The Sun, the central powerhouse of our solar system. A medium size, yellow star, composed principally of hydrogen - the lightest chemical element in the universe. Through a process of nuclear fusion - the union of atoms, the Sun forms denser atoms, releasing energy and heat in the process. This energy and heat radiates out, in all directions through the vacuum of space. All the planets in our solar system, intercept a small amount of this energy and heat, as they rotate around their axises and orbit around the Sun. The energy travels at the universal speed limit - known as the speed-of-light (c): which is approximately 3 x108 metres per second. Earth is just under 150 million kilometres from the Sun, and so it takes 8.3 minutes for energy leaving the Sun's surface to reach Earth's atmosphere.
Composition of the Sun
The Sun is made up of hot plasma (a partially ionised gas) interwoven with magnetic fields.
The Sun’s mass consists of about 71% Hydrogen (H), 27.1% Helium (He), and less than 2% is made up of heavier elements including Oxygen (O), Carbon (C), Neon (Ne), Iron, Beryllium-7&8 (8Be & 7Be), Boron-8 (8B), Lithium-7 (7Li) and others.
Below are the main neutral Hydrogen and Helium isotopes found in the Sun.
Forces in Atoms
An atom is a basic unit of matter, comprising of a dense, very small, nucleus at it's centre, surrounded at a relatively huge distance, by negatively charged electron(s) in orbit (the orbit is described as the electron shell or cloud), which are attracted by the electromagnetic force to the nucleus - mediated by protons.
The Nucleus consists of electrically neutral neutrons, and positively charged protons (except hydrogen 1H, which doesn't have a neutron), which are bound together by the Strong Nuclear Force - mediated by gluons. Neutrons and protons are collectively known as nucleons. The strong nuclear force overcomes the repulsion between positively charged protons (if there is more than one), to make the nucleus formation possible. The number of protons determines the different chemical element, and the number of neutrons determines the isotope of the same element. The number of electrons is normally the same as the number of protons - making the atom neutrally charged; hence if there are less or more electrons than protons the atom is charged; negative if there are more electrons than protons, and positive if the opposite is the case. Nucleons are made of elementary particles known as quarks, of which there are six types (or flavours); in the proton example below, it is made up of one down quark, and two up quarks, which have the lowest mass of all the quarks. Quarks are also held together by the Strong Nuclear Force - mediated by gluons.
The example atom used above is the Helium atom 4He isotope, which has two protons (like all Helium isotopes), two neutrons (which makes it this specific isotope of helium), and two electrons. The illustration uses the Bohr Model of an atom, which has been superseded, however, pedagogically it is still appropriate to understand the fundamentals.
Strong Nuclear Forces
Nuclear energy, is stored within the energy binding the nucleus together. It is the strong nuclear force (also known as strong interaction or strong force) that both slightly repels the nucleon particles (the protons and neutrons in the nucleus) from each other, but predominately holds them together (mediated by photons) - holding them, and in the case of more than one proton, overcoming the repelling electrostatic force between the protons. The nuclear force also, although in different ways (mediated by gluons), also holds the quarks together - the fundamental subatomic elements that make up nucleon particles.
The strong nuclear force is one of the four fundamental forces in the universe, and is estimated to be 1038 times stronger than gravity(another of the four fundamental forces); but, unlike gravity and electromagnetic forces, it only acts over very small distances (10-15m or 1 femtometer) - roughly the width of a proton.
The extreme pressure and temperature (above 10,000 Kelvin) at the centre of the Sun is often too hot for the protons and the electrons surrounding the nucleus to stay properly connected, and the electrons are consequently being ripped off the nuclei. This can be imagined like a soup of loosely connected and disconnected electrons and protons - known as plasma (the fourth phase of matter - after gas, liquid, and solid).
All particles that have the same charge (positive or negative) are naturally repelled by one another (and opposite charges attract). The force that drives them apart is electromagnetism. In these extreme conditions in the Sun's core, relatively small numbers of protons (the hydrogen nuclei disconnected from their partner electrons) are able to collide - they are able to overcome this repulsive force between protons with the same charge, and a initial fusion reaction can occur (less than 2% of the fusion process is through another reaction process known as the CNO cycle). In this process, the electromagnetism force has been overcome, as the collision speed has made the distances between the atoms small enough, so that the nuclear strong force can take over, and pull the two protons together to form a new larger nucleus.
This chain of events (explained in more detail below) lead to the conversion of Hydrogen into Helium. Helium is left as a byproduct (a form of "ash"), as the fusion of Helium (with another Helium) forms Beryllium-8, which is very unstable, and extremely quickly breaks part - back into two Helium atoms again, making further fusion into heavier elements impossible until later on in the life of star (also explained further below - if the star has sufficient mass). This build-up of helium means that the composition of the core is different from that of the rest of the Sun - an estimated 35% Hydrogen and 63% Helium (by mass).
Although Helium is heavier than Hydrogen, it is about four times lighter than the 4 protons required to create it. Some of the mass deficit is, impart, due to the loss of positrons and neutrinos, which are produced and released in the process; however, even accounting for this, there is still a small amount of mass missing. This remanding mass has been converted into energy, which can be shown in the famous equation: E=mc2.
Drivers of Nuclear Fusion
Gravitational Pressure - the compressive force
The Sun was produced by an initial gravitational collapse of a giant molecular cloud. Gravitational pressure acting on the gigantic mass, eventually compressed it enough - reducing its' size and increasing its' density - creating the conditions for fusion to occur in its' core. Due to the Sun’s mass (330,000 times that of the Earth) the gravitational force, acting within it, is incredibly high. The deeper into the Sun’s core (nearly 700,000 km from the surface to the core), the greater the column of mass above, and the greater the gravitational pressure: gravity has a gradient.
It is this same gradient in gravitational pressure, that means all Earth’s rocks are underfoot, with the denser elements (such as metals) accumulate at the core, and the atmosphere above containing the lighter elements, also spreading out in a gradient above (and not pressed down like a pancake on the rocks surface).
