Starplex

Rhombus's lights twinkled in confusion. "Boundless apologies, but that can't be right, good Jag. Those aren't spiral galaxies. They look more like disks." "I'm not mistaken," said Jag. "That is the Milky Way.

Since we are now six billion light-years from it, we are seeing it as it looked six billion years ago."

"Are you sure?" said Keith.

"I am positive. Once the pulsars had told me approximately where to look, it was easy enough to identify which galaxy was the Milky Way, which was Andromeda, and so on. The Magellanic Clouds are too young for any light from them to have reached this far out, but globular clusters contain almost exclusively ancient first-generation stars, and I've identified several specific globulars associated with both the Milky Way and Andromeda. I am sure of it — that simple disk of star is our home galaxy."

"But the Milky Way has spiral arms," said Lianne.

Jag turned to her. "Yes, without question, the Milky Way today has spiral arms. And, just as surely, I can now say that when it was six billion years younger, it did not have spiral

"How can that be?" asked Thor.

"That," said Jag, "is a vexing question. I confess that I would have expected a Milky Way even half its present age to still have arms."

"Okay," said Keith. "So the Milky Way gains spiral arms sometime in the interim."

"No, it is not okay," said Jag, his bark returning to its usual sharpness. "In fact, it has never made any sense. We've never had a good model for galactic spiral-arm formation. Most models are based on differential rotation — the fact that stars near the galactic center make several orbits around the core in the time it takes for those farther out to complete just one. But any arms that resulted because of that should be temporary phenomena, enduring at most for a billion years. Oh, we should see some spiral galaxies, but there is no way that three out of every four large galaxies should be spirals — which is the ratio we actually observe. Ellipticals should far outnumber spimls, but they do not."

"Obviously, then, there's a flaw with the theory," said Keith.

Jag lifted his upper shoulders. "Indeed. We astrophysics types have been limping by for centuries with something called 'the density-wave model' for explaining the abundance of spiral galaxies. It proposes a spiral-shaped disturbance that moves through the medium of a galactic disk, with stars getting caught up in it — or even being formed by it — as the wave rotates. But it has never been a satisfactory theory.

First, it fails to account for all the different types of spiral forms, and, second, we don't have a good answer as to what would cause these imagined density waves in the first place. Supernova explosions are sometimes cited, but it's just as easy to model such explosions canceling each other out as it is to get them to build up long-duration waves." He paused. "We've had other problems with our galaxy-formation models, too. Back in 1995, human astronomers discovered that distant galaxies, observed when they were only twenty percent of the current age of the universe, had rotational rates comparable to what the Milky Way has today — that's twice as fast. as they should have been rotating at that age, according to theory."

Keith thought for a second. "But if what we're seeing right now is correct, then spiral galaxies like ours must somehow form from simple disks, right?"

Another lift of the Waldahud's upper shoulders. "Perhaps.

Your Edwin Hubble proposed that galaxies each start as a simple sphere of stars, gradually spin out into a flat disk, then develop arms that open up more and more over time.

But although we now have observational proof that that sort of evolution does indeed happen" — he gestured at the disk of stars in the glowing frame — "we still don't have an explanation for why the evolution takes place, or why the spiral structures persist."

"But you say three quarters of all large galaxies are spirals?" asked Lianne.

"Wellll," said Jag, PHANTOM translating a hissing bark as a protracted word, "actually, we don't know much directly about the ratio of elliptical to nonelliptical galaxies in the universe at large. It's hard to make out structure in dim objects that are billions of light-years away. Locally, we see that there are many more spirals than there are ellipti-cals, and that spirals contain a preponderance of young blue stars, whereas our local ellipticals contain mostly old red stars. We've assumed, therefore, that any vastly distant galaxy that showed lots of blue light — after correcting for redshift, of course — was a spiral, and any that showed mostly red was an elliptical, but we really don't know that for sure."

"It's incredible," said Lianne, looking at the image.

"So — so if that's how it looked six billion years ago, then none of the Commonwealth homeworlds yet exists, right? Is there — do you suppose there's any life in the galaxy now?"