9. The Most Amazing Fact
Written February 2019
A few years ago, the American astronomer and science populariser Neil DeGrasse Tyson – the nearest thing we now have to a worthy successor to the late, great Carl Sagan – was asked what he thought was “the most astounding fact revealed to us by science”, and made a video with his answer. His choice of “most astounding fact” is one with which I wholeheartedly agree – and for that matter, I’m pretty sure that Carl himself would have said the same!
So this essay is my own attempt to convey what I truly believe is the most amazing fact which science has revealed to us. But I have to tell a bit of a story to lead up to it; it’s all about the birth, life and especially the death of stars.
Firstly, we need to consider a couple of terms, which are familiar to anyone with a knowledge of high school chemistry. An endothermic reaction is one which requires a steady input of energy to keep it going. An exothermic reaction is one which, once it has begun, releases energy. Fire is an example of an exothermic reaction; it takes a certain input of energy, in the form of heat, to start something burning, but once it starts, it releases a far greater amount of energy. ( Of course, energy cannot be created or destroyed; it’s more accurate to say that an exothermic reaction converts chemical binding energy into heat, while an endothermic one does the opposite. )
While these terms are most commonly used in chemistry, they also very much apply in nuclear physics – and that’s what is vitally important to this story.
The birth of stars could itself take up an entire essay of this length, but I’ll cover it briefly. Stars are formed by the gravitational collapse of gas clouds in interstellar space. The matter in these clouds is incredibly tenuous – just a few atoms per cubic centimetre – but in a region many light years across, it adds up to a lot.
Various effects cause some of this matter to “clump”, or collect into regions of slightly higher density than average. Very slowly at first, the mutual gravitational attraction of the matter in a denser-than-average region draws the matter towards the region’s centre; it begins to condense and increase further in density. Gradually, over many millions of years, a cloud of gas and dust accumulates, several light years across, which is significantly denser than the surrounding interstellar medium.
Naturally, the denser the cloud becomes, the more quickly it condenses. Typically, the cloud contains a few thousand solar masses of matter, but gravitational instabilities eventually cause it to fragment into many smaller clumps, each a fraction of a light year across and containing just a few solar masses of material. It’s these smaller clumps which will eventually condense into stars; stars are invariably born in clusters, with dozens or hundreds of stars packed into a relatively small volume of space.
As a cloud of gas and dust collapses inwards under its own gravity, increasing in density, its temperature also increases. ( A little basic physics here. As a given particle of matter “falls” towards the cloud’s centre of gravity, it loses gravitational potential energy and gains kinetic energy. Temperature is simply a measure of the kinetic energy of atoms or molecules. ) When it reaches a temperature at which it begins to emit a significant level of infrared radiation, we call the object a protostar. So the protostar becomes steadily smaller, denser and hotter, until something happens which prevents it collapsing any further.
That “something” occurs when the core of the collapsing protostar reaches a temperature of about ten million degrees; that’s the temperature required to enable the thermonuclear fusion of hydrogen into helium. Hydrogen fusion is an exothermic reaction; though it requires a huge amount of energy to trigger it, it releases an even greater amount. ( Consider a hydrogen bomb, which requires an “ordinary” uranium nuclear bomb to trigger it! ) Once fusion has begun, the matter in the protostar is heated to a greater degree than it could have been by gravitational collapse alone, and acquires enough kinetic energy to counteract gravity and resist any further collapse. So from then on, the protostar becomes a stable sphere of constant size; as long as fusion continues in the core, there will be a continuous supply of energy to maintain the equilibrium.
The thermonuclear “ignition” of its core is the point at which a protostar becomes a true star – and the delicate balance between gravity and outward energy flow is what governs the remainder of its life. It enters a stable state, in which it will remain for about 90% of its lifetime. In astrophysics, we say it enters the Main Sequence of the Hertzsprung-Russell Diagram – a graph which relates various properties of stars - but that’s a topic beyond the scope of this essay.
At this point, we can’t yet see the new star, as it’s embedded within an obscuring cloud of debris, left over from the original cloud from which it was born. This surrounding gas and dust is heated by the star and begins to glow, forming an emission nebula. Some of these nebulae are among the amateur astronomer’s favourite deep sky objects, such as the magnificent Orion Nebula. They are “stellar nurseries”, with entire clusters of very young stars hidden within them.
After a few million years, the stellar wind from the young stars disperses the nebula, and they become visible as an open cluster, or galactic cluster, such as the Pleiades. While stars are always born in clusters, they don’t stay together for ever. Over many millions of years, the random motions of the stars in a cluster eventually cause them to drift apart, and the cluster disperses.