Internal Pressure - the expanding force
The compression by gravity, is balanced by an outward pressure gradient force in the opposite direction, which is a combination of these three factors:
Temperature is a measure of the average kinetic energy (the random motion energy) in the system. The higher the temperature, the higher the amount of kinetic energy (they are, therefore, directly related). The kinetic energy can be translational motion (back and forth - as shown in the image to the left, with the arrows indicating velocity and direction), rotational motion, or vibrational motion.
Confined particles, like those in the centre of the Sun, are liable to move around very fast. This high motion of particles tends to be a reaction to confinement - suggesting an underlying “restlessness” (Capra and Luisi, 2014) at the atomic scale. And so, even materials, such as rock, that may seem life-less and inert to us, are, at the level of the nuclei, highly dynamic, and in motion at high speeds.
Pressure is the amount of force per area (P = F/A). Hot gas expands for instance, creating a greater pressure on the surroundings. When particles bounce-off the edge of a boundary surface, they apply an outward force on the surface. Faster (hotter) particles apply more pressure. For example, a ballon is inflated as the particles inside are pushing harder against the inside of the ballon, than the particles outside the ballon.
3) Density (Volumetric Mass Density)
Is the measurement of the amount of mass per volume (ρ = m/V). Gases can be compressed to smaller volumes and therefore, have higher densities per volume. As the volume reduces (goes down), the pressure increases (goes up); they are, therefore, inverselyrelated. This is because a decreased volume has a smaller surface area, and as it still has the same mass, and the same number of particles, there are now more particles (and mass) acting on a smaller surface area.
The Birth of a New Star
Barnard 68, a molecular (black) cloud of gas and dust, which will become a star, perhaps in the next 100,000 years. At the moment, it is still very cold (around 4 Kelvin), and so the particles are moving around very slowly, and gravity has not yet began to dominate. Once gravity dominates, the star will begin to collapse (known at this stage as a protostar), and eventually the core will be hot enough, and the pressure will be high enough for the star to ignite, as hydrogen fuses into helium.
Star Energy Production: Proton-Proton Chain
Phase 1 of the proton-proton chain reaction is the fusion of two repelling (same charge) protons (1H nuclei). For this to occur, the two individual protons have been able to get very close together - almost wrapping around each other - and have bonded together into an unstable Helium-2 - diproton (2He), which has two protons and no neutrons.
Now a very rare event can occur: one of the protons turns into a neutron. The change from a proton to a neutron is made possible by the third of the four forces of nature: the weak nuclear force - it basically changes one of the three quarks (the fundamental subatomic particles that make up a nucleon) within a proton from a 'down quark' into an 'up quark,' transforming the proton into a neutron, which releases an excess positive electric charge as a positron (e+ - these are identical to electrons except they hold a positive charge), a neutrino is also released (which has virtually no mass).
The resulting unstable diproton (2He) decays very quickly either back into a hydrogen-1 through proton emission, as is the most frequent scenario, or far more rarely into deuterium (2H) through beta-plus decay, the next element required for the proton-proton chain to continue.
These three rare events that need to occur: proton-proton bonding, weak nuclear force proton to neutron switching, and diproton to deuterium decay; make this first phase very rare, and therefore very slow.
The emitted positrons (e+) usually annihilate immediately, through impacts with free electrons (e-), which releases two gamma rays, with a combined mass energy of 1.02 MeV.
Phase 2 the resulting deuterium (2H) is able to fuse with another proton (1H nuclei), producing a lighter isotope of Helium (3He), and releasing the excess energy as a Gamma Ray Photon (γ) (5.49 MeV). This reaction can happen very fast - perhaps every second, on average.
Phase 3 there are four possible paths to generate the Helium isotope (4He), however, the following, so called ‘P-P I branch’ is the most frequent (83.3%). Here, the Helium isotope (3He) can react with another Helium isotope (3He), which produces two protons (1H nuclei) and the heavier Helium isotope (4He), and releases 12.86 MeV of energy.
The resulting Helium 4 atom (4He) has 0.7% less mass than the four protons from which it was created, as the lost mass was expelled from the core during the fusion stages as energy (26.73 MeV net), in the form of Neutrinos, and high energy Gamma Photons (γ).
This released energy will interact with electrons and protons in the core, with some being transmitted out, however, that which is retained will increase kinetic energy of fusion products and other elements, and increase the temperature of the plasma, helping to prevent it from collapsing under its own weight.
This energy expelled from the Core produces hydrostatic equilibrium, a corresponding outward force, that equally counters the Gravitational Inward Pressure, and suspends Gravitational Compression and collapse – making the Sun a stable star.
This hydrostatic equilibrium is self-regulating (see image to the right): when the rate of fusion is high, the core expands as the increased inner forces pushing outwards become greater than the gravitational pressure pushing inwards. This then reduces the relative density (mass to volume density) in the core, and therefore, decreases collision probability and therefore frequency, reducing the rate of fusion; and when the rate of fusion is low there is less heat produced, and a lower corresponding outward force, causing the core to cool and shrink, which increases density, and therefore begins to increase fusion again.
The released Neutrino particles continue almost directly, without resistance through the Suns' outer layers, as they have no electrical charge (although a very small mass), practically at the speed of light. The Positrons and Photons, on the other hand, do not make it very far as they enter into the Sun’s next zone: the Radiation Zone. The all important vibrational energy and/or gamma rays pass to the next stage.
Nucleosynthesis: The building of all the elements in the universe
“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.” Carl Sagan
During the Big Bang, it is theorised that temperatures were so high that fusion reactions took place, and made the very lightest elements - the base elements for all other elements - in the universe (hydrogen, two different isotopes of helium, deuterium, lithium, and trace amounts of beryllium) - known as Big Bang Nucleosynthesis.
