Over five billion years ago, our neighbourhood within the Universe, on the inner rim of the Orion Arm, was filled with a diffuse mist of hydrogen. This hydrogen formed an immense molecular cloud, and light took dozens of our years to cross this cloud. It was a brilliant, beautiful, resplendent stellar nursery – intertwined vespers of gas were lit by the energetic emissions of nearby, second-generation stars, and highlighted by the glare of the nascent Galactic core.
Today we look into the skies and see such giant molecular clouds in the Orion, Carina and Eagle Nebulas.
The molecular cloud which composed our area of the Galaxy was spun by its orbit and by its density into structures: clumps, bubbles, sheets, and filaments of gas orbited with the Galactic disc. The density of the cloud and its low temperature allowed these structures to agglomerate. Slowly, on timescales incomprehensible to the mind of the intelligence which would eventually arise here, these irregularities condensed and the clumps grew. After less than ten million years, gravitational forces began to exceed the pressure pushing outward from the clumps.
We can think of several causes for the molecular cloud to collapse: the cloud could have hit another, equally-dense broth of hydrogen; or, it could have, in its orbit of the Galaxy, passed through a dense region of the spiral arms, crowded by brilliant young stars already blazing into the night. We know, instead, that one of these nearby stars exploded, and that the uneven force of such a shockwave was the catalyst for collapse. We know this because of the presence of heavy elements in the star systems humanity has visited, and studied – gold, uranium, iron, nickel, lithium, and, crucially, carbon; almost everything heavier than hydrogen and helium, the soup of the Big Bang.
It was probably a massive second-generation star, and its death would have been violent and brief. Heavy elements elements could only have been formed inside the nuclear reaction at the heart of this star, or through neutron absorption; in either case, these elements were scattered by the supernova which marked its death. The high-speed impact of this shocked matter into the molecular cloud caused it to lose stability, and it collapsed.
As it collapsed, it fragmented. Chunks of that filament, clumping together into irregular balls, began to separate and disperse. We now call this turbulent fragmentation. The non-uniform velocities within the molecular cloud compressed the gas in shocks as the whole cloud collapsed, forming objects of varying sizes and densities. As these collapsing clumps of matter distinguished themselves from one another, some became gravitationally unstable, and fragmented again, into two or, in the case of Alpha Centauri, three major parts.
From this fragmentation came the material which would compose Sol and its attendant stellar system, the cradle of mankind, and the matter which would form the Alpha Centauri system.
Although these protostars could not yet create nuclear reactions, they did become warmer and thus began to glow brighter. By collapsing and contracting, they converted gravitational energy into kinetic energy – the closer their constituent atoms fell toward the centre of contraction, the less their gravitational energy, which increased those atoms’ thermal kinetic energy. These clumps warmed; as the hydrogen molecules contracted and collided they became excited and emitted radiation in the microwave and infrared spectrums. Much of this burgeoning radiation escaped, in the beginning, but as the contraction continued the molecular density increased, which began to trap these emissions, and a runaway heating effect began.
The protostars grew hot, quickly, and began to glow a dull, cherry red.
Over a hundred million years, the protostars began to spin, flattening the material which surrounded them into a fat circumstellar disc. This material continued to accrete, and would eventually, in billions of years, become the companions to these stars as they orbited the Galaxy – planets, moons, asteroids, and comets. Then, as young stars, Alpha A and B poured out a strong stellar wind. This pushed back the gases of the disc, and matter stopped falling into the star itself.
Parts of the disc began to clump together as had the molecular cloud: no longer falling toward the protostar, the gravitational heating slowed and the disc cooled, and grains of silicates and ice condensed. The grains of metals, water, ammonia, and methane – that 2% of the mass of the disk planted by the detonation which began its collapse – stuck together electrostatically, and as these clumps ploughed through the disc, they slowly grew into planetesimals. Bound together by a static force and a growing, weak gravity, they swam through the hydrogen and helium gases, and distorted the homogeneity of the disc as they orbited the protostars.
In what we would eventually call the Alpha Centauri system, the interactions of the protostars and their circumstellar discs must have been complicated, as are the orbits of the bodies in the system today. The clumping of the disc around each protostar was influenced by the gravity of the other, causing radial lines to spread from each protostar toward their mutual barycentre. These interactions prohibited the formation of the massive Jovian gas giants which grace the Solar system. The beginnings of these gas giants were pulled apart by the competing gravities of the two stars, or were dissipated by their combined stellar winds, leaving only heavy, silicate planets like Fram to form.
