Stars might seem like the most stable of objects, always shining bright in the same patch of sky night after night, but they too will come to end one day. This applies even to our own Sun, whose own fiery end in a few billion years’ time may consume the Earth. After a star’s final throes there is normally a final ember, present and visible, but far from the former brilliance of the star, commonly found nestled at the centre of a stellar nursery. These star remnants are collectively known as degenerate stars.
Degenerate stars come in several different forms, but here we’re taking the moral high ground and sticking to the astronomical kind. The bit of Physics key to understanding these special kinds of stars is called degeneracy pressure, and it comes from two important parts of quantum mechanics.
Firstly, there is the Pauli exclusion principle, which states that no two particles can occupy the same state. Then there is the Heisenberg uncertainty principle, which is a bit more complicated and is a classic example of quantum mechanics being really weird. This states that there is a limit to what can be known about a particle. This limit is fundamental and absolute; it does not come from measurement problems. It is simply impossible to know exactly where a particle is and know how fast its moving at the same time. In fact, the more you pin down exactly where a particle is at any given moment, the less you can know about its speed.
These two principles combine as matter becomes extremely closely packed. As the particles are forced closer they run up against the exclusion principle, which gives a well known position as the particles cannot overlap. As the uncertainty in the position is low, the uncertainty principle dictates the uncertainty in speed must be high. This means that the particles are moving fast, which generates an outward pressure, in exactly the same way hot air molecules in a tire move faster and can cause tire blowouts.
When stars run out of Hydrogen to fuse they can explode exceptionally violently in supernovae, or as the Sun will, expand into Red Giants. These will also eventually burn through all their Helium, however they are too small and light for the subsequent gravitational collapse to generate enough heat to begin Carbon fusion. Without nuclear fusion there is nothing to prevent the collapse, until the star runs up against degeneracy pressure from electrons. This is a White Dwarf star, likely to be the ultimate fate of our Sun.
With temperatures of around five times the Sun and enormous pressure, the conditions are sufficient for nuclear fusion of Hydrogen, but there is no Hydrogen present as it was used in earlier fusion processes. White Dwarves are typically made up of Carbon and Oxygen, the products of those earlier fusion processes, but the pressure is large enough to compress the star into a single vast diamond. Despite having a mass comparable to the Sun, White Dwarves have a radius of only around 10,000 km, comparable to the Earth, which makes them incredibly dense. In fact, a single teaspoon of a White Dwarf would weigh 5 tons.
If a star is 8-25 times as massive as the Sun, its collapse will not be stopped by the electron degeneracy pressure. Instead the collapse will continue until it runs up against degeneracy pressure from neutrons, which can be much more densely packed than electrons. Much more.
Neutron stars have all the mass of a typical star like the Sun, but are only 7-20 km across, about the same as a city, so the one pictured has a mass about 750,000 times that of the Earth. The surface temperature is about 1000 times that of the Sun and the density is frankly ridiculous. A single teaspoon of a neutron star weighs about as much as all the cars on Earth put together. Something dropped from a metre about the surface would hit the surface going at over 6 million kph, such is the gravitational strength. Because of the immense compression, Neutron stars are also the most perfect spheres in existence. On the whole, it’s fair to say they’d win most categories in a stellar top trumps set.
But even White Dwarves and Neutron stars don’t last forever. They will slowly cool over millennia, becoming black dwarves (none of which are yet thought to exist because the Universe hasn’t been around long enough for the cooling to finish), before evaporating away into the cosmic dark. Of course, you might be wondering what happens when a star is so incredibly massive that it overcomes even the neutron degeneracy pressure when it collapses. The answer is even more astounding than the Neutron star.
Ever wondered what makes the Sun (and all stars ever) shine? That would be nuclear fusion, the process responsible for lighting up our sky, day and night. It is absolutely key to the lives of stars, from their birth, to their explosive demise billions of years later. Ultimately it generates the heat that keeps our planet warm enough to sustain life. This is how it works.
At its simplest, nuclear fusion is just two atoms smashing together and creating a new, different atom, releasing energy in the process. Feel free to stop reading there, unless you fancy delving into some nuclear theory, with just a dash of quantum mechanics added for good measure.
All atoms consist of a nucleus, made up of protons and neutrons, surrounded by electrons. The nucleus is the important bit for nuclear fusion, so we can forget about the electrons here. The only thing that determines which element an atom is, is the number of protons in the nucleus. Hydrogen, the simplest element, has just one proton, Helium has two, Lithium three, Gold has 79, and so on. Protons have a positive charge, while neutrons are neutral, so atomic nuclei repel one another.
Another thing almost all nuclei have is some missing mass; adding up the mass of all the individual protons and neutrons gives a bigger answer than the actual observed mass of a nucleus. Thanks to Einstein’s famous equation, E = mc^2 (E being energy, m mass and c the speed of light), we can convert mass to energy. The missing mass is accounted for as energy. In fact, it is this energy that holds the nucleus together, which is why it’s known as the binding energy. It takes a different amount of energy to hold different nuclei together.