How long a star spends on the Main Sequence is heavily dependent on its mass; the smaller the star, the longer its life. This might not seem to make sense at first; don’t smaller stars run out of fuel sooner? No, because bigger stars need to generate energy at a far higher rate to balance their gravity. A star with ten times the Sun’s mass needs to burn its fuel at a thousand times the Sun’s rate; therefore it will exhaust its fuel in a hundredth of the time!
The biggest and hottest stars live for only a few tens, or at most hundreds, of millions of years. Sun-like stars live for around ten billion years; the Sun itself is currently about halfway through its Main Sequence lifetime. And the smallest and coolest red dwarfs live for much longer still; many are still around now, which were among the first stars to be formed in the early Universe.
Now, how exactly does a star generate its energy during its life on the Main Sequence? It is initially made mostly of hydrogen, which it fuses into helium, in the same reaction which powers hydrogen bombs. The process is known as the proton-proton cycle; for those interested in the details, it works as follows:
Two hydrogen nuclei ( which are simply protons ) fuse together to form a nucleus of deuterium, or heavy hydrogen, which contains a proton and a neutron. One of the protons actually turns into a neutron, by emitting a positron ( or antielectron ) and a massless neutrino. The deuterium then captures another proton, to become a nucleus of helium-3 ( two protons and one neutron ). Finally, two helium-3 nuclei combine to form a single nucleus of helium-4 ( two protons and two neutrons ), with the two excess protons being released. Then those protons in turn fuse with others, creating a chain reaction.
The net result is that four hydrogen nuclei are combined into one helium-4 nucleus. But the mass of the end product is very slightly less than that of the initial protons – even accounting for the positron. So where has that “missing” bit of mass gone? It has been converted into energy, in accordance with Einstein’s Principle of Equivalence, E = mc2. In a star’s core, hydrogen nuclei are being fused in immense numbers; that’s where the constant supply of energy comes from, which “holds the star up” against gravitational collapse, and causes it to emit copious amounts of light and other radiation. The Sun, in fact, is losing mass at the rate of four million tons per second – but don’t worry; it still has enough fuel left to sustain it for a long time yet – another five billion years!
Now let’s look at what happens in a star’s old age, as it begins to exhaust its supply of hydrogen fuel. You might think that it would simply keep on shining until the last of its hydrogen is used up, and then finally fizzle out, succumb to gravity and squash itself into some kind of dense, dead state – but that isn’t quite the case. In fact, a star’s death throes begin long before it actually runs out of hydrogen.
Fusion reactions only occur in the star’s central core; its outer regions aren’t hot enough. Remember that the temperature at its centre is measured in millions of degrees - but that of its surface is a mere few thousand. The helium produced by fusion is denser than hydrogen, so it naturally sinks towards the star’s centre. So the very centre of the core becomes a “dead” sphere of pure helium, with the hydrogen fusion taking place in a “shell” a little further out. This helium core produces no energy of its own, though initially it’s hot enough to resist gravitational collapse.
Slowly but surely, the helium core grows ever bigger, as more helium, produced in the hydrogen-burning shell, falls into it. Eventually, it reaches a certain critical mass, at which it can no longer support its own weight – and then the star is doomed. The helium core begins to collapse under gravity, while the outer layers swell up and are thrown off into space. When a star has recently undergone this process, we see it as what we call a planetary nebula ( a very silly name, due to what they looked like in early telescopes ), like the famous Ring Nebula in Lyra.
Then something happens which halts the collapse of the core. The collapse heats it to even higher temperatures; eventually, it reaches a temperature at which helium can itself begin fusion reactions, to form heavier elements. And this is the first vital fact which leads towards my “most amazing fact”; it’s believed that the only elements initially produced in the Big Bang were hydrogen and helium, and that all other elements which now exist have been produced in the cores of stars.
At this point, the star enters the next stage of its evolution. This new phase of helium burning causes its outer layers to swell to an enormous size – its radius increases about a hundredfold - while its surface cools and reddens. It leaves the stable state in which it has resided happily for the last ten billion years, and within a very short time – a mere couple of million years – becomes a red giant.
This, of course, spells disaster for any planets orbiting the star! When the Sun becomes a red giant, five billion years from now, its surface will be somewhere near the orbit of the Earth; the three inner planets will be completely destroyed.
