Starship Asterisk*

Wayne wrote:There are two schools of thought in galaxy formation (no, you can't escape it!).

The first one I'll discuss is "stars came first" that a cluster of stars used their gravity to draw in more material and so set off galaxy formation. The central black hole simply sank there and absorbed more mass as time went on. It has enough support to not be discountable, but doesn't have a majority of support.

The second is "black hole came first", that galaxies grew around the central black hole.

On to your questions.

Initial black holes need not have come from any stars at all: Primordial black holes (those formed before the surface of last scattering) may have accreted enough mass to become stable and and then become the seeds of galaxies. This is part of the "Black holes came first" idea. Perhaps some did, perhaps some didn't. Due to the destructive nature of black holes their age cannot be measured, only inferred, estimated or plain guessed.

The average size is not one solar mass. The Sun is more massive than 85% of all other stars. Most stats are dim K and M class red dwarfs (and theory predicts that even they are outnumbered by brown dwarfs ten to one).

The average spacing is not 4 lys, it's around 0.1 Msun/pc^3. This works out to a mean distance between stars of around 1.4 ly.

The Sun's neighbourhood is quite anomalous (don't read much into this, it was passing through a very dense region just 30 million years ago - The non-avian dinosaurs are over twice as old) in that it contains many massive stars and few normal stars. Related to this, the average spacing is much higher than normal, within 20 light years the average spacing is around 3 ly (counting binaries as one). The Sun itself is on the high side of spacings, but in just a million years, Gliese 710 will pass by at just over 1 ly.

Wayne wrote:That's by no means out of the question. The early universe was a pretty dense and very energetic place but we think that part of the initial formation of galaxies was around black holes (the "black hole came first" hypothesis).

While a supernova does cause giant molecular clouds to collapse in places (we believe that the formation of the Sun was triggered by a supernova or that at least the early solar system was very close to a supernova), they also sculpt out large voids. So such a "domino effect" would be present, but not really to the kind of spectacular level that every galaxy would have a supernova going on at the same time.

The domino effect that I refer to is not between galaxies but between neighboring stars in the same star field in the very compressed early universe. Perhaps scientists cannot possibly guess about this domino effect.

Once metals grew to enough of a level that the CNO cycle was able to provide substantial amounts of energy (population II) then the entire population III fireworks display plain stopped. This means that your 700 million years is a bit out: Population II was well underway as the globular clusters formed just 600 million years into the universe. The pop III stars (live fast, die young, no pop III star lasted more than half a million years and this is one of the problems we have in physics right now) were a feature of the 400 million year old universe, though they may have lingered in low mass irregular galaxies until much later.

Wayne wrote:No idea on the average spacing, but population III stars were quite odd to anyone who has a vague understanding of the various fusion processes.

A "normal" O or B class hydrogen fusing star uses the CNO cycle, a process involving catalysis by carbon, nitrogen and oxygen. However, these elements weren't present for the first population of stars, which instead had to use the proton-proton chain.

This meant that the cores of population III stars were much hotter than their population I analogues formed today (or in the last ten billion years or so) because the proton-proton chain is much slower than the CNO cycle, hence if the CNO cycle can produce the necessary energy, it will and prevent the core from heating enough to accelerate the proton-proton chain.

If the core was very hot, we know that the star would expand its outer layers. This means that early stars would not be blue at all, but would all be red.

We know extremely little about population III stars and it's perfectly possible they didn't form at all. The initial hydrogen collapse could grow out of control without stable fusion ever being reached and then explode as a pair-instability supernova without ever steadily burning hydrogen.