The Sun, like all other stars, is made up mainly of Hydrogen, the simplest of the elements. Thanks to the Sun being immensely massive (it weighs in at some 2 billion billion billion tonnes), there’s a huge gravitational force acting, compressing the Hydrogen gas, until it gets up to at around 15 million degrees Celsius, where nuclear fusion can kick in.
When two nuclei get close enough to one another, the strong nuclear force can take hold and grip them together. “Close enough” in this case turns out to be about 1 femtometre, or about 100 billion times shorter than the width of a hair. This is a bit of problem, as nuclei repel one another due to their positive electric charge. Even at the ridiculous temperatures at the core of stars, nuclei’s energies are insufficient to overcome the electric repulsion and allow nuclei to come close enough for the strong force to kick in.
Now this is where the quantum mechanics comes in. Quantum mechanics allows particles to occasionally be found where, according to classical theory, they shouldn’t be. Thus the nuclei can get close enough for the strong force. However, before quantum mechanics was discovered, nuclear fusion was still thought to be the source of the Suns power, with the temperature problem dealt with by legendary astrophysicist Arthur Eddington; “we do not argue with the critic who urges that the stars are not hot enough for this process; we tell him to go and find a hotter place.”
So now we’ve got two Hydrogen nuclei, i.e. two protons, at a distance small enough that the strong force can do its business. Lots of nuclear goings on follow, with a hopefully helpful diagram:
The two protons are bound together creating a Helium nucleus. One of these then switches to a neutron, which releases a neutrino, a wraith-like particle that very rarely interacts with anything, and a high energy photon, also known as a gamma ray. A positron, or anti-electron, is also emitted. We now have a Hydrogen nucleus again, a neutron bound to a single proton. The change in the mass and binding energy in two free protons becoming a bound neutron and proton releases the energy that the photon and neutrino carry away.
Another proton then joins the party, giving a Helium nucleus, with two protons and one neutron. Again the binding energy changes, releasing another high energy photon.
Two of these Helium nuclei then collide and fuse, but two protons are also ejected, so the final product is still a Helium nucleus as it still has two protons, along with the two neutrons. This nucleus is stable and will hang around in the star until all the Hydrogen fuel is all used up and gravity resumes its compression of the star, heating it and Helium fusion begins. But that’s more complicated and something to put aside for later.
All in all, this process of fusing Hydrogen into Helium releases 26.7 MeV of energy (or 4 trillionths of a single Joule). The Sun has an energy output of 380 trillion trillion joules per second, which means that every single second, for 10 billion years, the process of Hydrogen fusion above happens 1038 times. That’s 10 followed by thirty-eight zeroes. Every single second. For 10 billion years. The scale of these numbers is simply beyond comprehension.
Nuclear fusion doesn’t just keep the lights on in the sky, it also forges all of the elements up to Iron, as stars repeatedly exhaust their fuel, collapse and heat and begin fusion of the products of the previous stage. For instance, in about 5 billion years the Sun will run out of Hydrogen and then begin to fuse Helium into Carbon. It will also expand greatly and probably engulf the Earth and whoever’s left by then, but that’s definitely a problem for future generations.
As for the current crop of humanity, we first brought fusion to Earth in 1952 with the Hydrogen bomb Ivy Mike, which was about 500 times more powerful the fission powered nuclear bomb dropped on Hiroshima. Thankfully no fusion powered bomb has ever been used in warfare. Indeed, most fusion research is now carried out with the goal of harnessing nuclear fusion as an energy source in grand devices known as stellarators and tokamaks. Unfortunately, they remain inefficient, requiring more energy to be put in to initiate fusion than is released by the fusion, but work is very much ongoing to solve this. Nevertheless, humanity still has a great deal to learn before we can harness the vast power of the stars.
Almost a thousand years ago the brightest star in the sky was surpassed as the world was filled with wonder and light not seen before in recorded history. This doubtless shocked those living at the time, much as it would anyone today. The new bright object in the sky, which was visible even during the day, lasted for almost two years before fading away. We know this thanks to detailed accounts from Chinese astronomers, and the evidence points toward this “guest star” being the Supernova that formed the Crab Nebula we see today.
A Supernova is, in short, a really big explosion. As in big enough that the one seen in 1054 AD happened 6500 light years away, radiated only around 1% of its energy as visible light and could still be seen burning bright in the night sky all around the world (except maybe the then primitive continent of Europe where no records of it exist). NASA put it a bit more succinctly; a Supernova is “the largest explosion that takes place in space”.