Perhaps their discs merged at their edges, and material was swapped between the protostars and the lumps slowly building in their orbit. It is even possible that Alpha A and B were much closer, these billions of years ago, and swam like titans through a shared circumstellar disc, churning the glowing material about in complex tides.
Alpha A and B were spectacular sights, five billion years ago when they were T Tauri protostars. Their surface temperatures would have been similar to what they are now, though they would have been noticeably brighter, as their radii were smaller. Their discs would have glowed red-hot, and would have neatly bisected the stars themselves. From a distance, above or below the plane of the ecliptic, a dome sat at the centre of the disc – a hemisphere that was half a star, surrounded by a wall of slowly spinning matter. The light of the star would throw million kilometre shadows across the matter that was already clumping together in the disc, and ring systems would have developed in that disc as the interactions of the other star perturbed its orbit.
Across the sky, nearby stars were hot and young, filling space with violent stellar winds.
And then, probably within a million years of one another, and over two hundred million years before Sol, each of the twin stars of Alpha Centauri blazed to Main Sequence life.
Deuterium fusion ignition began, pouring out light, heat and radiation. This outflow slowed the collapse, and was channelled by the discs into bipolar streams. This flow imparted the angular momentum of the star to the material of the disc, just as the magnetic fields of their T Tauri stages had – forever, the planets which formed from the protoplanetary discs would orbit their parent stars on an equal plane of the ecliptic, at the equator of that star, and would match the star’s rotation.
Eventually the heat and mass of these stars would be enough to switch from fusing hydrogen to deuterium to fusing into helium instead. Very quickly, nuclear fusion found a balance where the energy exerted from the core balanced the weight of the collapsing matter which composed the star, and gravitational collapse ceased.
Alpha A accreted more mass than Sol, while Alpha B slightly less; Proxima, orbiting far from the barycentre, accreted about an eighth the mass of Sol. The lump of the molecular cloud from which Proxima developed was small, unstable; nuclear fusion in its heart was slow, fusing hydrogen into helium with much less efficiency than the furnaces at the hearts of Sol, Alpha A or Alpha B. It could not easily radiate photons from its core, an instead moved energy to its surface through convection.
Proxima was dim and isolated – it lacked its own circumstellar disc – and from the heart of the growing Alpha Centauri system, it was an insignificant, flaring bead, tracing an arc around the system thirteen thousand times as far away as Earth is from Sol.
Over a billion years, the clumps of silicates, metals, water, ammonia and methane began to build in size. Initially, they were carried by the turbulent motion of the gas disk itself, like debris carried by the swirling, seething motion of a whirlpool stirred by the two stars. When they collided with one another, they clung together, and grew. Soon they grew so large that they developed their own, shallow gravity wells, and attracted one another without the use of the currents foaming about them. Others formed by coalescing in the mid-plane of the disc, where the heavier material collected through the angular momentum of the disc’s rotation, and collapsed not unlike the molecular cloud had thousands of millions of years before.
Protoplanets kilometres across formed in these ways, and glided in languid orbits. And so began a period of intense violence: these planetesimals collided, smashed together, blasted one another apart, coalesced, and, eventually, formed a stellar system recognisable to us today. Close to the Alpha A and B, volatiles like water and methane could not form, and instead bodies of silicates and metals settled into orbits – we now call the largest of these Nebu and Fram. Both of these planets were created through countless impacts, which imparted mass to their subjects, and altered their orbits. Nebu found a comfortable orbit close to Alpha A; Fram, its satellites and rings, was hammered, pushed and pulled into a looping orbit between the two Main Sequence stars.
Stellar winds forced the gaseous hydrogen and helium of the disc far from the stars, and icy volatiles which could not form close to the heat and energy of Alpha A and B found stability here also. Thus formed a massive Oort Cloud – trillions of inert lumps of dirty ice, slung from the warm heart of the system by their hyperbolic orbits, or coalesced from the cooled gas, gathered dozens of times further from the barycentre as was Proxima. The Oort Cloud of Alpha Centauri was of much greater mass and density than Sol’s, for here could be found that material of the circumstellar disc which had formed the cores of Jupiter and Saturn, and the entirety of Uranus, Neptune, and the Kuiper Belt around Sol. These objects, while the best source of water for light years, were also the greatest challenge for the Quoqasi to navigate as we decelerated from our interstellar slingshot, and arrived in Alpha Centauri…