The red giant phase is relatively short lived; it lasts just a few tens of millions of years. Of course, it now has a new core, composed of the denser products of the helium burning, and surrounded by concentric shells of helium and hydrogen burning. When this core reaches a critical mass, the star finally loses the long battle against gravity. The core collapses, the tenuous outer layers are thrown off, and what remains of the star, with nothing left to support it, shrinks inexorably to become a white dwarf.
A white dwarf is a very strange object. With no energy source to resist gravity, it collapses into an extremely compact and dense state, with the mass of the Sun compressed into a sphere about the size of the Earth! It’s now composed of a bizarre kind of matter, which is found nowhere else in the Universe, with a staggering density of around a million tons per cubic metre. A piece the size of a sugar lump would weigh a ton!
At such a density, the familiar laws of classical physics break down; the behaviour of white dwarf matter is governed by the laws of quantum mechanics. The more massive a white dwarf, the smaller its radius – which doesn’t appear to make sense! This is because the forces between atoms try to resist further compression, but the stronger the object’s gravity, the more easily these forces can be overcome, and the closer the atoms can be squashed.
But there is still a limit to how far a white dwarf can be compressed. There’s an effect of quantum physics, called electron degeneracy pressure, which halts the collapse, and causes the white dwarf to stabilise at a particular size. The electrons in each atom can only exist in certain discrete energy levels, and no two can occupy the same energy level; that’s like trying to put two pegs into the same hole. In a white dwarf, all electrons are forced into the lowest energy levels; after that, no amount of further compression can force the atoms closer together. This effect resists gravity and prevents the star being squashed any smaller.
Of course, the compression of matter to this dense state has made it very hot. The surface temperature of a white dwarf is around 10000oC; that’s why it glows white! But after the collapse has been halted by electron degeneracy pressure, there’s no longer any source of heating, so the star gradually begins to cool again. Very slowly, over billions of years, it becomes steadily cooler and dimmer, fading from white to red, until eventually, it ceases to shine at all, and ends its days as a black dwarf - cold, dark and thoroughly dead.
Going back a few paragraphs, we have the second vital fact. When the star’s core collapses, and its outer layers are thrown off, those heavier elements which were produced in its core are dispersed into the interstellar medium, and mixed into the gas clouds from which a new generation of stars will eventually form. The first generation of stars – some of which still exist to this day, those long-lived small red ones – consisted only of hydrogen and helium. Those of the second and later generations – including our Sun – also contain heavier elements; everything other than hydrogen and helium was produced in the cores of earlier, long-dead stars.
What I’ve described so far is the death of an “ordinary” star like the Sun. Apart from a brief blaze of glory in its red giant phase, it’s fair to say that it finally goes out “not with a bang, but a whimper”. But the same isn’t true of much bigger stars…
There is in fact a limit to the size of a white dwarf. If the mass of the star – or rather, what’s left of it, after it has blown off its outer layers during its red giant phase – is greater than 1.4 solar masses, then its gravity is so powerful that it overcomes even electron degeneracy pressure, and it’s compressed to an even denser state. This limiting mass is called the Chandrasekhar Limit, after its discoverer.
When a star whose mass exceeds the Chandrasekhar Limit collapses, the end result is something even weirder than a white dwarf. It’s another remarkable fact that normal matter – even solid matter – consists mainly of empty space! The nucleus of an atom is an incredibly small and dense object, whose diameter is a tiny fraction of that of the whole atom, with its orbiting electrons. But how tiny might surprise you. If you imagine an atom of a heavy element, such as uranium, to be the size of a football stadium, with the outermost electrons orbiting around the back of the stands, then the nucleus is the size of a pea on the centre spot!
But when a big star’s gravity overcomes electron degeneracy, the electrons in each atom are forced into the nucleus, where they combine with protons to turn them into neutrons. Separate atomic nuclei, now consisting entirely of neutrons, are forced vastly closer together than is possible in normal matter, resulting in an object many orders of magnitude denser even than a white dwarf!
This bizarre object is called – not surprisingly – a neutron star, since it consists mainly of a mass of neutrons and not much else! Its density – now comparable to that of a gigantic atomic nucleus – is truly staggering; for a star whose mass just exceeds the Chandrasekhar Limit, the resultant neutron star has a radius of only about ten kilometres. A cubic centimetre of it weighs 100 million tons!
So what finally halts the collapse of a neutron star? Well, neutrons also exhibit a kind of quantum behaviour which prevents them being forced too close together; as a white dwarf is “held up” by electron degeneracy pressure, so a neutron star is “held up” by neutron degeneracy pressure. The existence of neutron stars was predicted by theoretical physicists in the 1930’s, three decades before the first ones were actually discovered. The physics of neutron stars is truly bizarre, but is beyond the scope of this essay.