Supernovae are the fate of only the largest stars, those with masses 8 to 15 times that of our Sun, which has a very different fate in store. There are two types of Supernova, with just type two explained here (typically they’re a bigger explosion). When stars run out of Hydrogen fuel they collapse and heat up until the core is hot enough to fuse the Helium left over from Hydrogen fusion. When they then run out of Helium they collapse again, heating up, until the leftover Carbon can be fused and so on, up to Silicon being fused into Iron. The stages past Helium fusion require such high temperatures that only the most massive stars can reach them, with Silicon burning occurring only at temperatures 2000 times greater than at the centre of the Sun. No elements past Iron can be fused in the hearts of stars as to do so requires more energy to be put in than is released, which is known as endothermic.
Each of these stages becomes progressively shorter, from 10 million years of stable Hydrogen burning to just 2 days of Silicon burning. This is for two reasons, firstly there is simply less of the heavier elements (2 Hydrogen nuclei become 1 Helium and so on). Secondly, at the insane temperatures in the later stages, the photons released in fusion actually have enough energy to disintegrate the products of fusion, undoing the work done millions of years before. This is known, unsurprisingly, as photo-disintegration and it is also an endothermic process. The energy removed in photo-disintegration critically weakens the core.
Without enough pressure generated in the core, the star will succumb to crushing force of its own gravity. An exceptionally rapid collapse begins, the star falling inwards at some 70,000 km/s, crushing the star until it is three times as dense as an atomic nucleus. Then the strong nuclear force takes hold, causing an enormous repulsive force, which violently throws the star into a rebound, releasing an enormous shockwave as it does so. This shockwave strikes the outer layers of the star, which causes more photo-disintegration, releasing a truly immense number of neutrinos. These are wraith like particles that very rarely interact with anything (billions are streaming through you right now), but in the extreme conditions of a supernova a lot of them are captured, which releases a ridiculous amount of energy, more than the entire energy produced by a typical galaxy. This accelerates the gas cloud even more, to become the biggest explosion humanity has ever witnessed. The star has gone Supernova.
It is in this last hurrah of a star that the elements beyond Iron are created. When the shockwave strikes the outer layers some of the energy is used in the endothermic fusion processes needed to create all the 75 or so elements heavier than Iron. In the ensuing explosion these are scattered far across the cosmos. We can be sure about this because we ourselves are formed of some of these elements, as is the Earth. Supernovae also leave beautiful nebulae, from which other stars, like our Sun are later born in huge cosmic events. These events of incredible violence lead directly to some of the most stunning scenes in the night sky, as well as the life which witnesses them.
What about the star’s core? Well its fate depends yet again on its mass, but there are two known paths it could take; neutron star or, for the larger cores, a black hole. These degenerate stars are fascinating in their own right and are be explained here.
In a galaxy the size of the Milky Way we expect to see a Supernova once every 100 years or so. The last witnessed was the Kepler Supernova in 1604, so we are a touch overdue for one of these phenomenal events, and you can be sure that you’ll know when it happens.
It was 1964, before man had gone to the moon, when Peter Higgs first published a paper proposing a mechanism detailing how the property of mass came about in the Universe. Nearly half a century passed before there was any experimental verification of his ideas, and it took the biggest and most complicated machine ever built to procure it. It is perhaps the single most important confirmed breakthrough in Physics this century. But what exactly is this “Higgs Boson” that captured attention the world over?
Basically it’s a quantum excitation on the Higgs field, which is one that breaks certain gauge symmetries of the electroweak field. Simple.
For those who have absolutely no idea what that means, the following is much more accessible explanation that doesn’t require a degree or two in theoretical Physics.
In particle physics there are 16 well established fundamental particles – fundamental here meaning that they cannot be broken down into any further particles and these 16 particles can combine in a variety of ways to make up everything in the entire universe. Together with one equation which governs exactly how these particles interact, they comprise the excitingly named Standard model, which describes everything in the Universe, the way it moves and essentially everything – except Gravity. Both the Standard Model and its problems are themselves worthy of stand-alone pieces but that’s for another time.
The properties of these 16 fundamental particles and where they come from are all very well understood, with the exception of mass. Before Higgs, there was no established mechanism showing where particles got their mass from, it just was. Trying to explain exactly why stuff that just seems to appear out of nowhere can exist and where it comes from is not a bad definition of Physics, so naturally people began working out some mechanism by which particles, and by extension everything in the universe, could have mass. Peter Higgs is the man credited with being the first to solve this problem, though like so many other famous problems, there were several others who figured it out at very similar times.
In practice of course the mechanism is exceptionally complex and is governed by similarly complex equations. It can be simplified somewhat to give an understanding of what’s going on without all the maths parts. The central part of the theory is an all permeating field that spans all the Universe. This may sound like a strange idea and it is, put it’s fairly common in Physics. The most common example is probably the Earth’s magnetic field: for every point on and around the Earth there is an associated number, which is tells your compass where to point. Another possible way of looking at the concept of a field is a room. At every point in this room there is naturally a temperature. Collectively, all the data on the temperature at each and every point is a temperature field. The Higgs field is fairly similar except that it spans the entire Universe and deals with mass rather than magnetism or heat.
