The Small Matter of Our Universe PDF

3. The Small Matter of Our Universe 3.1 Earth in the Universe The Earth is just over 4½ billion years old (or to be more precise, 4.55 billion), whereas the Universe itself is estimated to be around 13 billion years old. It very much depends on who is asked, as some place it as...

3. The Small Matter of Our Universe 3.1 Earth in the Universe The Earth is just over 4½ billion years old (or to be more precise, 4.55 billion), whereas the Universe itself is estimated to be around 13 billion years old. It very much depends on who is asked, as some place it as older and anything up to about 17 billion. However, for geological purposes, assuming the more conservative value of about 13 billion is perfectly acceptable. The main 'entities' in the Universe are Galaxies which are simply large concentrations of stars. Our own Galaxy, the Milky Way probably formed with other galaxies just after the 'Big Bang'. However, Planet Earth didn't exist at the beginning of the Universe and there's 8½ billion years of Universal history to account for before we arrived on the scene. So, before looking at our Earth in more detail it's worth spending a little time thinking about what went before. Where did Earth come from? What was there before there was an Earth? What about our own Solar System and the Sun, when and how did they form? 3.2 Origins The origin of the Universe is widely thought to have been from a 'Big Bang', producing in the intense heat and pressure of the first 29 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH second of existence, the element hydrogen. We could spend a large proportion of this book discussing what happened to various weird sub-atomic particles during those initial first few moments of the Universe. However, suffice it to say that as far as geology is concerned the crucial result of what happened is that the simplest element of the periodic table was fused in abundance, as will become clear a little later. So why do we think there might have been a 'Big Bang'? To help explain this, imagine being in the centre of Dublin late on a Saturday night. An ambulance races along the streets. As it approaches you the pitch of its 'der-der' siren increases. Passing by and disappearing into the distance the pitch drops sharply and then slowly decreases as it tails-off. This change in pitch of the sound is known as the Doppler Effect. The important point is The Doppler Effect in sound and light that light behaves in a travelling towards, and then away from you. similar way, ‘bunching- up’ and decreasing its wavelength as an object approaches you but ‘stretching it’ to increase it as it travels away. What has this got to do with the 'Big Bang'? Edwin Hubble was the first to realise that when he looked at galaxies and the light emitted from them, they were moving. Not only that, but they were showing a redshift, increasing wavelength 30 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE Doppler Effect, so must be moving away from each other and us. In other words, the Universe is uniformly expanding everywhere and hence the idea of an original 'Big Bang' to send all newly forming stars and galaxies off in different directions. 3.3 What’s in Space? It is tempting to think that 'Outer Space', as the name might suggest, has nothing in it and is essentially a vacuum. So it may come as something of a surprise to find that 'Space' is not empty but actually has quite a lot going on in it. If we think of the great melting pot that is the Universe, it has four main ingredients in it. The first three are energy (E), light (c) and mass (m). These are inextricably linked together by Einstein's simple but perfectly-formed formula at the heart of his 'Special Theory of Relativity'; E = mc2. Einstein followed this groundbreaking theory by a second which he called his 'General Theory of Relativity'. It's more complex, but key in that it links Energy to the fourth of our ingredients, Gravity. In essence, if there is energy then there will be gravity. This couples gravity to the 'Special theory' equation such that if there is gravity then it will affect anything with mass or light. Gravity will pull on something with a mass, but in theory it can bend light as well. Why is all this important? Because a swirling mass of gas and dust out in space will create its own gravity. By swirling it must have energy and therefore gravity. This motion in tandem with the gravity it creates is crucial to star birth, driving the stellar life cycle. All stars have a life cycle; they are born, they live and then they die, sometimes quite spectacularly. 31 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH When we look out into the Universe, we can see galaxies, and within these galaxies we can see clusters of stars. Stars can actually be classified according to their brightness and temperature. Rather than there being a random assortment, most stars lie on a line of similar brightness and temperatures, known as main sequence stars. Apparently larger stars, the 'giants' and 'supergiants' are in fact cooler than main sequence stars, glowing red as opposed to burning white. Super-hot small stars, such as white dwarfs, are hotter than those in the main sequence. This predictable Star classification in terms of size and temperature. pattern or spread of stars belies at what stage they are at through the stellar life cycle. The origin and existence of our own Solar System is inextricably linked with the life cycle of our star, the Sun. Therefore understanding how stars work provides the key to understanding the origin of our own Solar System and consequently our planet. 