A star whose mass is a few times greater than that of the Sun suffers similar “death throes” to that of a Sun-like star – except that its end product is a neutron star instead of a white dwarf. But the biggest and hottest stars – the blue supergiants, with masses of about eight or more solar masses – end their lives in much more dramatic fashion. They go out not with a whimper, but with a very big bang indeed! In fact, you could say that they commit a spectacular suicide; the demise of such a star is marked by one of the most violent events in the Universe, a supernova.
More accurately, the event I’m about to describe is called a Type II supernova. A Type I supernova is a completely different kind of stellar explosion, which is equally dramatic, but unrelated to the death of a blue supergiant.
The term “supernova” is actually a pretty silly and misleading one, but unfortunately, we’re stuck with it! Firstly, as I’ve just said, we use the same word to describe two totally different and unrelated phenomena. Secondly, the word is derived from nova, which is itself short for nova stella, or “new star”. A “classical” nova is a stellar eruption on a much smaller scale, which causes a star’s brightness to increase greatly for a time; it’s so called because it sometimes results in a normally faint star temporarily becoming visible to the naked eye, giving the appearance of a new star having suddenly appeared in the sky. The word “supernova”, as you can imagine, was invented to mean an especially bright nova; however, we now know that novae and supernovae are totally different and unrelated phenomena!
Such adjectives as “spectacular” and “dramatic” are really not adequate to describe a supernova. When a massive star explodes in this manner, its brightness suddenly increases by a factor of a billion or more; for a very brief time – usually just a few days – the star can outshine the rest of the entire galaxy in which it is situated. In fact, almost all our knowledge of supernovae comes from observing them in galaxies other than our own; they are so bright that they can be detected at distances of hundreds of millions of light years.
Supernovae are very rare events; within any given galaxy, they occur at an average rate of just one every couple of centuries. By sheer bad luck, none have occurred in our own Galaxy since the invention of the telescope. Luckily, in 1987, astronomers were blessed with the next best thing – Supernova 1987A exploded in the Large Magellanic Cloud, one of the small “satellites” of our Galaxy. Apart from that one, we have only been able to study them in distant galaxies, many millions of light years away.
So what causes a supernova? Well, it’s an inevitable consequence of nuclear physics. I explained earlier how a star generates its energy, during the main part of its life, by the nuclear fusion of hydrogen into helium, and how the fusion of helium into heavier elements causes a Sun-like star to swell into a red giant - the beginning of its death throes.
But in more massive stars, things don’t stop there. All those heavier elements which are being produced also sink into the star’s core. The greater the star’s mass, the higher the temperature which is produced in its core by this infalling of denser elements – and the higher the temperature, the further the process of nuclear fusion can go. The carbon and oxygen sink into the centre, where they are heated to even greater temperatures; then they fuse into neon, and next to silicon. So now the star’s core consists of a series of concentric spherical shells, each sustaining a particular fusion reaction. The outermost shell still consists of hydrogen; inside that is a denser and hotter shell of helium, then further shells of carbon, oxygen, neon, and finally silicon in the centre. ( Other elements are also produced, but those are the ones which dominate in each of the shells. )
Remember that such a big star is short-lived; as it has had to consume its hydrogen fuel at an extravagant rate to counteract its huge gravity, it has taken only a few tens of millions of years to reach this state.
Finally, if the star’s mass is eight or more solar masses, its centre reaches a temperature – over a billion degrees! - at which silicon nuclei can fuse to form iron – and when that happens, it spells disaster for the star!
A work colleague once asked me in puzzlement, how it is that the fusion of light elements into heavier ones – as in stars and hydrogen bombs – releases energy, while the fission of heavier elements into lighter ones – as in nuclear reactors and “ordinary” nuclear bombs – also releases energy. He couldn’t see how that’s possible; surely if going one way releases energy, then going the other way must absorb it!
Well, the answer is that both release energy, only so far. The most stable atomic nucleus is that of iron – iron-56, to be exact. The fusion of light elements into heavier ones is exothermic, until iron is produced – but going beyond iron becomes endothermic. Similarly, the fission of heavier elements is also exothermic until it gets down to iron, and splitting that any further becomes endothermic.
So up to now, all of the fusion reactions have been exothermic, and have therefore continued to provide a supply of energy to “hold the star up” against gravity. But once iron is produced, going any further suddenly becomes endothermic; the energy required to trigger the reaction is greater than the amount released. This energy is provided by that immense heating of the star’s core. This is the only process in the Universe which can produce nuclei heavier than iron; every existing atom of every element heavier than iron – the gold or silver in your jewellery, the copper or lead in your water pipes, the mercury in your tooth fillings, the uranium used in nuclear reactors - was produced in the core of a supergiant star, in the final moments of its life! And that’s my next vital fact.