So now we have a set of fundamental particles and a field that covers the whole Universe, but how do these two ideas lead to particles having mass. Well, one analogy of the Higgs field involves syrup. Imagine a room full of syrup. With nothing better to do, and presumably lacking pancakes, you decide to walk through the room. As you set out on your walk, you begin to feel the syrup resisting your attempts to pass through it, dragging you backwards and slowing you down. It is as if you are heavier; i.e. have a greater mass than normal. That’s a very simplified picture of how the Higgs field interacts with particles, slowing some down more than others to give some particles more mass than others. It is important to note that in reality the Higgs field does not work by slowing down things that are moving, but on all particles in the field. The field of course spans the whole Universe, so it acts on all particles everywhere, and all objects, including the screen you are reading this on and you yourself, which is why everything in the Universe has mass.
Except that last part isn’t quite true. There are two particles, the photon and the gluon, that do not interact at all with the Higgs field, so they are massless particles. This in turn means they travel at the speed of light, which is handy in the case of the photon as photons constitute light.
So there it is, particles have mass as they interact with an all permeating Higgs field in a way that can be thought of as objects moving through syrup, except without the particles doing any of the moving part. Hang on a minute though, this piece isn’t titled the Higgs field, it’s called the Higgs Boson, so what’s that?
To answer this, a slightly closer look at those 16 fundamental particles is required. 12 of these particles are termed fermions and it these particles that make up everything you have ever known. But without the other 4 particles the Universe would be a pretty boring place – just a load of particles sat about, not forming atoms, stars or moons for humans to go walk on, or possibly the Universe would be entirely devoid of any form of matter. These 4 particles are known as Bosons, and they each mediate a fundamental force. The photon and the gluon together represent the electroweak force (previously thought of as the electromagnetic force and the weak nuclear force which were subsequently shown to be two aspects of the same thing), while the Z and W particles represent the strong nuclear force.
Like the Higgs field, these forces have fields. Excitations in these fields are what cause the bosons to spring into being to mediate the force associated with the field. The Higgs boson is the same in this regard; it springs into being when the Higgs field is sufficiently excited. Note that excitation here does not refer to a literal state of being excited but rather is a term commonly used in Physics relating to the energy level something sits at.
So there it is – the Higgs Boson.
As for why it took so long to verify this model experimentally and find the Higgs Boson, the reason can be explained fairly easily. To produce particles, it is necessary to smash other particles together at sufficiently high speeds and in just the right manner. For the Higgs Boson, the energy levels required for this process are far larger than for any other particle previously discovered, as the Higgs Boson has a much much larger mass than almost all other particles, some 126GeV, compared to say an electron at 0.000511GeV (eV, or electron volt, is a unit of mass used for subatomic particles, while G is the prefix for giga or 1x10^9. 1eV = 1.78x10^-36 kg = 0.00000000000000000000000000000000000178kg).
Of course there are many things we still do not know about the Higgs, including its exact mass, whether or not it has multiple manifestations just to name a couple. There are also numerous questions left unanswered questions of the Standard Model itself, and it is these that Physicists at the Large Hadron Collider at CERN are now turning their attentions to. Those asking what practical applications the discovery of the Higgs are have missed the point of Physics and are likely going to be disappointed for a while to come, as there are no obvious ways that the Higgs can be used practically. That most certainly does not mean there won’t be any, after all Heinrich Hertz, the man credited with discovering radio waves, famously said “I do not think that the wireless waves I have discovered will have any practical application”. Of course they turned out to be one of the key building blocks in the formation of the modern world.
Hopefully, somewhere along the way, this has provided a reasonably clear explanation of what the Higgs Boson is, where it comes from, what it does and how it fits into the wider framework of the Standard Model of particle Physics. Perhaps that first technical definition of the Higgs Boson looks a much more manageable now, but if it doesn’t its worth knowing anyway, as it’s a great way to pass yourself off as intellectual at dinner parties.
For almost 400 million years the Universe was dark. There was simply no light. There were no stars to light up the heavens. Until the first stars formed; the first step in billions of years of birth and fiery death and birth again for stars great and small. The search for that first light has been going on for years, but we can be fairly certain about how stars’ form, both in the early days of the Universe and today.
Clouds of gas are common in the vast space between stars in today’s Universe, known as the interstellar medium. Commonly these are nebulae, immense gas clouds that are the remnants of supernovae, produced as huge stars reach the explosive end of their life and throw off their outer layers into space. In the early Universe these gas clouds were almost entirely Hydrogen, the simplest and lightest of the elements, but the current clouds contain more complex elements, fused in the heart of stars. It is these gas clouds that collapse to form stars.