From this point on, it has been up to the Stars to continue building most of the other elements in the Universe. As described in the previous section, the Sun is able to transform the very light hydrogen into helium through fusion; building a heavier element - an element with more protons (in particular) and neutrons in the nucleus, which makes it a different, and heavier, element. In stars smaller than our Sun, the reactions like the proton-proton chain are the only ones that takes place. However, in a star larger than our Sun, near the end of a its' life, it can also fuse helium into carbon and oxygen; and in very large stars, this can continue to build silicon, all the way to iron. These elements are released (ejected) into space during different periods of the stars collapse. This process is known as Stellar Nucleosynthesis.
The elements beyond iron are not created in stars, as these fusion reactions require energy. Instead, the heavier elements in the periodic table are formed in supernovas - the last evolutionary stage of a massive stars’ life, where the end of it’s life is marked by one huge final explosion - where neutrons are captured, leading to the heavier elements, which are also finally expelled into space (known as Supernova nucleosynthesis).
The very heaviest elements, such as Gold, Silver and Platinum, are also the rarest, as they are only able to form under the extremely rare conditions found in the supernovas generated after the collapse of the largest stars in the universe.
We are all star (and supernova…) dust...
Nuclear Binding Energy
Binding energy is the energy needed to separate a nucleus into its constituent nucleons (protons and neutrons). A system of distinct, unconnected protons and neutrons has a larger mass than an atom of equivalent bound nucleons. The binding energy of a nucleus is equal to the amount of energy released when forming a new nucleus or nuclei through fission or fusion.
Within a nucleus there is an interaction between the electrostatic force repelling the protons - wanting to explode the nucleus apart, and the strong nuclear force - wanting to bind the nucleus tightly together. The electrostatic force can act across the entire nuclei, but the strong nuclear force can only act across the distance of around one nucleon.
In a very small (light) atom, with only one proton like hydrogen (H1), the repulsive electrostatic forces are very low, and the binding strong nuclear forces are, correspondingly, much higher. In a very large (heavy) atom, such as Uranium (U238), the opposite is true - the electrostatic forces are much higher, due to the large number of protons (and neutrons); and the strong nuclear force across the larger nucleus, is correspondingly much lower (as it is only 'strong' over very short distances).
Nuclei that contain more than 209 nucleons are all very unstable, and are subject to spontaneous decay. Atoms with a number of nucleons around 60, such as Iron and Nickel have a balance between these two counter-acting forces (electrostatic force and strong nuclear force) - and can be considered the most stable elements ('Goldilocks' elements) - as they are the most tightly bound.
All elements lighter than Iron and Nickel release energy when they fuse - and become more stable through fusion. By adding nucleons to iron, the product becomes heavier (not lighter), and so more mass means more energy is needed to be added to the system, and so energy is not released. All elements heavier than Iron and Nickel release energy when they break apart - and become more stable through fission. These heavier elements ‘prefer’ to loose nucleons, particularly during impact with other elements, releasing energy, as the products become lighter.
The amount of energy released by one fusion reaction (the proton-proton-chain example) is not as great as by one fission reaction (of uranium 235), however, if considered by energy per mass, then fusion releases far more energy then fission. For example, deuterium (2H) releases 275 million kcal of energy per gram, compared to 20 million kcal of energy per gram of Uranium 235; and also keeping in mind that fossil fuel is limited to about 10 kcal of energy per gram.
As mentioned in 'Nucleosynthesis,' the binding energy also helps explain why Iron and Nickel are the last elements to be made in Stars (through fusion), as they are stable, they need energy from outside (like all the other following heavier elements) to fuse them with other elements to create the rest of the periodic table.
Fusion vs. Fission
The scale of energy released through a fusion reaction (measured in millions of electron-volts - MeV) is around a million times greater than that of the energy released through the breaking of chemical bonds - the bonds between atoms (measured in eV), which is the source of energy for all non-photosynthetic heterotrophic life on Earth - such as Animals and Mushrooms, and that, which is in contained in fossil fuels.
Fusion energy, does not (currently) happen on Earth, but human made fission energy does.
Fusion - the fusing (or synthesis) of light atoms (i.e., hydrogen) into larger, heavier atoms (i.e., helium), releases energy. The final product (i.e., helium) is slightly more stable, and has slightly less mass than the sum of the original two atoms (i.e., two hydrogen atoms); this slight loss of mass has been converted into energy (E = mc2). 'Light' (as in low mass) atoms can be defined as those lighter than Iron (Fe56).
Fission - the splitting of heavy atoms (i.e., uranium and plutonium) into smaller, lighter atoms (i.e., barium and krypton), generating energy. As with fusion, the final product is slightly more stable, and the two products (i.e., barium and krypton) are also very slightly lighter than the original single nucleus (i.e., uranium 235); this slight loss of mass has also been converted into energy (E = mc2). 'Heavy' (as in high mass) atoms can be defined as heavier than Iron (Fe56).
Nuclear Power Stations use the fission reaction to release energy from heavy atoms, which basically, in turn, converts water into steam, turning a turbine, that spins a generator, that creates electricity (the same basic process for all forms of electricity production, except photovoltaics).
The fact that both fusion and fission generate energy may not, at first sight, seem obvious: if splitting larger atoms into smaller atoms, produces energy, then it could be logical to assume that fusing atoms together in larger atoms, would require energy; however, depending (mostly) on whether the nuclei are light or heavy, this is not the case.
Fusion as an Energy Resource
The debates over nuclear fissions continued use for energy are multi-faceted and polarised, even within the environmental community; and across state lines - i.e., France currently produces over 77% of it's electricity from Nuclear Energy, and Germany, produces less than 14%, with older reactors being decommissioned.
The basic negatives for using fission power is the radioactive waste (which will basically exist forever); it's links to atomic weapon production; when it goes wrong it has huge, devastating effects; the large bodies of water which are required are heated and it changes local ecosystems; and it's power stations are very expensive. The positives are mainly that it can produce a large amount of centralised, continuous, base-load energy (from a relatively small amount of material), and doesn't produce any carbon dioxide (or others toxins - except the radioactive waste, which can be contained) during energy production. However, nuclear power plant construction, and the extraction of uranium or plutonium and other elements, all use fossil fuels, and so the overall CO2 reduction is open to debate. Pro-nuclear can also point out that fossil fuels (taken as the alternative), such as coal and 'natural' gas, also contain radioactive elements, which are not contained, and when burnt can be released in the fly ash, as with the soot, that can be both carcinogenic, darken land surfaces (such as snow) reducing the albedo effect (reflectivity) of Earth, and contain heavy metals such as lead and mercury for example.
