Level 5: Systems — \(400\text{M}\) years
Molecules
Size: \(0.6\text{nm}\)
Initial state: Heavy atoms
Production process: Cosmic chemistry
Meta-stable equilibrium: Heavy molecules
Stable future: Noble gases and most stable molecules.
Galaxies
Size: \(50 \text{kPc}\)
Initial state: Large-scale structures
Production process: Galactogenesis
Meta-stable equilibrium: Galaxies
Stable future: Sterile gas haloes
Level 5 is set under the sign of systems. That surely seems very generic and deserves some explanation. As we’ve moved up on this ladder of levels, more and more complexity has developed. Granted, an Iron atom is certainly a complex systems of quarks and electrons. But here we focus on the macroscopic \(\Omega\) level: the mesosphere, where the production of the higher levels is brewing. Up to Level 3, the theater of operations was still universal, with the cutoff still spatiotemporal in the broadest sense: the matter-dominated 4D universe is formally included in the most general post-Big-Bang higher-dimensional universe, yes. But for denizens of the epochs described in Level 3, the higher dimensions and past epochs were inaccessible. The mesosphere was then not a specific identifiable object that entities may be in or out of, but the whole world at that point. Level 4 is somewhat intermediary: it was significantly labelled as the level of structure, and not of structures. As much as we can try to divide the cosmic foam into clusters, walls and filament, we’re still talking about one ur-object, not easily dividable into a coalition of smaller ones.
At Level 5 there is marked change, since the new focus will be on clearly identifiable ordinary objects: galaxies. What makes each of them singular is not just their composition or physical characteristics, in which case two galaxies near enough to each other could be deemed to form the same "stuff"; instead it is the distinction between inside and outside, and the feedback processes governing their relationship that make a galaxy a individual complex system. There is a clear distinction, beyond sheer scale, between a galaxy (assemblage of stars and gases) and a cluster (assemblage of galaxies). The latter is a mere set of objects that are mechanically and thermodynamically connected to each other, without the self-perpetuation feedback loops obeserved in galaxies.
Systems, which are familiar to us from our experience of many other levels, as we’ve seen, can then be deemed an emerging quality. Systems will make the production of more and more complex entities in successive mesospheres possible. Galaxies, solar systems, biosopheres, etc. will form more and more hospitable havens where the march of entropy is temporarily halted, even reversed. Galaxies are a perfect example. Although the universe as a whole is very cold (less than \(3\text{K}\)), galaxies are not hot spots in a uniformly cold environment. They are a cold spot in the high-energy furnace that is cosmic gas after re-ionization. Voids within the cosmic foam are cold, too cold and empty to see anything interesting emerge. But the reionized foam itself is scorching, its particles to energetic to allow even molecules to form. Galaxies provide us with the best of both worlds: dense enough but cold enough that stars can form in a consistent way over many generations, without hindering their own birthrates, like the first populations did.
The galactic epoch : from \(400\text{M}\) years
Our definition of epochs is also changed compared to how we’ve proceeded so far. Cosmic dawn, or reionization, were definite spans of time with a beginning and an end, both in our past. The galactic epoch has not ended. And even though we will attempt to predict their demise, there is no set date we can assign.
When we last discussed the state of the universe, it was being fully ionized by the radiative activity of its stars, quasars and black holes, while frequent super- or hypernovae kept exciting even further this already hot brew. Star formation found itself completely hindered, and the story of complexity in our universe could well have ended right there, with a hot plasma soup, slowly cooling down for eternity.
Luckily, this didn’t come to pass. How, then, did we get out of this ionized trap? At temperatures between \(10^6\) and \(10^7\text{K}\), cooling is highly inefficient, and Jeans volumes the size of our current clusters collapse at free-fall time scales, over hundred of billions of years. In fact, this is the collapse process that clusters are still going through right now! It will never come to fruition though, since we’ve seen that universal expansion will bring about structural ruin much earlier...
How did smaller structures emerge, then. The key reason is chemical enrichment. Before the first stars, the universe was only hydrogen and helium. Towards the end of reionization, metallicity rates are still low (below \(10^{-4})\)), but every little helps, as far as cooling is concerned. Not only is cooling efficiency positively correlated with metallicity, heavier elements tend to coalesce into dust particles, and "dusty clouds" cool even more efficiently than clouds that are chemically similar on average but made of a homogeneous gas.
Of course the metallicity and dust distribution is far from homegeneous: novae, winds, and general thermal agitation have spread heavier elements randomly. We find ourselves with a similar gaussian distribution as previously when we considered dark matter haloes, except that this time, we’re interested in both mass density and chemical composition.
For a given density, metallicity and temperature, there is a Jeans volume within which a cooling-supported collapse could happen. Such volumes start cooling rapidly, becoming proto-galaxies.