Once an area of the cloud becomes dense enough, it will generate a net gravitational force, pulling the gas together and the cloud will begin to collapse. It may be that an external shock, like a nearby star going supernova, triggers the collapse, but the result is the same. Once the collapse has begun the cloud might fragment, becoming a few separate collapsing clouds, destined to become stellar neighbours and siblings. The collapse accelerates as the gas cloud is compressed as this increases the clouds density and thus increases the gravitational force acting to compress it.
The very centre of the cloud is heated immensely by the continued and ever faster collapse, such that the heat and pressure in the core is sufficient to halt the collapse and the cloud becomes a protostar. Now the protostar sucks in more and more gas and debris from the surroundings, some of which may not become part of the star, but instead become planets or asteroids in orbit around the star.
Eventually the gas ignites, burning deuterium, a form of Hydrogen, which further heats the star, until, at last, Hydrogen fusion begins in the core. A star is born. The properties of the star; its brightness, its temperature and how long before its inevitable fiery death, are determined by the initial state of the cloud. The birth of a star is the start of a long journey, one that lasts billions of years, and lights up our sky and that of distant worlds.
In ancient times the Sun was revered in cultures around the world as a God, worshipped for its light and life giving powers. Thousands of years of research and vast improvements in technology have led to the realisation that our Sun is in fact a very average star, just one amongst billions in the Milky Way, which is in turn just one of trillions of galaxies in the Universe. And like all these trillions of stars, the Sun, which has given rise to so much life, will one day, inevitably, die.
It will not be a slow process, nor do you need to worry as it won’t happen for a few billion years. Happen it will though, and there is, as yet, nothing that humanity can do to prevent it.
Like all stars, the Sun generates its power by fusing billions upon billions of Hydrogen nuclei into Helium nuclei every second in its core, which requires immense pressure (~3.84 trillion psi) and temperature (15 million degrees Celsius) in the solar core. The energy released by this fusion is what bathes our tiny world in light and has fuelled the planet for untold millennia. More importantly for the Sun, it prevents the Sun from collapsing under its own weight due to gravity. The Sun weighs in at 2 x 10^30 (2 followed by 30 zeros) kg, or around 330,000 Earths, and generates a strong enough gravitational field to keep all the planets in orbit around it, so the struggle to overcome its own gravity is a pretty tough one. It’s a struggle that the Sun has been able to win so far, thanks to nuclear fusion, but gravity always wins in the end.
The Sun has only a finite amount of Hydrogen in the core and once it’s all been fused into Helium, in about 5 billion years, the Sun will enter its final stages. Gravitational collapse will begin, heating the stellar material and compressing it further until the regions around the core are hot enough for Hydrogen fusion to begin. The energy released by this fusion heats the outer layers of the core significantly, causing them to expand greatly. The Sun will become a Red Giant, around 100 times larger than it is currently, swallowing up and vaporising Mercury, Venus and likely Earth. The material ejected in this expansion will form a beautiful iridescent nebula that may birth stars of its own in the distant future.
As for the core, gravity will continue its merciless compaction. Temperature and pressure will rise and rise until the Helium fused in the main stage itself fuses, becoming Carbon, which may also fuse with Helium to become Oxygen. Much as before, the energy released by the fusion will stave off further collapse for a time, though for a far shorter time now, as Helium fusion is less efficient than Hydrogen fusion and there are simply fewer Helium nuclei to fuse. Once the Helium is exhausted, gravity will once more assert itself.
The core, now comprised of Carbon and Oxygen, will collapse further, becoming a White Dwarf. Now collapse is finally halted, but not by nuclear fusion now. Instead the collapse has forced electrons so close together that Pauli’s exclusion principle from quantum mechanics comes into play. Simply put, this principle states that two electrons cannot occupy the same space. This leads to what is known as electron degeneracy pressure, as the electrons exert a force to comply with the exclusion principle. This will hold the final structure of the Sun up against gravity, at a fraction of its current size and brightness. In time, even this dead remnant will cool and fade, to a hypothetical black dwarf. The Sun will be no more.
This is no special path, no unique destiny; our star is exceptional only in that it is very average. Indeed, this is why we can know so much about the fate of the Sun, as we can study similar stars across various stages of their lives. Such a detailed knowledge of future events on the grand scale of the cosmos is a testament to the millennia of development of scientific advancement and development. Even Gods may die, but knowledge and ideas cannot.
Dark matter is definitely one of the most sci-fi sounding names in current Physics and it has been puzzling astronomers for years. But what is all the fuss about?
Well, the story goes back nearly fifty years to a woman called Dr Vera Rubin who began studying the speeds of stars in galaxies. Most of the mass in a galaxy is at the centre so those stars closer to the centre should move fastest, and the speeds fall the further from the centre of the galaxy a star is. This is what can be predicted from Newton’s law of gravity and it is very well documented in other systems; Pluto moves a lot slower than Mercury for instance, because Pluto is a lot further from the Sun. But, to everyone’s surprise this was not at all what Rubin found; it appeared that all the stars in a galaxy orbited the centre at the same speed, regardless of their distance from the galactic centre.