Ever since the 1930's, many scientists have also been working on trying to make fusion energy possible on Earth - but as yet, none have succeeded. Although, at least one company - Tri Apha Energy (TAE) in California, USA, seems to be getting closer in recent years, with a hydrogen-boron reactor; it's potential is said to be three to four times more energy per mass of fuel than fission, with virtually no waste, and no risk of melt-downs - watch this space (on Earth)…
Electromagnetic Radiation - an Introduction
The energy and heat that reaches the Earth ultimately comes from the Sun, with vastly smaller amounts from other astrophysical sources (i.e., Pulsars and Quasars, and radio waves emanating from Jupiter).
The energy and heat released from the Sun comes in the form of electromagnetic (or EM) radiation, propagating outwards in all directions, and from the more sporadic solar winds.
Electromagnetic (EM) energy, is a form of radiation, as it travels out as rays in straight lines, and is ‘electro’ ‘magnetic’ - a combination of both electric and magnetic - as it distinctly consists of two harmonic waves oscillating (‘swinging’ back and forth) perpendicular (90º) to each other - electric-waves, and magnetic-waves.
All EM waves travel at the fastest known speed in the universe - the universal speed limit: (c) 299,792,458 meters per second - known as the speed of light, as light also happens to travel at this speed limit.
The energy within EM radiation is held and transported equally in the perpendicular electric and magnetic fields propagating out into space.
This energy is measured in its irradiance or intensity. The irradiance energy from the Sun that reaches the top of Earth’s atmosphere is about 1350 Watts/m2, known as the solar constant. (Around half of this energy actually reaches the Earth’s surface).
EM energy also transports momentum (although it has no mass), which means that it is also able to exert a very tiny mechanical force on other objects: known as radiation pressure. It is this radiation pressure from photons escaping the Sun’s core, combined with thermal pressure that balances the compressive pressures from Gravity; and could also be harnessed to move a solar sail in space. This effect is virtually non-existent on Earth, however, so is not discussed any further in the main text below.
EM radiation can be understood as a wave of propagating electric and magnetic fields, and also as distinct masslessenergy ‘packets’ (known as photons).
EM radiation, therefore, is also understood to be particles as well as waves: known as the wave-particle duality. The energy in this packets (photons), for visible light, is very small: around 4 x 10-9 Joules - or between 1.8 electron volts (eV) for red light to 3 eV for blue light. It is important to note here that all atomic particles, such as electrons, have particle-like and wave-like aspects to their natures - not just EM radiation.
To understand this range of interrelated phenomena in more depth it is easier to first look at some of the basic concepts: electrostatics, wave-particle duality, and then magnetism; to understand them first as separate phenomena, before putting them all back together again, as part of a integrated emergent phenomena, as they are now recognised to be.
There are three types of electric charge: positive, negative, and neutral. Protons have a positive charge, electrons have a negative charge, and neutrons have a neutral charge - so no charge.
Atoms and molecules that have an equal numbers of protons and electrons also have a neutral charge. Those atoms or molecules that have more electrons than protons have a net negative charge (and are known as anions), and those atoms or molecules that have less electrons than protons have a net positive charge (and are known as cations); both negatively or positively charged atoms or molecules are collectively known as ions.
Particles with the same charge repel each other, opposite charges attract, and neutral charges have no repulsion or attraction.
Even though electrons have a much smaller mass than a proton, they have the same level of charge. A proton has a charge of +e, and a electron has a charge of -e. And all electric charge is quantised - it comes in discreet units - small distinct packets of energy (meaning that half charges are not possible).
When a particle enters into the electric field (which is explained below) of a charged particle, it experiences a force - either attracting it or repelling it. This is the physical property of an electric charge. Electric charge is measured in coulomb (C) in engineering, and as elementary charge (e) in chemistry. The electric charge of a particle reduces across a distance - just like gravity; and can be calculated using Coulomb’s law: the force is proportional to the product (sum) of the charges, and inversely proportional to the square of the distance between them.
Coulombs law also shows that the electric force increases as the charge increase, and it can also be used (by chemists) to predict ionisation energy: the energy required to create an ion; in other words, the amount of energy needed for an atom or molecule, to loose an electron. As Coulomb’s Law states, that the force reduces over distance, this law shows how electrons close to the nucleus need a lot of energy to separate them from the nucleus (and those further away, less energy), and that the more protons in the nucleus (the greater the positive charge), the greater the energy required to pull electrons away from the nucleus.
When an electric charge is in motion it is known as electric current, with the intensity (amplitude) usually measured in amperes.
Examples of electric current in nature can be observed in lighting, static electric discharge (i.e., by rubbing a ballon on ones head), and the solar winds effect on the Earth’s Ionosphere (creating the polar auroras).
Current can consist of any moving charged particle, such as an electron flowing through a conductive material (such as a metal), or a photon or electron through the vacuum of space, or ions (charged atoms) flowing through an electrolyte.
Creating a electric current creates a magnetic field.
This is known as Ampère's law. To the right, the image basically shows a green cable with a current (I) moving from a positive charge to a negative charge, with the generated circular magnetic field (B) produced around it, perpendicular to the current.
The relationship between electrostatics with magnetism in electric current, showed scientists that they are related phenomena, and more recently, that they are part of the same phenomena.
More about magnetism is discussed below, and in the page about the magnetosphere, however, it can be said here, that the early understanding of the relationship led to the invention of electromagnets - a technology that is used to run motors, generators, loudspeakers, and MRI machines for instance.
All positively or negatively charged particles produce, and are influenced by electric fields.
