Star Formation In Pictures
Star formation is arguably the central astrophysical process. The only thing that could compete would be what we might call ‘universe formation’, but as far as we know, that’s only happened once. Star formation, however, is still ongoing. And star formation dictates everything you can see, from the stars in our skies to the shapes of galaxies. (And planet formation is effectively a side-effect of this process as well.)
So, this is a post about star formation. We’re going to follow a generic main sequence star through from its birth to its middle age. And there will be pictures:
(There’s a much, much larger version of this image available at this link, but beware – it’s huge!)
Nebulae are clouds of gas and dust. Well, perhaps ‘cloud’ is too strong a word – nebulae are vastly more diffuse than any terrestrial gas. A cup full of the Earth’s atmosphere might contain 25 million million atoms, whereas a similar cup full of a nebula may contain perhaps 10,000. For a human being, they would be little different from a hard vacuum. Nonetheless, it’s in these cold, thin clouds of gas that stars begin. Nebulae are fragile things, existing in an uneasy equilibrium between their feeble gravity and their internal pressure. It doesn’t take much to upset them. Now, the note the bright bluish star near the bottom…
…and here we see the nebula, being rocked by a supernova. You can see the shockwave of gas, being blasted out from around the supernova itself. Remember the hot, young blue star above, an O or B-type, many thousands of times brighter than our Sun and perhaps a few dozen times more massive? Well, as these stars do, it’s gone supernova. (They go bang fast – O-type stars, for instance, live for as little as 3 million years. They pay a savage price for their tremendous luminosities. More gentle stars, like our Sun, can live for billions of years.)
The Long Fall
And now we see the consequences of disturbing the nebula. The pressure wave has compressed a region of gas a little bit too much. Now that the density has risen, its weak heat – the thermal motion of its atoms – is no longer enough to hold it up against its own gravity. It begins to collapse. You can see a little swirl as streamers of gas plunge inwards, like water spiralling down a drain. Already there’s a little glint of something there on the lower right…
And here is a close-up. What is this bizarre, cool, fat, red and bloated thing? It’s a protostar – the stage immediately before fusion begins. Protostars glow not through fusion but through gravitational contraction – as the cloud of gass collapses, it shrinks. The atoms are forced into proximity, rubbing up against each other. This friction heats the gas, making it glow. And around the protostar is a vast disk of gas and dust. Already pebbles and grains of ice have started forming inside this disk.
Accretion of planets
The protostar has shrunk to its minimum size. It now glows a smug orange-yellow rather than the cooler, ruddy pink it had before. It is exactly on edge, an instant away from critical density. In mere moments, its core will ignite as the pressure becomes high enough to fuse hydrogen atoms. Away from the star, we can see that the debris disk is being depleted. Planets are accreting. They’re still not finished, and they’re young – the nearest one to us is big, so it has yet to freeze. Its entire surface is molten magma. The smaller planet in the middle-distance has already developed a solid crust, though, due to its smaller size. (Smaller orbjects have a higher surface area to mass ratio, so they will radiate heat more efficiently. This is why the Earth has plate tectonics but the Moon doesn’t – the Moon is cold[er] inside!)
A small accident
And – oh my, what’s happened here? It appears the two planets were too close together. Their orbits weren’t quite stable. An interaction has happened – they’re colliding! This is a shocking event, and it brings home just what a violent process planet formation is. A planet is built out of millions of collisions like this, most of them smaller but a few possibly even larger.
As for the star, it is now burning hydrogen. It has stabilised onto the Main Sequence and many happy billions of years of fusion are ahead of it.
A ring of smoke
The planetary collision is over. The two bodies have merged. A debris ring has formed around the planet, mass ejected during the collision. The planet’s surface now has a solid crust (the collision melted what had been there). The crust remains thin, however, and the land is wracked with earthquakes. Vast volcanoes loft plumes of gas and dust into the air. You can see them glimmering redly on the nightside. But their fury serves a purpose – the gases they vent are building up into an atmosphere. Already the planet is wreathed in clouds and a torrential, worldwide rainstorm is under way. It will rain for thousands of years. When the sky is no longer saturated with water, the low-lying parts of the crust will be flooded. The planet will have oceans.
Moons and middle age
And here we see what became of the smoke ring. It has condensed into a moon! Meanwhile, the planet is now blue with water and an active weather system fills its skies. Who knows, perhaps even the beginnings of life are stirring in those seas? The monster vulcanism of the world’s early days has died down somewhat, although the planet remains an active place.
If you look very carefully, the star is slightly redder, slightly brighter and slightly larger than it was before. As they age, main sequence stars redden a little and expand somewhat, presaging the changes that will eventually grip them toward the end of their lives. This star is now entering middle age – it has billions of years to go yet before its final expansion. But eventually, expand it will…