This seemed impossible at first, as the stars toward the edge of the galaxies had speeds that were far too fast for the galaxies gravity to hold onto them. By rights, they should have been flung deep into intergalactic space. They were simply going too fast but were just happily orbiting, seemingly unware that they were in complete violation of Newton’s law of gravity. This meant one of two things; Newton’s age old, well tested and pretty darn accurate gravitational laws were wrong, or our understanding of galaxies was very wrong.
Unsurprisingly it seemed easier to update our views on galaxies than to tear down Newton’s law (admittedly Einstein kind of did this anyway but Newton remained good enough for all astronomer’s purposes). So, to ensure there was a reason why all the stars in a galaxy didn’t just fly off into deep space, there had to be some extra mass in the galaxy. This is exactly what Dr Vera Rubin postulated in the 1970s. Not just the sort of extra weight you put on over Christmas though, such was the speed of the outward stars that there had to be nearly 5 to 10 times as much extra mass in a galaxy as there was from the stars alone (which is a lot of pigs in blankets). It is this extra mass that was termed Dark Matter, a name first proposed by Fritz Zwicky in the 1930s. Observations proving the existence of this new kind of matter soon followed.
The name actually has pretty mundane origins really; Dark Matter is so called because it is 1. Matter and 2. Dark, in that we can’t see it. We know that Dark Matter is dark, i.e. cannot absorb or emit light, otherwise we’d have seen it ages ago and nobody would’ve needed to bother revolutionising our view of galaxies and indeed the universe.
We can map out the distribution of Dark Matter using Einstein’s gravitational lensing of light, whereby huge masses are capable of bending the light that passes nearby, which Dark Matter does plenty of. But what actually is this dark matter, because it can’t be any type of matter that we already know about because they all interact with light, which Dark Matter doesn’t. Well, nobody really knows, but a load of proper clever people are working to try and figure out exactly what is holding our galaxy, and all the others, together.
WIMPs are the strongest candidate currently for being dark matter. These Weakly Interacting Massive Particles tick all the boxes as to the properties that Dark Matter particles must have, i.e. being weakly interacting and massive. This proposed particle lies outside of the currently accepted standard model of fundamental particles. It does link up nicely with another theory, that of supersymmetry, which predicts the existence of particle with properties nearly identical to the WIMP. However, experiments at everyone’s favourite particle accelerator, the Large Hadron Collider, have not produced the results supersymmetry predicts they would. Further experiments directly aimed at observing the WIMPs themselves have also failed. These results put the theory of Dark Matter being WIMPs, at least in their simplest incarnation, into serious doubt.
There are many, many more possibilities for Dark Matter particles that are currently being explored with vigour across the world, all of which could turn out to be completely wrong of course. We know where Dark Matter is, we know what it does and we know how any particle should behave, but for now the question of what it actually is has the most exciting of answers: nobody knows. Yet.
Simple Harmonic motion is a physical phenomenon first observed by Galileo. The story goes that he was in mass one Sunday, and, bored with the service, noted that the chandeliers above the altar were swaying slightly. So bored he was in fact, that he took it upon himself to count out how long each swing of the chandelier took. To his surprise, he found that the time remained the same for each swing. Another Sunday, another boring mass and Galileo observed another chandelier and found that the time for each swing of this chandelier was also a constant. Galileo was later put under house arrest for life by the church, but this wasn’t just for being bored during mass.
Let’s break it down first, so that we have an initial idea about what simple harmonic motion might be. Simple – well that just means it’s easy (so far, so good). Motion – that’s just things moving (still pretty good). Harmonic – The idea of things being in harmony might sound familiar to musicians in terms of pure notes (a wave with constant amplitude), and the idea is the same here. So, simple harmonic motion is just an easy description of things moving with a constant amplitude (amplitude is the maximum distance from the starting point).
One of the best examples of simple harmonic motion is a mass hanging on a spring.
Initially the mass will be still, as the force of gravity acting on it will be cancelled out by the tension in the spring, the forces are said to be in equilibrium, and where this happens is known, unsurprisingly, as the equilibrium position. But, if you give the mass a pull downwards and then release it, it will move upwards, as the tension in the spring will have increased. However, the mass won’t stop moving when it returns to its original equilibrium position, even if there is no resultant force acting on it.
When the mass is moving above the equilibrium position, gravity creates a force in the downward direction. The resultant force acting downwards means that the mass will slow down, until it has stops moving for an instant, at the maximum displacement. Gravity will then continue to accelerate the mass downward back toward the equilibrium position.
Once the mass reaches the equilibrium position again, there will be no resultant force acting, but the mass will carry on moving down. As the mass passes the equilibrium position on its way down, the force of the spring acting up will overcome gravity, slowing the mass down, until it is at rest for an instant. The mass will then move upwards as the tension in the spring overcomes gravity. Then, well, skip up a couple of paragraphs.