At the time these objects formed, star formation rates were severely depressed, but there were still lots of Population II stars around. Early galaxies were then made of hot, but quickly cooling, ionized gas, as well as population II stars and other objects (most current galaxies seem to be endowed with an active galactic nucleus (AGN), in practice a giant quasar or a supermassive black hole).
The formation process of galaxies is complex, as it involves thermic, mechanical (angular momentum in particular) and chemical effects vastly beyond the reach of this quick overview, and is still an active topic for research. These effects’ interplay leads to the familiar shape of galaxies, spiral (like our Milky Way), elliptical or lenticular.
As the gas inside a galaxy cools, the collapse of small, dense clumps accelerates, and star formation rates pick up again! Galaxies act as cosmic nurseries!
A galaxy is not like a biological cell and doesn’t have a clear membrane separating it from the outside. The change of medium is observable in terms of phases. We can recapitulate the different mediums in the cosmic foam after the first generation of galaxies has formed:
The Intergalactic Medium (IGM) is the same reionized gas that was pervasive in the cosmic foam before galaxies first appeared. It still represents 90% of the universe’s ordinary matter. It is then a hot ionized gas (\(10^5\) to \(10^7\text{K}\)) of mostly hydrogen and helium, with small amounts of dust. Its density is low at less than 1 atom per \(\text{m}^3\).
A subset of the IGM, the Intracluster Medium (ICM) is even hotter, at \(10^8\text{K}\). It is a byproduct of galaxy formation since the ejection of gas following supernovae, and of radiation from stars and other bodies, can heat up the IGM, and make it denser (about \(1000\) times more). The more galaxies, the more gas and radiation coming out, so the ICM, as the name suggests, is primarily present within galaxy clusters.
The Interstellar Medium (ISM) is the gas that is present between stars, within a galaxy. It is far from uniform and we can arbitrarily distinguish three phases:
The coronal gas, or Hot Ionized medium is \(10^5\) to \(10^7\text{K}\), with a density between \(1000\) and \(10^4\) atoms per cubic meter. Its characteristic scale height is about \(3000\) parsecs (in the Milky Way). We can see that despite its presence within the locus of the galaxy, not much separates it from the ICM. Its mass is about \(50\%\) of the total ISM mass.
The Warm Ionized medium has a temperature around \(8000\text{K}\), with a density between \(0.2\) and \(0.5\) atoms per cubic centimeter. Its characteristic scale height is about \(1000\) parsecs and its mass is about \(30\%\) of the total ISM mass. It is a much denser, much cooler medium than the HIM. Unlike the coronal gas, it is present in the galactic disk (for disk galaxies, the most frequent type), within which it then consistutes the most massive component of the ISM.
The Warm Neutral medium has a temperature around \(6000\text{K}\) and a similar density to the WIM. Its characteristic scale height is about \(400\) parsecs and its mass is about \(15\%\) of the total ISM mass. As the name suggests, this phase has cooled enough that the gas is not ionized anymore: it is a neutral, atomic gas.
The Cold Neutral medium has a temperature around \(100\text{K}\), with a density between \(20\) and \(50\) atoms per cubic centimeter. Its characteristic scale height is about \(200\) parsecs and its mass less than \(5\%\) of the total ISM mass. It is a hundred time denser and colder than the WMN, but shares the same chemical composition.
Lastly, the Molecular clouds (MCs) show temperatures that don’t exceed \(20\text{K}\), with a density up to one million atoms per cubic centimeter. Their characteristic scale height is about \(80\) parsecs and their aggregate mass less than \(1\%\) of the total ISM mass. These clouds are cold enough to contain molecular.We’ve seen examples of that pre-reionization, when dihydrogen molecules helped bring up cooling efficiency dramatically. This time, as we’ve seen, the chemical enrichment undergone in past eras allows the emergence of much more diverse molecules: the very first chemical compounds.
90% of ordinary matter mass is in the IGM, while 10% is today in galaxies. At first, these 10% were just capsules of gas inside proto-galaxies, that progressively evolved into the phases we’ve described. But star formation is still strong enough, even today, that 90% of galaxies’ mass is contained in astral bodies (mostly stars, but also, quasars, black holes...) and the remaining 10% in the ISM.
What characterizes the workings of galaxies, and partly explains their longevity as stellar nurseries, is a feedback loop that is only partially understood but involves these elements:
thermic feedback from stars and black holes: radiation from these astral bodies is bound to heat the surrounding medium. Star formation takes cold gas and turn it into very hot and luminescent objects, which are bound to re-ionize their birthplace. That is why molecular clouds are now thought to be only transient. In a given galaxy, they disappear and pop back up somewhere else at a rapid clip.