An electric field is a measurable effect generated by any charged particle.
An electric field can be illustrated as a vector field (these do not really exist - it is a form of visualisation): that is to a say - a field that permeates out into space at all times, like spokes in a bicycle wheel, in all directions from the centre of a particle, to an infinite distance; and, like gravity, the effect gets weaker the further the distance is from the particle centre.
The orange ‘equipotential lines’ represent this reduction in measurable effect over distance (closer circles have a greater potential force than circles spaced further apart).
An electric field can be calculated (and visualised) by placing a positive point test charge (which is a very small positive charge) in various places around the charged particle, which shows its’ electric field direction and magnitude.
The arrows indicate weather the electric field is repelling or attracting - outwards for positively charged particles, and inwards for negatively charged particles. For instance, when a positivelycharged particle interacts with a very small positive point test charge it will repel it (therefore outward pointing arrows), and when a negatively charged particle interacts with a very small positive point test charge it will attract it (therefore inward pointing arrows).
These vector arrows should not to be confused with an electric force, however, as a charged particle can only cause an electric force on (and with) another charged particle; a particle on it’s own in space has an electric field (and the vector arrows showing how it will
effect - and be effected by - a positive test charge), but it does not create a force by itself.
Positively charged field lines, never cross, and always travel from positive to negative charge. As can be seen in the third image scenario, a positive charge (in this case a proton) has been attracted to a nearby negative charge (in this case an electron), and the field lines that were previously going straight outwards or inwards like wheel spokes have come and bowed and joined together; and the equipotential lines have been ‘squeezed’ in the middle and stretched on the outside. This third image becomes an electric field of a electric dipole. The fourth image shows the electric field lines for two repelling protons with the same positive charge, which, if there are no negatively charged objects around, will flow outwards for infinity, ‘in search’ for a negative charge.
The electric field combines with the magnetic field to form the electromagnetic field (this will be explained at the end of this section).
Magnetism - A Short Overview
So far, it has been illustrated that EM radiation is a propagating electric field that travels like a wave and a particle through the vacuum of space at the speed of light. However, at the beginning of this section, it was also highlighted that there are actually two waves propagating through space, together - and this second wave, not yet discussed, is the magnetic field. Magnetism is covered in more depth in the next webpage on the magnetosphere, so this section on magnetism will be brief.
Magnets, instead of the ‘positive’ and ‘negative’ charges (monopoles) for particles, magnets have north and south dipoles. However, just like charges there is a repulsion between similar poles (like charges) and attraction between opposite poles (opposite charges). And this force also reduces over distance, and can also be calculated using Coulombs Law. Compasses can be used like the ‘test charges’ in electric fields, to understand magnetic fields, which again, look very similar (see image to the right).
The main difference in the two phenomena is that a proton (positive charge) an electron (negative charge), for example are individual elements - they are monopoles, however, it is impossible to split a magnetic into one individual south and north monopoles - and so magnets are always dipoles.
The first clear proof that both electrostatics and magnetics where part of the same phenomena was when scientists (such as Hans Christian Ørsted) found that by moving charge (making a current), a magnet would be deflected. And so: a moving electric field (a current) generates a magnetic field.
Therefore, a ring of compasses placed around a wire with a current flowing through it, made the magnets collectively point in a circle around the wire (see image in electric current section): In other words the magnetic field is perpendicular to the current, and therefore the electric field. The greater the charge, the greater the magnitude of the magnetic field, and the direction of the current determines the direction of the magnetic field.
When an electric field is moving a magnetic field is created (discussed above), and the scientist Michael Faraday proved that the opposite was also true: a moving magnetic field can generate a charge. This is called electromagnetic induction, and will be discussed more in the next page on magnetism.
Bringing it all Together
Therefore, it is currently understood that the energy that is released from the fusion reactions in the Sun, and is transported, in straight lines across space, is a combination of oscillating electric fields and magnetic fields, perpendicular to each other, in discreet ‘ packets (protons); and in a spectrum of wavelengths (discussed in the next section).
There are two main types of waves, mechanical waves and electromagnetic waves: and these are the main ways that energy moves from point A to point B: in other words, all waves transport energy as they travel.
Mechanical waves need a medium for energy to be transferred through - they can not travel through a vacuum. Mechanical waves, include ocean waves, seismic waves, or sound waves for instance - i.e., sound needs water or air to vibrate through. The fact that sound cannot travel through space, is probably a good thing concerning the Sun, as the noise given off by the Sun has been estimated to be around 100db, and if it could travel through space to Earth, it would be as loud as a mechanical hammer - all day-long, every-day… Mechanical waves can be both longitudinal (which is basically a compression or compaction being transmitted through a medium) or transverse waves.
Electromagnetic waves, such as visible light, do not need a medium - they can travel through the vacuum of space, but they can and do travel through mediums, and are effected by mediums - such as water, or air for instance - where they can be slowed-down, absorbed, reflected or refracted for instance. Electromagnetic waves are only transverse waves, and therefore, can also be polarised.
Although these forms of energy are different, they can all be represented as waves, as they all exhibit wavelike properties.
A wave can be defined as a disturbance propagating through space - which usually transfers energy. A single periodic wave, can be visualised in the image to the right.
A few properties that characterise a wave: Hertz (Hz), is the number of full wavelengths (λ - in meters) that pass a particular point (Point A in the image to the right) every second (s). It holds that the shorter the wavelength, the higher the frequency (ν - in Hertz) - they are inversely proportional - and as all forms of electromagnetic radiation travel at the speed of light (c: 3 x 108 m/sec (or 300 million metres per second) - the relationship is described in the formula c = λν (same as λ x 1/T or λ x f).
Period (T) is the number of seconds per oscillation, and as frequency is the number of oscillations per second; frequency (f) = 1/T.
The second image to the right shows two waves with the same wavelengths, but different amplitudes (heights). With mechanical waves, amplitude normally means more energy (larger oceans waves carry more energy than smaller ones), however with electromagnetic waves, it describes more brightness, but not more energy, as will become clearer shortly.
