We know that stars evolve: the dust and gases accrete under gravity, stimulated by the winds of expanding shells of previous supernovae; under compression their interiors “ignite” with thermonuclear fires; they evolve along the “main sequence” of the Russell diagram for most of their careers; and when their hydrogen fuel in the centre runs out, they implode and explode as supernovae of different types, leaving behind either a white dwarf or a neutron star (pulsar) or a black hole, depending on their size.

We know that planetary systems evolve. Actually we only know one example, namely our own solar system. It has experienced changes, such as when a planet broke up creating the asteroid belt, or when Pluto escaped from being a satellite of Neptune and became a planet on its own.

We know that planets evolve. Possibly Venus was not al-ways so hot and Mars was not always so dry. But again, we know most about our own planet Earth, and its tectonic plate movements, which probably go in a cycle of periodically assembling the continents into one and disassembling them again.

We know about the evolution of the biosphere on Earth, and the evolution of the human noosphere and sociosphere. We know something about historical developments and the evolution of technology and culture.

We have been moving, in this discussion, toward the micro-end of the “cosmic zoom” scale. Let us now reverse direction, and move toward the macro-end from the stars up. What do we know about the evolution of galaxies?

An article on this in Scientific American, January 1993, stresses the many remaining ambiguities and contradictions in our knowledge of galactic evolution. Probably my conclusions here are premature and will be modified in the light of future research; but I would like to try to describe the pattern as I see it now.

A spiral galaxy like our own Milky Way consists of a disk with a bulge around it, a rim around the disk which contains the spiral arms (our Sun is located in one of these), and a halo around the rim. It seems that stars were generated first at the centre of the disk; these were stars somewhat larger than the Sun, which at the end of their career became Type Ia supernovae, which produce mainly iron in their last stage of nucleosynthesis.

The second stage of star formation occurred in the bulge and halo, skipping the rim. (Perhaps there was too much “wind” in the rim from the supernovae in the disk.) These were much bigger stars, that eventually became super-novae Type Ib, Ic, or II, producing largely elements between helium and iron (peaking at carbon and oxygen) in their last stages. (What produces elements higher than iron? The article did not say.) Later, stars formed from the halo super-novae dust in the rim and arms; this is why our solar system is rich in carbon and oxygen.

Meanwhile, in the centre of the disk, many of the old black holes and neutron stars coalesced into a giant central black hole. (That is not definitely proven yet.)

An article in Science, December 11, 1992, tells us that in distant (i.e. early) galaxies, spirals are much more common (30%) than elliptical galaxies, while in closer (more recent) galaxies, spirals form only 5% of the total. Spiral galaxies are the active star-formers; with time, they use up all their gas and become elliptical (burned out). Some spirals (maybe including our own) may have rejuvenated themselves by colliding with other galaxies and swallowing them up or merging with them.

However, new young spirals full of young blue stars are still forming, which poses a mystery.