This abrupt transition from exothermic to endothermic reactions has fatal and dramatic consequences; it means that, almost instantaneously, the star loses its energy source, and loses the battle against gravity. While a supernova is commonly described as a stellar explosion, it actually begins with an implosion; the star’s core suddenly collapses inwards within a few seconds, heating itself in the process to a staggering temperature of about 50 billion degrees.
Note that this sudden collapse only occurs in the star’s core. Astronomer Phil Plait coined a wonderful description of what happens next; “the star’s outer layers then experience a Wile E. Coyote moment” – as in the joke where he runs off the edge of a cliff, suddenly realises that he’s hanging in mid-air, then begins to fall! Momentarily, the outer layers really are suspended above empty space – then they in turn fall inwards under gravity towards the core, becoming heated to immense temperatures.
Then, due to this intense heating, the infalling material “bounces” outwards again, and the star blows itself apart in a colossal explosion. For a few days, it shines more brilliantly than an entire galaxy – then it gradually fades over a period of months or years. So again, all those heavy elements produced by the core’s collapse are dispersed into the interstellar medium, to become the material from which later stars, and their planets, will be born. And that’s my final vital fact.
What remains of the star’s core now collapses under gravity to become a neutron star. Meanwhile, the material which was blown off from its outer layers continues to expand at a rate of thousands of kilometres per second. This expanding shell of incandescent gas can be seen from thousands of light years away, and remains visible to astronomers for thousands of years, as it gradually cools and fades. There are many examples, in our own Galaxy, of these supernova remnants – glowing gas clouds with neutron stars ( some of which we detect as pulsars ) at their centres. Some of these are the remains of stars which exploded many millennia ago.
During recorded human history, in the pre-telescopic era, we know of eight supernovae which occurred in our Galaxy, between AD 185 and 1604. More accurately, the historians of various cultures recorded the appearance of very bright “new stars” in the sky, and their descriptions are consistent with supernovae. For some of these, where the position of the phenomenon was recorded with sufficient accuracy, astronomers have been able to identify the corresponding remnant.
The best known example is the supernova which was seen in AD 1054; it was recorded by Chinese astronomers, who called it a “guest star”, and left very detailed descriptions of its position, its brightness and the time over which it remained visible. At its peak, it was so bright as to be visible in daylight!
The remnant of this event is the famous Crab Nebula in Taurus, about 6500 light years away. ( Of course, AD 1054 was the year in which the light from the supernova reached the Earth; the explosion actually occurred 6500 years earlier. ) The glowing shell of gas is now about 14 light years across; it shines partly because it’s still hot, and partly because it’s excited by radiation from the neutron star at its heart. The latter was one of the first known pulsars, and is by far the most studied.
The Crab Nebula is regarded by astrophysicists as one of the most important objects in the sky. As well as being one of the closest supernova remnants, it’s also one of the very few for which we know precisely when the explosion occurred; studying the structure of the remnant, and knowing exactly how old it is, can tell us a lot about how it has evolved. It has been said, with only a little exaggeration, that modern astrophysics can be divided into two parts - the Crab Nebula and everything else!
I said earlier that what remains of the star’s core after the explosion collapses to form a neutron star. That is usually the case, but not always. The mass of this remaining core is only a fraction of the giant star’s initial mass, but it can still be equal to several Suns. If its mass is greater than about three solar masses, then its immense gravity will overcome even neutron degeneracy pressure – and then there is absolutely nothing which can halt its collapse. In this case, the end product is one of the most bizarre objects in the Universe – a black hole.
A black hole represents the ultimate state of gravitational collapse – an object which has literally crushed itself out of existence. In theory, it shrinks to what we call a singularity – a point mass of zero dimensions and infinite density. It’s like a bottomless pit of gravity, from which nothing, not even light, can escape. The physics of black holes is well beyond the scope of this essay!
So finally, we put all this together. Our Sun and its planets, with all the heavy elements which make up the rocky planets such as the Earth, formed from the cloud of debris left by an ancient supernova – the violent death of an earlier massive star. And apart from the hydrogen, every atom which makes up you and me was created in the core of some long-dead star!
That, in my and Neil Tyson’s opinion, is the Most Amazing Fact revealed to us by science. In the immortal words of Carl Sagan, “We are star stuff”!
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