Light Intensity (brightness) reduces the further away from a light source (just like gravity), the lit object(s) are from it. As shown in the image to the right, there is an inverse relationship to the square of the distance (inverse-square-law). Light must spread out over a larger surface area from the point source, over increasing distances, resulting in the reduction of brightness over a distance. For instance, the light intensity from the Sun on Mercury is estimated to be 9126 Watts/m2 (on average), compared to the greater distance to Earth, which receives ‘only’ 1367 Watts/m2. And so the threefold increase in distance, between these planets and the Sun, results in an approximate ninefold decrease in light intensity on Earth.
Light also redshifts if the light emitting object is moving away from the lit object (or viewer). Which is how scientists have proven that the universe is expanding.
The fourth image to the right, illustrates two waves with the same amplitudes (A), but with different wavelengths (λ). As all electromagnetic waves travel at the same speed – the speed of light (in a vacuum) – the number of waves (per second) that travel past point A depends on the wave length. This number is known as frequency (Hz), and so the shorter the EM wavelength the higher the frequency, and the higher the energy; and inversely, the longer the wavelength the lower the energy.
Electromagnetic radiation, can therefore, be described by its wavelength, amplitude (brightness), frequency, and period. But what is actually waving up and down?
These wave images, are not attempting to show photons flowing through space on a wave. Instead, the wave, is the oscillation of the value of the electric field (and magnetic fields as we will be showed later), as the photon travels through space at the speed of light.
Physicist Max Planck found that EM Radiation is not continuous - like the line in the wave image shown previously, instead energy is transferred in individual “packets” - or particles (of the size hv, ‘h’ being the Planck constant), oscillating as a wave. This was the discovery that light is both wavelike and particle-like - depending on the scale it is viewed at.
This “packet” became known as a ‘photon,’ and as described in the previous section on the transfer of energy through the Sun, photons interact with atoms and molecules, by being absorbed or emitted, or breaking molecules, or changing the energy properties of atoms and their electrons. Just as atoms can absorb photons, by energising outer electrons to a higher orbit; atoms (or molecules) that loose energy, also emit photons. The energy lost or gained by the atom or molecule, is directly proportional to the frequency of the photon, shown in Planck’s famous equation: E = hv. ‘E’ being the energy of the photon emitted or absorbed, and h being Planck’s constant, given in Joules (J).
Energy is microscopically deposited in discrete amounts - on a macroscopic scale it looks like a smooth continuous wave, but at the view of an electron, light deposits discrete - specific size chunks: which means they are quantised - they have clear quantities.
With the understanding that atoms absorb and emit energy only as discreet quantities (quanta), it was also proven with the photoelectric effect, that EM Radiation only travelled in quanta too; and so quantum physics was born.
An experiment first made by Hertz in 1887, showed that blue light could eject an electron (once ejected, is called a photoelectron) from a material (in this case potassium); but, red light couldn’t, even if the intensity (the amplitude) hitting the material was increased, and for long periods of time.
This went counter to classic wave (mechanical) theory that understood that the ‘bigger’ (the higher amplitude of) the wave, irrelevant of its’ frequency (wavelength), it should heat up an electron until it was eventually freed from the metals surface. But the opposite was happening.
Einstein proposed correctly that, in fact, EM Radiation was actually travelling in quanta (photons), and not in continuous waves. And so, the photons have, like the wave, a specific frequency (wave length), but as a short ‘pulse’ or ‘packet’ their amplitude is better thought about in the numbers of photons; for instance, increasing the amplitude, increasing the number of photons travelling through space, it doesn’t generate (at least visually) a higher single wave. And so, unlike mechanical waves, EM waves do not increase in height with increased amplitude, and so a weak photon will not be able to knock an electron out of a metal, until the mimimim frequency is reached (threshold frequency), irrelevant of how many have been directed at it. And when the minimum threshold is met, electrons will be released, and an increase in intensity (more photons) will now release more electrons. This proposition, that was built on the work by Planck, was finally verified in experiments by Robert Millikan in 1916.
A standard 100-Watt light-bulb, for example, emits around 1020 photons per second.
The Double Slit Experiment
One famous experiment that shows how light is (also) a wave, is Young’s Double Slit Experiment.
The first image to the right showed what the experiment would (should) show if light was indeed purely a particle (the orange dots): those particles that travel through the slits would predominately group very close to each other on the light detector screen inline with the slits.
The second image shows a light source on the left, which in modern experiments could be a laser, The laser is shone at a screen, which again, has two very small slits in it. The two holes are really close together - about the same distance apart as the wavelength of the laser light (making it possible for one single light ‘beam’ to be shone through both holes, to guarantee they are in phase). The orange arrows represent the light as waves (photons), and the larger orange curves represent the peaks of the photon waves, from a imagined top view. Beyond the light blocking screen, is the cavity, with a light detector at the back.
The second image shows how, the light that does goes through the slits, diffracts (bends), and spreads out, splitting into two new waves. The two new waves interfere with each other. As can be seen in the third image to the right: when a wave peak meets another wave peak, it creates a wave with a greater amplitude (more brightness) - known as a constructive wave; and when a wave peak meets a wave trough, they effectively cancel each other out (darkness) - known as a destructive wave. And so when the wave reaches the light detector screen, there is an unexpected wave pattern (known as an interference pattern) of particles, with the greatest intensity in the middle where there is the most constructive waves crossing prior to reaching the light detector screen, and effectively dark patches where the destructive waves have cancelled out the light.
More modern experiments have shown that there are even stranger things going on, like when a single photon (or electron) is fired at a time; the overall pattern after time on the light detector screen looks like the original interference pattern… is the electron predicting the future interference - or is it splitting in two and interfering with itself - or?
