365daysDate: May 18, 2009

Title: The Life of a Proton


Podcaster: Nik Whitehead

Description: The universe around us is made up of many different types of atoms, yet only two types, hydrogen and helium, were created in the Big Bang. Other elements have been formed by a wide variety of fascinating astrophysical processes since then. This podcast will take you on a quick tour of some of these processes by following the life story of one of the simplest things in the universe – a proton – from its birth in the Big Bang to its death, swallowed up by a black hole.

Bio: Nik is a lecturer in computer science at the University of Akureyri in northern Iceland… but computer science is not her passion. She has a Bachelors degree in astronomy and astrophysics then took her Masters and Doctoral degrees in computer science when she realised that there are not enough jobs in astronomy to go around. What she’d really like to be when she grows up is either the navigator of the starship Enterprise or maybe a space traffic controller.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Richard Drumm, The Astronomy Bum, HTTP://, in memory of Captain Roland Jones, who first introduced me to the stars.


Hello! Welcome to the International Year of Astronomy podcast for May 18th. I’m Nik Whitehead, talking to you once more from Akureyri in Iceland, where I teach computer science at the university. Although computing is my day job, astronomy is my first love, and I’m delighted to be able to contribute again to the 365 Days of Astronomy series.

The universe is a fascinating place, full of interesting objects and amazing processes. I’m going to give you a quick tour of some of these by looking at the life story of one of the simplest things in the universe – a proton. We’re going to follow it from its birth in the big bang until its death, swallowed up by a black hole. In between these we’ll look at some of the different things that might happen to it.

Most cosmologists today are agreed that all matter in the universe was created approximately 13.7 billion years ago in the big bang. Atoms were not created immediately – the first things to come into existence were the elementary particles like quarks and electrons, and it took about a millionth of a second for the universe to cool down enough for protons and neutrons to form. Our newly-born proton was moving at relativistic speeds and was in serious danger of hitting other particles. At this point there were almost the same number of protons and anti-protons in the universe; they then annihilated each other when they collided. Fortunately our proton was one of the tiny proportion – thought to be about one in thirty million – that survived this great annihilation period.

Within a few minutes of the Big Bang the universe had cooled enough for the free protons and neutrons to start combining into deuterium and helium in a process known as Big Bang nucleosynthesis. By now all of the matter in the universe had formed; all that was left for it to do was to undergo the different processes that would turn it into the familiar stuff we see today.

Our proton was one of the roughly 75% of all protons to remain free, able to travel across the universe until it felt the slight gravitational attraction of a region of gas that was a little denser than the rest of the gas around it. More and more protons and other nuclei were captured by these clumps of gas, and by 500 million years after the Big Bang these clumps had become proto-galaxies. The dense clouds of hydrogen and helium begin to collapse under their own mass to form protostars. As they collapsed they heated up until their cores were hot enough for nuclear fusion to begin and for the star to be born.

Nuclear fusion occurs within the core of stars, and in main sequence stars like out sun this takes place via what is known as the proton-proton chain. Two protons fuse to form a deuterium nucleus made up of one proton and one neutron. This then fuses with another free proton to form a Helium-3 nucleus with two protons and one neutron. At this point normally one of two things can happen. Two Helium-3 nuclei can fuse together with two free protons to form two Helium-4 nuclei. This process accounts for about 86% of the Helium formed. The other common way in which Helium is formed is for two Helium-3 nuclei to fuse together to produce a single Beryllium-7 nucleus, which decays into a Lithium-7 nucleus before fusing with another free proton to form two Helium-4 nuclei.

Our proton, though, managed to avoid getting involved in any of these processes. Perhaps it spent its time in the star’s outer atmosphere, far from the stellar core where fusion takes place. These first generation stars were massive and evolved very rapidly, fusing heavier and heavier elements in their cores until eventually they exploded as supernovae, ejecting not only their atmospheres but also their cores out into space, creating heavier elements in the explosion than they could ever create by normal fusion alone.

Cast out into space in the supernova explosion, our proton eventually finds itself as part of another giant molecular cloud. These clouds are typically about 100 light years in diameter and contain millions of solar masses of material. In the cooler environment of the cloud the proton pairs up with an electron to form a hydrogen atom, which then pairs up with a similar atom to form a hydrogen molecule.

The clouds contain not only molecular hydrogen, but also heavier elements than were in the original clouds formed just after the big bang, so the second generation stars formed from the cloud will contain more of these heavy elements in their atmospheres. As it happens, the section of the cloud containing our proton collapsed into an average-sized star. Only the largest stars end their lives as supernovae, and this one will instead eventually swell up into a red giant star before shedding its atmosphere as a planetary nebula, throwing out our proton at a velocity of several kilometres per second together with about 40% of the star’s mass before settling down as a white dwarf.

Yet again our proton floats around for a while before becoming part of yet another molecular cloud. Here it joins up with another hydrogen atom and an oxygen atom to form a water molecule. Eventually the cloud collapses to form a third-generation star surrounded by a large protoplanetary disc. Here molecules of heavier elements clump together to create the dust grains that eventually form planetesimals, the building blocks of the planets. Some water molecules end up in the minerals making up these rocks, while other form clumps of ice which eventually go to form the outer planets and their moons, Those clumps of ice and rock that don’t form planets end up careening around the solar system as comets. The water atom containing our proton crashes into a small rocky planet in the star’s goldilocks zone and becomes part of a great ocean, where it is eventually taken up into a sequence of living creatures and excreted again.

This brings out proton from the Big Bang to the present day, through three generations of star formation. But what will happen to it in the future?

Eventually the star expands, heating up the its planets and our water molecule gains enough energy to evaporate from the planetary surface and is eventually ejected from the solar system to float in yet another molecular cloud. This cloud, called a Bok globule, is much smaller than the others, less than a few hundred solar masses. When it collapses it forms two stars in close orbit around each other – a binary system. As the collapsing gas heats up our water molecule is split apart and the proton can go free once more, into the atmosphere of the smaller of the two stars.

The star’s larger partner is a huge star of over twenty solar masses. A star this large evolves rapidly, first becoming a red giant and then burning ever-heavier elements in its core until it eventually goes supernova and collapses. Even after the supernova explosion the stellar core is over three solar masses, much too large to collapse into a white dwarf, a neutron star or even a quark star. The star therefore collapses into that most exotic of objects, a black hole. When the smaller star finally evolves to its red giant phase, its extended atmosphere begins to be pulled towards its binary companion forming an accretion disc around the stellar remnant.

If the companion star was a white dwarf or neutron star, our proton might eventually become part of the thin layer of hydrogen on the star’s surface, or become part of a nova eruption and either fused into helium or shot off into space again to join yet another gas cloud. But our proton’s fate is more final than that. Over time its orbit will change and the proton will get closer and closer to the black hole until it eventually crosses the hole’s event horizon and disappears from the visible universe.

What happens to it then? We don’t really know. If we were to stand a safe distance from the event horizon and watch our proton being drawn towards it we would never actually see it cross the event horizon due to the effects of time dilation caused by the mass of the hole. There are a lot of theories about what might happen within the event horizon, depending upon the structure and physical properties of the black hole, but for now these are just that – theories. Some of them include exotic ideas such as wormholes and white holes by which our proton might reappear in our universe in some other point in space and time, but there is no way as yet that we can prove that these exotic things exist.

This mysterious ending only adds to the excitement of our proton’s story. Maybe in time we’ll advance our scientific knowledge enough to be able to be more certain about our proton’s fate. For now, though, we’ll have to end with a cliffhanger and the thrilling prospect of Life of A Proton: The Sequel.

End of podcast:

365 Days of Astronomy
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