32 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE 3.4 Star Birth So, what is there in-between stars if it's not all an empty vacuum? In fact, large areas of 'Outer Space' are occupied by solar nebulae. These are giant, swirling, amorphous clouds of gas and dust perhaps best thought of as space junk. These clouds are the rubbish left over from when a previous star exploded at the end of its life and the energy from this explosion causes the cloud to move. The presence of energy in solar nebulae also means that they must create their own gravitational field, rather than, say, relying on the proximity of a star and its associated gravity. As a nebula cloud swirls or spirals, matter is naturally concentrated towards the centre of the spiral at the point of rotation. The cloud as a whole will exert a gravitational field, but by beginning to concentrate a little more mass towards the centre, it begins to exert a greater gravitational pull than the outer reaches of the nebula. Thus, gravity starts to pull matter towards the centre of the cloud. Increasing mass towards the centre results in a corresponding increase in gravity. This leads to a runaway positive feedback loop; more and more gas and dust is sucked in to the centre of the rotating cloud and this increases the gravitational pull on more gas and dust to the centre. This continues for some considerable time, but eventually the matter at the core of this rotating cloud begins to heat up as more and more matter is crunched into the centre. Finally, the heat and pressure at the core of the solar nebula crosses a threshold and triggers what is known as the proton- proton chain reaction. This is the basic chemical reaction 33 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH that burns at the heart of every star, keeping it 'alive' and is the reason that they shine so brightly in the night sky. The proton-proton chain reaction that burns in every star: explained step-by- step in the following section. e+ 1 H 1 2 3 H H He 1 H 1 H  4 He  1 2 3 H H He 1 H 1 H 1 e+ H Proton  gamma ray e+ Positron Neutrino Neutron 3.5 The Stellar Engine: The Proton-Proton Chain Reaction A star is a bit like an internal combustion engine. You need to put a certain amount of fuel in to start with, then you initiate the combustion reaction and motor on happily for a while, then finally you start to run out of fuel and eventually the motor dies. The starter fuel in a star is very specific; it's hydrogen. Hence the importance of when hydrogen was produced in the 'Big Bang', because without it stars will not form. As it is so critical, 34 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE it's worth having a quick look at what actually happens in the proton-proton chain reaction, and to do that we need to think about what's in and around the nucleus of an atom. In the chemical Periodic Table of elements, each element is given an atomic number and atomic mass. The atomic number is the number of positively-charged protons that the element has in its nucleus. For example, hydrogen with a single proton, has an atomic number of one. However, the atomic mass of an element in the Periodic Table is the average number of protons plus neutrons a particular element contains in its nucleus. Neutrons, as the name suggests, are a subatomic particle similar to protons but with no charge. For something so fundamentally precise as a table of all the elements that exist, why should the atomic mass of an element be just an average? The problem is that we can have different flavours of the same element, known as isotopes. For instance, the element hydrogen has three isotopes, hydrogen, deuterium and tritium. They all have the same atomic number, consisting of a single proton in the nucleus. However, deuterium also has a single neutron and tritium has two of them. Thus, the atomic mass for the element shown on the Periodic Table has to take into account the natural abundance of each three isotopes on the planet, thereby giving an average. However, when we are discussing an individual isotope, we can be precise about exactly how many protons and neutrons it has in its nucleus and this is often shown as a number before the elemental code for that isotope, for instance deuterium is 2H. Having this difference in mass and charge within the atomic nucleus gives isotopes of the 35 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH same element slightly different physical and chemical properties during reactions. Other particles can reside around the nucleus of an atom. For instance, negatively charged electrons 'buzz' around in successive layers, or 'shells'. An increasing finite number of electrons can be held in each shell out from the nucleus. Electrons can be 'excited' by absorbing the energy from an incoming photon, which bumps them up into the next shell. Often this is only temporary and dropping back down a shell will result in the electron giving-off a burst of light energy. Sharing electrons in an incomplete outer shell can help stabilise elements and they then become bound together as in a chemical reaction. The behaviour of electrons; whether they can get excited or can be shared with another element is what determines how reactive elements are. Elements whose outer shell is filled to Proton-Proton Chain Reaction: Step 1, Hydrogen to Deuterium. capacity are chemically inert, such as the gas helium. The proton - proton chain reaction starts by taking two hydrogen atoms, 1H (each consisting of simply 1 proton) 36 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE and crunching them through a series of reactions until they form the stable, inert form of the gas helium, 4He (2 protons + 2 neutrons). Initially, the two hydrogen atoms are forced to bond together because of the intense heat and pressure in the reactive core of the star as it is forming. This produces the second isotope of hydrogen, deuterium, 2H (consisting of one proton and one neutron). The reaction alters one of the original hydrogen protons to become a neutron in the resulting deuterium and this produces two weird subatomic particles. Firstly, a positron is given-off. This is the positively charged equivalent of an electron. An even more strange second particle lost from the hydrogen proton is a 'ghost' particle called a neutrino, which has neither mass nor charge. This spontaneous fragmentation of one of the reacting hydrogen atoms also produces light energy. Once deuterium Proton-Proton Chain Reaction: Step 2, has formed, it is Deuterium to 'light' Helium. immediately forced into a second reaction with another hydrogen atom. This time though, the deuterium and hydrogen are effectively fused together to form the light isotope of 37 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH helium, 3He, which contains two protons and one neutron in its nucleus. This reaction again produces light but also generates some rather nasty gamma radiation. Finally, the third spontaneous step in the chain reaction is for this 3He to crunch with another 3He atom which has been forming close-by in a parallel chain Proton-Proton Chain Reaction: Step 3, 'light' reaction up to this Helium to inert Helium. point. The two react again to produce light, but this time two hydrogen atoms are ejected to leave behind two protons and two neutrons to form the inert gas helium, He. This 4 terminates the chain reaction. The proton - proton chain reaction obviously occurs in parallel reactions en masse in the centre of every star. But a single chain reaction will consume six atoms of hydrogen to produce 1 inert atom of helium as the waste product. However, during the reactions, two hydrogen atoms are released back out and are then free to be used in a subsequent reaction. Thus the net loss of hydrogen is only four atoms for every one helium fused. The three steps in the reaction each produce light energy and this is why a star looks bright as it ‘burns’ its hydrogen. 