The Sun emits electromagnetic radiation in many different wave lengths. The actual EM radiated by the Sun, is known as the solar spectrum, and is made up of some ultraviolet (A, B, and C), but mostly ‘visible light’, and infrared (A, B, and C) - as heat. It extends from around 290 nm to more than 3200 nm. Some stronger rays are emitted by the Sun (such as X-Rays), as mentioned in previous sections, but these do not make it through the Earth’s lower atmosphere. Small amounts of radio waves are also given off by the Sun and other stars. The energy spectrum given off by a star depends upon its size and temperature.
The ‘visible spectrum’ - that part of the spectrum that we can detect with our eyes is only a small part of the entire spectrum of electromagnetic waves. To the left of visible light are wavelengths that increase (higher) in frequency - short waves, and to the right are waves that decrease (lower) in frequency - and therefore long waves. As stated above, all forms of electromagnetic energy travel at the speed of light, but the amount of energy of a photon differs across the spectrum - from left to right, from more powerful gamma rays (i.e., 106 electron volts), X rays and Ultraviolet (and more harmful to life) on the left, to less powerful infra-red, and radio waves (less harmful for life - i.e., 10-8 election volts) to the right.
As is shown in following web pages, the Earth’s atmosphere filters out nearly all the most dangerous forms of electromagnetic waves.
All objects have charged particles moving around within them and interactions occurring - which changes their velocities, and elements vibrating and rotating, therefore emits (and absorbs) EM radiation (unless the object’s temperature is OºKelvin - absolute zero). Therefore all objects, such as ourselves, the air, walls, and an apple pie, emit (and absorb) electromagnetic energy, and the Sun can be said to produce EM radiation, as it emits, but absorbs virtually no EM radiation.
The Importance of Temperature (Thermal Radiation)
In understanding, and being able to calculate objects emittance of EM radiation, scientists developed experimental test rigs using empty boxes, internally covered in black soot, for instance called a blackbody, creating an idealised model, to compare against experimental results.
What scientists (such as Planck and others) eventually found with these experiments, was that the amount (emission rate or intensity), and type (spectral distribution) of EM radiation emitted and absorbed, depends on temperature. They found that:
Hot objects emit energy at a higher rate (greater intensity), cold objects far less.
And that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelengththat is emitted the most (known as lambda (λ) max) - and:
Hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.
To understand what this means, and the terminology described above, the first picture to the right shows a light bulb (which is connected to an electrical circuit for power…).
Incandescent lamps glow bright as electric current flows through a filament. The filament is a conductor, which allows electricity to flow through it. However, filaments are made short and thin, which increases the resistance, and so the flowing current basically bumps into the atoms in the filament a lot, as they ‘try’ to travel through it, increasing the temperature. As already stated above, hot objects emit (more) EM radiation, and so the objective of the incandescent bulb is actually to get the filament really hot - although this means a lot of energy is wasted as heat (IR radiation).
The first image ‘Off’ (to the left) shows that there is a measure of EM radiation emitting from the bulb even though the light is switched off. As the atoms are being accelerated and/or decelerated in the bulb, like in any object, they are emitting some EM radiation to the room, however, as the light bulb is off and cold, it is only giving off very, very small amounts of long-wave infrared into the room, that is too cold to feel as heat.
The next image ‘Slightly On’ shows that when the light bulb is slightly on, the emitted radiation shifts to the left - or as scientists say - shifts to the blue as it starts to go towards the blue spectrum of visible light. Here the bulb does emit small amounts of visible light in the red spectrum (as well as more IR), and so part of the EM radiation is now visible as low intensity red light, and perhaps a small amount of heat can be felt.
The last image ‘Fully On’ shows, again, how the curve shifts further to the left, now that the light is fully on, and the mix of the light spectrum now being emitted gives the lamp a “warm” white glow. The amount of emitted IR radiation has also increased again, and so the bulb is hot to touch.
This basic light bulb example confirms that hot objects emit energy at a higher rate (greater intensity), cold objects far less. It also shows that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelength that is emitted the most (known as lambda (λ) max). And it shows that hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.
The experiment also shows that the intensity spectrum is a bell curve - there is a max intensity, and after which, short-wave length emission drops to zero. This was not obvious, at least in theory to scientists for a long time - although experiments showed bell curves as observed results. Their calculations showed that the curve should keep increasing from the right: as temperature increased, intensity would increase to infinity as wave length shortened (known a the UV catastrophe). It took Plank, to suggest that light was not a continuous wave, and instead ‘packets’ (as discussed in part 2) to make the maths work, and later Einstein to confirm that light was actuallypackets (also particles) in the photo-electric effect experiment that made the calculations (which are relatively complex) show what was observed in experiments.
Black body radiation is an ideal model (black bodies do not really exist) for the EM radiation given off by an ‘ideal’ (or ‘perfect’) radiator. In general, an object can absorb the same EM radiation as it can emit, however, as a perfect absorber, it will reflect no light(as all is absorbed), and therefore will look black (hence ‘black body’). Black bodies also emit EM radiation at all wavelengths.
Black body radiation calculations purely consider temperature (which must be constant - in thermal equilibrium), and it ignores size, and material composition (discussed below). As a model it can be used to compare against what is observed, and although it is a theoretical model, surprisingly, the Sun is very close to a black body concerning absorption, as any light that hits it’s gas/plasma surface is scattered and totally absorbed. However, it is not a perfect black body, concerning emittance: The sun does emits light across the entire EM radiation spectrum, but it doesn’t follow exactly the ideal model, due to different dynamics happening inside the plasma, but it can be considered a black body for most calculations. The first of the images to the right shows the ideal black body radiation, with all the properties discussed with the light-bulb example above. And the second image to the right shows that actual emittance of the Sun (orange line filled with yellow. As can be seen from this graph, the ideal black body radiation for an object of this temperature (5778 Kelvin) follows very closely to the measured emittance.
The Importance of Material Type
As mentioned above, most objects are not black bodies, in that others factors, particularly what the object is made of is also very important. For instance, when iron is heated to the same temperature as glass, it emits and absorbs a different amount and type of EM radiation - it has a different emission spectrum.
