While observing the open clusters M6 (the Butterfly cluster) and M7 (the Ptolemy cluster) in the tail of Scorpius (whilst dressed as Dr Who) this week, I realised that, whilst I had waxed lyrical about the bits of star forming regions visible in the night sky, I had never actually explained why or how stars form! Now, I can understand those that say this knowledge isn’t hugely useful on a day-to-day basis, but it does provide a huge insight into our own place in the universe, and it certainly humbles me! So for that reason alone it’s worth going into and without further ado here is my intro to star formation!
Stars are the ‘cells’ of galaxies. They are the units that ultimately control everything, from the formation of planets and life to the supermassive black holes and quasars at the centres of galaxies. Yet for such beautiful, important bodies, they have incredibly humble origins. Throughout space there are pockets of hydrogen gas, collected in vast ‘molecular clouds‘ due to gravity. All of this hydrogen is left over from the Big Bang, although it may have participated in a few other events since then, and occasionally these clouds are dotted with regions of heavier elements, left over from the deaths of older stars. Regardless, these clouds are stable – by that i mean that the forces acting on the individual hydrogen atoms are balanced, so that the clouds are neither drifting apart or collapsing inwards. The only two important factors here are gravity and kinetic energy (i.e. how quickly the individual molecules are travelling) and as long as these two balance each other out, the cloud will remain stable (and rather boring!) for billions of years.
But of course, these clouds don’t live in isolation, they have gravity due to their mass and so they can attract other clouds of hydrogen an hence increase in mass, thus increasing their gravity and allowing them to accumulate more mass and so on until they reach the ‘Jeans mass‘. This is the point at which gravity becomes strong enough to overcome the kinetic energy of the individual hydrogen molecules and so, like loosing a tug-of-war, the entire cloud collapses in on itself. Up until this point, the hydrogen atoms have been crashing into each other occasionally, generating small amounts of heat that we can see as infrared or microwave radiation in our telescopes, but as the cloud collapses collisions become more frequent and intense as the molecules accelerate due to gravity, and so the radiation produced also increases in intensity and energy. Even this process might be stable if the energy produced by these collisions was free to escape but, like clouds blocking the light from our sun, there comes a point when the hydrogen atoms are so dense that the radiation produced by these collisions can no-longer escape. Now this energy is expressed as heat, and the temperature at the centre of these collapsing cloud skyrockets.
Regions of space where this is occurring are known as protostars, and visually this is what is occurring inside the “Pillars of Creation” from the Eagle Nebula that I mentioned last week, and within the above image. Of course, this collapse cannot go on forever either. Eventually the soaring temperature reaches a point where individual atoms have enough energy when they collide to fuse together through nuclear fusion to produce helium and a HUGE amount of energy in the form of radiation. At this point, the cloud bursts into life as a fully fledged star, a chaotic balance between the gravitational forces trying to collapse the star in on itself, and the out-pouring of radiation from this fusion process preventing the collapse. Depending on the stars size this balancing act can last for billions of years (as with out sun) or only a few million for the most massive stars. Regardless, this is the formative process of everything within our entire universe – this balance between radiation and gravity – as without it, there would only be hydrogen and yawning empty space. Oh and maybe dark matter – but we’re not too sure about that!
Over the next few posts I’ll go through the whole stellar and galactic life cycles – but bear in mind that they all hinge on this process. Everything stems from here: light, energy, elements, even people eventually! This is the beginning of the ‘story to end all stories’ as we are all – at some level – made from stars.