven't we observed this 'gravitic chemistry'?"

Rees nodded approvingly. "Good question. Hollerbach calculates that the right conditions must prevail: the right temperature and pressure, powerful gravitational gradients—"

"In the Core," Nead breathed. "I see. So perhaps—"

There was a soft boom.

The Bridge shifted slightly, as if a wave were passing through its structure. The image in the monitor broke up.

Rees twisted. A sharp smell of burning, of smoke, touched his nostrils. The Scientists were milling in confusion, but the instruments seemed to be intact. Somewhere someone screamed.

Fear creased Nead's brow. "Is that normal?"

"That came from the Library," Rees murmured. "And, no, it's not bloody normal." He took a deep, calming breath; and when he spoke again his voice was steady. "It's all right, Nead. I want you to get out of here as quickly as you can. Wait until…" His voice tailed away.