Understanding how EM radiation (energy) interacts with materials (matter), at the atomics level and at the molecular level will be discussed further in some of the further web pages; as the EM radiation begins to penetrate the Earth’s atmosphere and interact with the Earths surface.
Life's Window to the World
Electromagnetic Energy is not only the primary source (or secondary source after gravity…) of energy on Earth, ‘light,’ the smaller fraction of the electromagnetic spectrum, is also an important source of information for many forms of organisms. The different properties of light, such as intensity, duration, polarisation, and spectral composition, can all be used as sources of information.
“In all, light sensing is connected to movement in some way so that, once signalled, the creature can respond.” 
Life on Earth has developed three principle forms of light detectors, known as photoreceptors: flavin-based blue-light photoreceptors (i.e., cryptochromes), retinal-based green-light (such as rhodospin), and linear tetrapyrrole-based red-light sensors (i.e., phytochromesin green plants).
In animals the detection of light leads to vision. In its simplest form, light-detecting cells in worms for instance, are scattered across the skin (although concentrated near the head), helping them detect warmth and sunlight, as too much of both will dry them out, and UV light which kills the delicate nerve-endings, causes paralysis.
Other more complex visual systems, such as compound eyes can be found in insects and crustaceans for example, that consist of several thousand light detectors called ommatidia. These are particularly good at sensing movement, and a broad range of colours (i.e., the Mantis Shrimp, which possesses ‘hyperspectral’ colour vision).
Single-lens eyes, such as our own, have also evolved into various forms in fish, birds, mammals, and spiders for instance. Some snakes (such as pit vipers) also have the ability to sense infrared thermal radiation (IR) between 5 and 30 μm, through sensors located in their noses - ‘smelling’ heat - giving them the ability to not only locate prey, but also vulnerable body parts in the dark. However. this is not strictly a photoreceptor, it is more a temperature sensor.
Plants, as well as being able to use light as an energy source, are also able to detect light (due to light’s importance for energy), which can result in changes in growth and morphology (form) - known as photomorphogenesis. Some plant seeds, such as many lettuce varieties, need light to germinate (positive photoblastic), or when detecting a lack of light will continue to lay dormant. Photoperiodism is a physical response, such as flowering, which is made by red-light detection during a 24 hour period. In this case, plants only start to produce flowers when they detect a critical night-length, which signals the appropriate time of the year to bloom.
Phototropism is a physical response, where blue-light detectors, help direct the plant to grow shoots towards the sun; much like heliotropism, which describes the ability of some plants (such as sunflowers) to track (and turn with) the Sun’s motion on a daily basis - and some other plants ability to raise and lower their leaves on a daily basis (i.e., leaf heliotropism of many legumes). Light detectors in plants can also determine the quality of the light - helping plants to avoid growing or even germinating (like oaks) in the shade. Finally, chloroplasts - the organelles, mostly located within plant leaf cells, where photosynthesis takes place - are also able to move towards or away from light within the cell - again optimising the amount of light that the plant can catch. 
Most fungi, are able to sense and react to light, as many have all three of the major classes of photoreceptors (exceptions may include some forms of yeast - single cell fungi, for instance). Some fungi, such as Cyathus stercoreus require light to initiate fruiting body development. Other fungi, such as sporangiosphores, grow towards the light (not bend towards light like plants) - phototropism, and the fruiting body of basidiomycetes depends on light at different development stages - and in:
“…ascomycetes, light also has a strong impact in morphological and physiological processes, including the regulation of conidial germination, hyphal branching, sexual and asexual development, and secondary metabolism.” 
Photosynthetic bacteria, such as Cyanobacteria (among the world’s most important oxygen producers) are able to detect different wavelengths, as they migrate up and down water columns in the worlds oceans - at the water surface light still has a wide spectrum, but at depth, blue light dominates.
Cyanobacteria, and two slightly larger organisms, protists, Erythrodinium, and Neodinium, are able to use their entire bodies, as a microscopic light-sensitive focusing lens device (much like a camera lens), a kind of single-cell eye. Using the information to move towards or away from light (phototaxis), through patches of motor proteins, forming…
“…on the side of the cell facing the light source. Pili [hair like appendages on the surface of many bacteria] are extended and retracted at this side of the cell, which therefore moves towards the light.” 
Light as Communication
Many different species within all the kingdoms of life, are able to manipulate light as a form of communication. Some plants, for instance, have the ability to create a 'bluehalo' of light (known as structural colours) around their flowers to attract pollinating bees ; or, have the ability to use specific pigments (pigments absorb and/or reflect light in certain wave-lengths), such as those that reflect UV, which are visible to insects (but not to us), and act as landing lights, guiding the insects towards the flowers nectar (and therefore, pollen). Bioluminescence, is the production of light through a chemical reaction, in an organisms body . Fireflies, and many marine animals, from algae to jellyfish and crustaceans (and symbiotic bacteria living in some of these organisms), are able to control the chemical reaction, which is used for attracting mates, feeding and protection.
Humanity also uses light for communication. Beacons of fire (sometimes positioned in a network), have been used in different parts of the world for centuries, as a form of communication across distances upto 100 km. In the 3rd century BC, ancient greeks were transmitting and receiving different messages by varying the combination and position of lit touches . This form of communication evolved to the more modern, Morse code, invented by Samuel Morse in 1832 (which can also be transmitted via radio waves and electrical pulses), which is able to transmit the entire alphabet and numbers 0 to 9 by varying the duration of the transmission: generating a series of dots and dashes (a dash lasts three times longer than a dot).
Modern technology also include the transmission of data via optical fibres, and the use of laser beams to read information from pitted patterns in a spiral track on CD's.
A more recent technology invention, known as Li-Fi, uses LED lights, much like Morse code, to transmit data (and position) through very high flickering rates (invisible by the human eye), which are able transmit the equivalent to the '0''s and '1's of digital code.
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 Schuergers, Nils et al. (Feb 9, 2016) ’Cyanobacteria use micro-optics to sense light direction.’ eLife 2016;5:e12620 DOI: 10.7554/eLife.12620
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