There are a few key points in simple harmonic motion. The energy in the system is always the same, just constantly flipping from potential, locked up in the spring and the height of the mass, to kinetic, in the movement of the mass. The acceleration of the mass is always in the opposite direction to the displacement from the equilibrium. The amplitude of the system is fixed, as is the frequency of the mass.
There are a few equations that fully define simple harmonic motion, which are described here: https://isaacphysics.org/concepts/cp_shm
Recently I attended a lecture at Jodrell Bank, home to the iconic Lovell telescope, on the subject of the continuing search for the first light in the Universe. The talk, from Professor James Dunlop of Edinburgh University, was very well delivered and interesting throughout. Here is a condensed version of the talk, in my own words:
Humanity has always been fascinated by the stars. Our ancient ancestors looked up at the night sky and doubtless marvelled at its brilliance. They saw patterns and figures; the constellations, and gave great weight to the predictions based around their movements. Thousands of years later, we still look up into the sky, but now we are searching for something; the very first light in the Universe.
A crucial part of this search is the Doppler effect. This is the same effect that causes the noise from an ambulance siren to change pitch after it passes you. It happens because the source of the sound waves, the ambulance, is travelling away from you. As it emits a sound wave, it then travels further away from you before emitting the next one.
This means that fewer waves reach you in a given time; the frequency of the sound, the pitch, has decreased.
The same effect happens with light. But, instead of the pitch increasing, it is the wavelength of the light that changes. If the light source is travelling away from you, the light will be stretched, its wavelength will increase, shifting the light to the red end of the spectrum; redshift. If the source is travelling toward you, the light waves will be compressed, the wavelength will decrease, shifting toward the blue end of the spectrum; blueshift. Physicists always seem to come up with the most inventive names.
Knowing that stars emit specific wavelengths of light, depending on the elements they contain, it is possible to calculate how fast a star is moving away from the Earth by how redshifted its light is. By doing just this for stars varying distances from Earth, Edwin Hubble was able to conclude that redshift increased with distance. The further away a star was, the faster it was moving away! This was hugely significant as it indicated that, if the clock were wound back, all the stars in the Universe would have been at a single point in space. This is one of many pieces of evidence that supports the famed Big Bang Theory.
The Universe is known now to be almost 14 billion years old. Thanks to experiments at CERN and other similar facilities around the world, we have a good understanding of the very early stages of the Universe, up to a few hundred thousand years. And, because we can see it, we have a good understanding of the more stable Universe, a Universe filled with trillions of stars in billions of galaxies. However, we do not (yet) have a brilliant picture of the Universe in the infancy of the stars – around 400,000 years after the Big Bang to a few million years later.
This is the hunt for the first light in the Universe, a search for the very first of the stars so that we may better understand how they formed out of the early dark Universe and paved the way for the next generation of star, and ultimately, the Sun and the inhabitants of a small rocky planet nearby.
Key to this search has been the Hubble Space telescope, which has produced a number of stunning pictures over its many years in service, some of which have been featured as the Picture of the Week. By looking at one tiny patch of the sky hundreds of times over a matter a weeks, Hubble was able to produce a famous image; the Hubble Ultra Deep Field. Again, Physics covering itself in glory in the naming game.
Another important piece of information must be mentioned here. Light does not travel infinitely fast. It certainly seems it does in our daily lives though, but this is because it travels fast. Really, really fast. 299,792,458 metres in one second fast. That’s going around the world 7 times in just under a second, or to the moon and back in under 2.
But space is big. Really, really big. It takes light 8 minutes to arrive from the Sun, while on Pluto, the warmth of the Sun takes around 5 hours to arrive. It takes light 4 whole years to arrive from our nearest star, Proxima Centauri. So the distances involved are absolutely phenomenal, or, as my old Maths teacher would say in a distinct Yorkshire accent, proper big.
And the finite speed of light has profound consequences. It means that the light that arrives on Earth shows us an old picture of the star that sent it. As you look up into the sky, you see the Sun as it was 8 minutes ago, if you catch a glimpse of Proxima Centauri, you are actually seeing our neighbour as it was 4 years ago. And if you look deep enough into the sky, you will be able to see the light from the earliest stars, ending its 13 billion year journey across the cosmos by hitting the eye of a creature on a planet that didn’t exist when the light set off.
And this is what Hubble did to produce the Ultra Deep Field image. It captured the light from the earliest stars and galaxies that we can see. But there is a problem. Remember that redshift effect? Well, for the light that originates from the earliest stars, its effect is massive. The stretching of the light toward the red end of the spectrum makes it harder for Hubble to detect the light, meaning that Hubble can only see starlight that has undergone less than a fixed amount of redshift. Any redshift greater than this and Hubble is unable to properly capture the light. This limits how far Hubble can see back in time, as it is the earliest stars have light that is redshifted the most.