That picture is also complicated by magnetic fields and other effects which can direct the bulk of the radiative energy out into the IGM, thus playing a regulating role.chemical feedback from star formation takes the form of metallic enrichment. That’s an evolution that is not reversible. The more generations of stars, the higher the metallicity of the universe, no matter what happens after that. Not only is the ISM naturally enriched as the locus of star formation, supernovae winds ensure that the IGM’s chemical composition tracks that of galaxies, creating a chemically homoegenous environment. We’ve already talked of the enhancing effect of metallicity on cooling efficiency. This goes a long way into prolonging the life of galaxies.
The previous generations of stars were short-lived and, starting out with very low metallicity, they could only have a modest impact on chemical enrichment. This was not enough to prevent their super-luminescent radiation from re-ionizing the stellar nuseries. Now, stellar lifespan and metallicity are high enough that a virtuous cycle can set off where the heat from each new generation of stars is absorbed in a cooling process by the newly formed dust from their parents’ supernovae.mechanical feedback is also crucial: the interior of a galaxy is a turbulent domain. In other words, a given volume undergoes different phases of compressions and relaxation, which is exactly the kind of thermodynamic process that enhances stellogenesis.
Supernovae and AGN activity generate supersonic winds that can rage within the ISM or eject large amounts of matter into the IGM via galactic fountains (matter from the IGM also continually gravitates towards the galaxy, so the cycle goes on and on).
The intrinsic geometrical and mechanical properties of a galaxy also play a role. For instance, elliptical galaxies, which tend to be older, seem to have formed most of their stars early on, and those are also bigger (and then will be shorter-lived: their stars tend to be close contemporaries to each other, and by now are all older and redder). Spiral galaxies like our Milky Way however, appear to have formed fewer star in their early years. However their star formation is still going strong.
It may be that none of this is a concidence: the mechanical and magnetic properties that are believed to enable the gestation of proto-galaxies into spirals might well be only possible in second-generation galaxies; for instance, ones that are borne out of a merger between two earlier ellipticals.
A spiral galaxy’s arms are not rotating group of stars. Stars can move through them unaffected. They are waves of compression that move around the galaxy, "temporarily" changing the pressure in the local gas volume (the spiral pattern from the Milky Way takes more than 200 million years to complete a lap). These compression/relaxation patterns may keep star formation alive, but could also play a role in keeping it subdued enough that the galaxy’s fuel won’t burn as fast as in an elliptical. Blue, bright, short-lived stars are born and die within the time that the spiral arm is passing by. Since these stars are the most visible, that is why these density waves appear as shiny arms around the galaxy center. Smaller, longer-lived stars like our Sun are alternately in and out of spiral arms (they also rotate around the galactic nucleus, but with a different angular speed).
Ultimately, this equilibrium can cease, and a galaxy can "go red", suffering the fate of the universe after re-ionization. The ISM can get heated to a point where no new stars are forming. When all existing stars have reached the end of their life, the galaxy progressively dissolves and goes back to the IGM. Others suffer a more violent death, colliding or merging with one of their brethren, or with a supermassive black hole.
Be that as it may, galaxies are extremely successful systems at creating a stable environment for star formation. It is estimated there are \(10^{11}\) galaxies in the observable universe, each containing betwen \(10^7\) and \(10^{11}\) stars.
The stellar epoch : from \(1\text{bn}\) years
Within galaxies, star formation then hits its strides in earnest around one billion years after the Big Bang, reaches a peak around two, after which point it started a slow decrease. The first generations of stars, with near-zero metallicity, were very massive and very short-lived, as we’ve seen. Higher metallicity over time makes it possible for smaller clouds to cool into a protostar, and some of them, much smaller than their ancestors, will make it to star, status, although not all.
Below 1.25% of our Sun’s mass (\(0.0125 M_\odot\)), the cloud gets warmer and is called a sub-brown dwarf (not dissimilar from a gas planet, except that it’s not orbiting another star; that cutoff mass is 13 times the mass of Jupiter).
Betwen \(0.0125\) and \(0.08M_\odot\), hydrogen can be fused into deuterium at some point during the object’s lifetime. It is then called a brown dwarf.
Above \(0.08M_\odot\), the protostar’s core can become hot enough (\(>10^7\text{K}\)) for the nuclear fusion of hydrogen into helium to start. A static equilibrium is soon reached and the object is officially a star!
At this point, the star is said to enter its main sequence, which represents roughly 90% of its life. The temperature and luminosity of the star stay constant during that phase, although the parameters depend on its mass: heavy stars are shorter-lived, hotter and more luminescent (they "burn brighter" indeed). Our Sun will have been in its main sequence for about \(10^{10}\) years, so it is about half-way through it’s life. The largest known stars, about 100 times heavier, live only for a few million years. The smallest ones, right above the brown dwarf cutoff, have a theoretical lifespan of trillions of years, although in practice other cosmological events will put an end to their life before.
