A Long Time Ago in a Galaxy Far, Far Away

Have you ever looked out on a damp and dreary January day and considered that everything you see around you -- from the bare trees and the frost-fringed asphalt to the discarded newspaper tumbling on the breeze -- is the culmination and product of nearly 14 billion years of cosmic history? The same is true of everything around us, from bacteria to buildings to our bodies.

I often take solace in the idea that the protons, neutrons and electrons that make up every atom in every molecule in my body were around, in some form or another, long before I existed. They will continue to exist long after I am gone. Their story is intertwined with the history of the Universe and the formation and evolution of galaxies.

Our solar system is about five billion years old, or about a third of the age of the Universe. Five billion years ago the Sun ignited in a spark of nuclear activity as a cloud of hydrogen gas gravitationally collapsed into a dense core. Temperatures and pressures in this core rose to a level that allowed pairs of protons (the single particle of which hydrogen nuclei are comprised), which are normally repelled by each other due to the electrostatic force, to become glued together by the strong nuclear force. Nuclear fusion. Fusion releases copious amounts of energy, and the sustained "burning" of hydrogen over the lifetime of a star is what causes them to shine.

A natural consequence of nuclear fusion is the synthesis of new, heavier elements. For example, the fusion of hydrogen can form helium, which has two protons and two neutrons. When all the hydrogen is used up, the helium nuclei can fuse to form beryllium. This can go on to form carbon. Other elements are also synthesised by nuclear reactions in the star. At the end of a star's life, when nuclear fusion is no longer sustainable, its substance is dispersed back out into the interstellar medium, releasing these newly formed elements like seeds to the wind.

This process of enrichment (or pollution as some call it) has played out since the first stars burst into life just a few hundred thousand years after the Big Bang. It follows that the original hydrogen from which the Sun was kindled was part of a larger cloud of gas already enriched with other elements that were formed long ago. Some of this material condensed out of the solar nebula into a dusty disc around the newborn Sun. Within this disc the planets formed. We owe our existence to previous generations of stars.

In fact, in order to achieve the correct mixture of elements we see in the solar system requires the blend of the debris of many, many stars. Moreover, for any element heavier than iron, we need something with a little more punch: explosive nucleosynthesis, or the formation of elements during the extreme conditions of a supernova.

The physical size of individual stars and their immediate environment is  tiny compared to the size of entire galaxies. If the distance between the Earth and the Sun could be shrunk to just one millimetre, the distance to the nearest star would about 270 metres, and the distance to the centre of our Milky Way galaxy would be over 1500 kilometres. When stars die they disperse their star stuff into their local environment; this detritus can spread through the galaxy because galaxies as a whole are dynamic places: their contents are in motion. Our own galaxy, for example, is a spiral galaxy, shaped like a disc. It is a disc because it is undergoing rotational motion: all the stars and gas and dust in the disc are orbiting a central hub (called the bulge) like a spinning plate. At the position of the solar system, the rotational speed of the galaxy is just over 200 kilometres per second. Motion within galaxies is in part responsible for helping to distribute new elements throughout the interstellar medium. Winds blown by massive stars and the explosion of supernovae also drive gas through the galaxy. In some galaxies that are forming lots of new stars, these winds can actually blow gas right out of the galaxy like a fountain. This material can later rain down on the galactic disc, peppering it with heavy elements.

On a grander scale, the interstellar media of entirely separate galaxies can become intermingled. Galaxies coalesce into new systems in spectacular events called mergers. Mergers are driven by gravity. In general, matter in the Universe likes to cling together: we call it 'clustering'. When two or more galaxies are in close proximity, there is the possibility that the two will become gravitationally bound and eventually collide. In these violent episodes the discs of galaxies can be totally destroyed as they are teased out in long streamers of gas and stars. This triggers bursts of formation of new stars, with the whole mess eventually settling into a new 'relaxed' configuration after a couple of billion years. The Milky Way will suffer such a fate just before the death of our Sun: in less than five billion years our galaxy will collide with M31, the Great Galaxy in the constellation  Andromeda, which is another spiral galaxy of comparable size to our own and part of our Local Group. As a guide to its distance, on the same scale as our shrunken Earth-Sun separation of one millimetre, the distance to M31 would be around 160,000 kilometres. That distance is decreasing all the time as the two galaxies race towards each other.

It is fascinating to think that the iron atoms within haemoglobin proteins in your blood were forged in the hearts of  now long-dead stars. These ancient Suns scattered their ashes like spindrift through the galaxy when they ended their lives in supernovae. One day, not long after the collision of the Milky Way with M31, the Sun itself will finally exhaust its fuel, swell to a red giant and then shed its atmosphere in a blossoming nebula, taking out the inner solar system in the process. That same iron now in your blood will be recycled and recast, to become part of new star systems, or to drift for aeons in interstellar or intergalactic space. We are the antecedents of myriad possibilities of future galactic evolution. Far in the future, new planets and civilisations that form within the emerging cosmic wreckage will be crafted from atoms that were once formed in stars that shone a long time ago in a galaxy far, far away.

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