Around about 5 billion years ago something truly remarkable happened here. I mean here at this unremarkable position in this unremarkable spiral arm of this unremarkable galaxy. At the time this place was a region of interstellar gas and dust, a very tenuous cloud with no particular place to go and no particular thing to do. The remarkable happening was that something disturbed this aimless chaos. It might have been something as simple as a star sweeping past on its own way in its own orbit; it might have been something as dramatic as a star exploding in the vicinity. Either way the gravitational disturbance was enough to give this region of that gas cloud a slight swirl, just the merest touch of concerted angular momentum. And that was enough to start the entire story of life on Earth.

Of course another few hundred million years or so was needed to establish a habitable planet. That initial disturbance set the gas and dust swirling and the resulting interactions caused the dispersed particles to begin to come together into a rotating disc, which retains most of the disturbed cloud’s angular momentum. This is a solar nebula: the beginnings of a star and its planetary system. A nebula is an interstellar cloud of dust, gases (and as we will see later, in Chapter 4, organic molecules). Originally, the word nebula was applied to any astronomical object that looked diffuse and cloud-like and many distant galaxies were called nebulae for this reason; indeed, the Andromeda Nebula (= Andromeda Galaxy) was so named even before the nature of galaxies was established. This is old-fashioned usage of the term, which should now be reserved for the clouds of dust and gases that are often star-forming regions.