The ultimate fate of stars is almost entirely determined by their initial mass:
Below \(0.3M_\odot\), stars on the main sequence are called red dwarves. They simply burn down all of their hydrogen into helium. At that point there is no pressure from fusion anyomre, since it has entirely stopped. Gravitational collapse then proceeds unfettered, until the helium turns into a degenerate plasma where protons, neutrons and electrons move freely. Fermi-Dirac statistics predit that electron degeneracy pressure manifests, which re-stabilizes the objects and halts the collapse again: it has now become a white dwarf, whose low luminosity is just a consequence of thermal activity. In the absence of fusion, its temperature just slowly gets down, until it becomes a black dwarf: a lifeless husk of matter. That has not happened yet, given the current age of our universe.
Above \(0.3M_\odot\), a star that burns all the hydrogen in its core is large enough to enter the red giant phase: the core contracts and becomes much hotter, which triggers the burning of hydrogen in the star’s shell, resulting in a massive expansion (hundreds of time). Red giants are not in a stable equilibrium and become colder but more luminous as times go by. Red giants whose initial mass was below \(0.6M_\odot\) just become white dwarves once hydrogen shell stops to burn.
Above \(0.6M_\odot\), but below \(10M_\odot\), stars follow a different path after the initial red-giant phase. When the helium core finally collapses on itself, it doesn’t have time to become degenerate, as the temperature is high enough to trigger the fusion of helium into carbon and oxygen. The star enters a state of quasi-equilibrium for a while, before a new red-giant phase called the asymptotic giant branch kicks in: this time, fusion stops in the carbon and oxygen core but starts again in the helium shell, and the star becomes even more luminous.
The thermic conditions mean that the helium burning shell is very loosely held, and massive stellar winds end up exposing the core more and more, which ionizes the surrounding matter, creating a nebula. At the same time, a process of neutron capture, combined with the decay of unstable isotopes, creates nuclei heavier than oxygen, up to lead. Towards the end, the central core becomes a very bright white dwarf, surrounded by the nebula, marking the end of the star’s life (except in some cases, where it appears that white dwarves that are part of a binary system can still be disrupted enough that they end up exploding in a supernova, producing elements up to zinc).High-mass stars (above \(10M_\odot\)) evolve differently. Their transition to the giant state is smoother in the sense that helium fusion can start concurrently with hydrogen burning in the shell. The are called red supergiants. The increase in luminosity is then lesser, although they did start from a point of very high luminosity in the first place. When the carbon and oxygen core is massive enough, fusion can actually continue, producing neon, magnesium, silicon,... The end point is iron, although small amounts of nickel are also produced. Any heavier element would cost too much energy. When this unfusable core reaches a critical mass, it collapses, unable to support itself. The fusion of the whole carbon core into iron is so fast (hundreds of years) that the star looks unchanged and has no time to adapt and find a new equilibrium. It won’t evolve into a white dwarf. Instead, it will end in a supernova event.
It is worth remarking this story is a bit simplified. Very heavy stars, above \(40M_\odot\), cannot become red supergiants straight away. They burn so bright and are so hot that the intensity of the winds battering their shell won’t allow it. They may spend some time in that unstable blue supergiant phase, following which they will sometimes directly explode into a supernova. The mass loss from the shell may also allow them to enter the red supergiant track.
We’ve already encountered supernovae when discussing the end of hypermassive primordial stars. The mechanism was different though, triggered by pair instability. In modern, smaller, more metallic stars, the process is driven by the iron-nickel core collapse. The core will collapse in a fraction of a second, at a speed around 20% of the speed of light. The resulting shockwave then bounces back, transferring a massive amount of energy to the outer layers of the star, triggering fusion reactions that go beyond the iron peak, generating all the heavier elements that are known to naturally occur (up to the heaviest one, uranium). Meanwhile, the burst of energy drives a powerful explosion of radiation, and expels the matter at high velocities, bound to scatter the cosmos.
The remaining core continues its collapse. For stars with an initial mass below \(25M_\odot\), the collapse is finally stopped by neutron-degeneracy pressure when the radius is just around 30\(\text{km}\) and the surface temperature \(10^{11}\text{K}\). Electrons and protons have merged to form neutrons, and what remains is called a neutron star. An initial release of energy (about 10% of the object’s rest mass) in the form of neutrino-antineutrino pairs is called the neutrino pulse. An exotic event, the merger of neutron stars is now believed to have produced and ejected most heavy elements, from niobium to uranium.
For heavier stars, the final outcome depends on their mass and metallicity combination. Some will leave a neutron star remnant, while for others, neutron-degeneracy pressure cannot halt the collapse, which ends up in a black hole.