Looking at what Hubble can see though, we know that we have not seen the very first stars yet. By analysing the spectrum of light from the oldest stars that Hubble is able to detect, it has been seen that they have some heavier elements, such as Carbon, Nitrogen and Oxygen present. These elements cannot be formed by the stars in the main stages of its life, but only as they are dying. Thus, these stars are not the first stars in the Universe, but only the children of stars that have gone before.
We must keep looking for first light then. And that is exactly what is happening, with NASA planning to launch the James Webb telescope in 2018 (although the launch has already been delayed several times). This brand new telescope will orbit the Earth much further than Hubble, meaning it will experience less interference from the Earth, will have a much bigger mirror to focus the light and, crucially, will be able to detect light much further along the spectrum than Hubble. The James Webb telescope then, may be the first time we have a real chance of seeing the very first light from the oldest of the stars and galaxies.
However, the James Webb telescope is hugely ambitious. With Hubble, the first mirror that the telescope was equipped with was not at all fit for purpose, rendering the telescope entirely useless until astronauts replaced it. In fact, almost the entirety of Hubble has been replaced and upgraded over the years, which is what has made the telescope so long lived. But with James Webb, no such repairs are possible; the telescope is planned to venture too far out into space for astronauts to be able to journey to for repair missions. If something breaks, the game is over. And that is only if the unfolding procedure more complex than IKEA furniture that must be undergone for the huge mirror to assemble is successful. There are serious doubts then, if the search for first light will be finally ended with the James Webb telescope.
A space mission looking at some of the most violent explosions in the Universe has recently marked its 10th anniversary.
Gamma Ray bursts. They’re pretty nasty stuff.
“They’re the most luminous, high energy explosions that have happened since the Big Bang,” says Neil Gehrels, the leader of NASA’s Swift programme. “It’s like a beam of gamma radiation that’s flying through the Universe.”
What would happen if one of these cosmic death rays of high frequency electromagnetic waves hit the Earth?
Well, as you might have guessed by the whole cosmic death ray part, it wouldn’t be too great for life on our planet. “For a planet 1000 light years away, it would destroy the ozone layer. If it was just 100 light years away it could blow the atmosphere off,” says Gehrels matter-of-factly. Thankfully, he then adds that “The chances of that happening to the Earth are fairly small, about once in a billion years,” he adds. “It’s certainly not as great a threat as a giant asteroid hitting our planet.”
But still, it’s probably a good idea if we have some way of detecting them round the clock. Thankfully, we do.
There is an international team of scientists, led by Gehrels, that operates and monitors the Swift satellite, which keeps an eye on these cosmic death sentences. Named after its ability to respond instantly to the around 90 high-energy flashes of radiation it detects each year, Swift has been in orbit since November 2004.
Before Swift, it wasn’t known for sure what caused Gamma Ray bursts, but thanks to Swift, astronomers now believe that the longer bursts, which last more than two seconds, are caused by the death throes of the most massive stars, as their centres collapse in on themselves. When the stars then explode into oblivion, a jet of gamma rays is emitted to travel the cosmos, hopefully not to strike the Earth.
The shorter of these explosions (less than two seconds) is categorised as short bursts. The Swift team has concluded that these are caused by the collision of two dense neutron stars. Neutron stars are the diameter of just a small city, but have the mass of the entire sun, which is one of the reasons these collisions are so violent.
Surprisingly, the Swift scientists have also discovered is that gamma ray bursts are vitally important to the evolution of the Universe. “When a gamma ray burst goes off near a star with a planetary system, it can have a very important and destructive influence,” says Gehrels.
One of the interesting phenomena that Swift demonstrates; because light from the other side of the Universe takes so long to reach the Earth, some gamma rays bursts spotted by Swift actually began their journey towards us shortly after the Big Bang 13.7 billion years ago. This means that when a blast goes off, it lights up that particular region of space allowing astronomers to get a glimpse back in time to the birth of the very first stars 500 million-or-so years after the Universe came into existence.
This has helped astronomers to confirm some pictures of the early Universe says Gehrels. “When the Universe was born, the only elements were hydrogen and helium but explosions started to seed the galaxy with higher elements like carbon, nitrogen and iron – the elements that make up our bodies.”
Not only do we owe our very existence to cosmic explosions, there is some evidence that the Earth’s ecosystem has been directly affected by these bursts of energy. Research published in 2013 suggested that a blast of radiation that hit our planet in the 8th Century may have been the result of a gamma ray burst, though Gehrels is inclined to reserve judgement.
Excitingly, he believes that even after 10 years of observations, Swift will be able to function for at least another five years, before it becomes just another piece of space debris, but it has already transformed how astronomers see the Universe.
“Before Swift, astronomers used to think the Universe was a steady set of stars and galaxies,” he says. “But if we put on our gamma ray glasses and look up at the sky, it’s always popping and bubbling and flashing – it’s a very different kind of violent Universe.”
And that is most definitely an exciting picture, which hopefully Swift and other programmes will be able to build up further in the coming years.