38 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE The upshot of this stellar engine is that every star must have a finite amount of fuel which it can ‘burn’ and turn into the inert waste product, helium. Once the fuel supply starts to run out, the star begins to die. Our own star, the Sun, has enough hydrogen to burn for about 10 billion years. However, as we said right at the beginning of this Chapter, the Earth is just over 4.5 billion years old, so our Sun is fast approaching its mid-life crisis. 3.6 Death Star As a star approaches the end of its life, the type of death it undergoes is largely dependent on how much mass it had in life. The burning ball of gas we see as a star is held uneasily together by two opposing forces. The chain reactions going on in its core are attempting to blow the star apart. However, because of the immense mass of the star, it creates enough gravity to keep a lid on the reactions. A delicate balance is Balanced forces within a star during mid-life. created between the outward force from the core reactions and the inward force of the gravitational pull. This balance is known as the neutral line, where the inward 39 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH and outward forces are equal. Problems arise as the star begins to run out of hydrogen fuel. As the number of simultaneous proton-proton chain reactions begins to decrease, the outward force begins to lessen and the neutral line starts to contract towards the core as gravity begins to gain the upper hand. This is the beginning of the end. In a star with a small to moderate mass, as the fuel gradually runs out the neutral line slowly shrinks towards the core of the Star Death 1: Stars with small or moderate mass - star. As it does neutral line gradually shrinks as fuel runs out, and so, the matter on the outer shell expands to produce a Red Giant. the outside of the line is effectively allowed to expand out as the outward force is greater than the gravitational pull. This produces a red giant, which is something of a misnomer as although the star looks bigger it is simply because the outer shell of gas has expanded. As we remarked earlier, red giants are actually cooler than other stars, reflecting the death throes of their life cycle. Far more spectacular is the death of a star with large mass. In this case, the neutral line begins to move in towards the core as 40 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE the hydrogen starts to run out as before. However, in this scenario, because of the immense mass of a larger star, all matter is kept within the neutral line. Worse, as the engine begins to slow, the huge mass and corresponding gravitational pull rapidly take over and all matter is sucked in towards the stellar core. As the entire star begins to collapse under its own gravity the Star Death 2: Stars with large mass - high temperature in the gravitational pull keeps all matter within neutral core begins to heat line and core collapses, triggering further chain up. Eventually, the reactions. super-heated and super-compressed core spontaneously triggers further chain reactions; not using hydrogen as a starter fuel this time, but helium and as heavier elements are produced, these too are crunched, and so on up the Periodic Table. In fact, these superheated dying cores can produce every element in the Periodic Table up to iron (atomic number 26). These runaway chain reactions cause the star to expand and for a time the outward force exceeds the inward gravitational pull. But again as the reactions waiver and slow, gravity takes over 41 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH and the star again collapses. This expansion / contraction cycle repeats again and again, with each time the stellar core gradually ramping up the temperature and pressure. Eventually the Star Death 2: Stars with large mass - if collapse and threshold point is expansion cycles continue, star may eventually reached during explode in a supernova. one last crunch and the star finally blows itself apart. Successive, spiralling shells of matter are blasted out into Space to leave behind a small, shrunken core. This event is known as a Supernova and the intense pressure shock waves that ripple through the shells can go on to form every naturally occurring element in the Periodic Table. What happens to all that matter that is blasted out into Space during a Supernova? Well, after the initial violence of the explosion, the ejected matter starts to slow down and become a swirling, glowing mass of gas and dust. A solar nebula, in fact. The circle has closed; the life cycle is complete and the stage set for star birth once again. There is one final passing comment worth making before we go on to think more specifically about our own planet's origin and 42 INTRO TO GEOLOGY 3. THE SMALL MATTER OF OUR UNIVERSE evolution. We can find many naturally occurring chemical elements here on planet Earth; 117 to be precise. 91 of these lie higher than iron in the Periodic Table. Our own star, the Sun, whilst it does its best, is unfortunately nothing more than a moderately-sized star. Given the previous discussion it is clear that Earth's Periodic Table of elements cannot have been produced by the Sun. In fact, the only way we can have all the naturally occurring elements we see on Earth today (other than hydrogen and helium!), is from our Sun and Solar System having formed from the resulting solar nebula of a previous Supernova. So everything we see around us today, every atom in our bodies, must have come from the spectacular Supernova death of a previous star. *** 43 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH 44