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Chris Ruiz | TRIUMF | Canada

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How to do experiments with a star (I)

I’ve had a few requests to explain exactly what ‘Nuclear Astrophysics’ involves, for the layman. In my first post I mentioned that, since we can’t touch a star only observe its light, all we can do is to try and make a ‘little piece’ of a star in the laboratory to study. That is still a cartoon of what we really do. I will try to explain more clearly here and in future posts what is involved in studying the inner workings of stars.

Nuclear Astrophysics relates astronomical observations (top left) to models of exotic stellar phenomena (bottom left, classical nova) by studying the microscopic nuclear reactions involved (top right) by experiment (bottom right)

Nuclear Astrophysics relates astronomical observations (top left) to models of exotic stellar phenomena (bottom left, classical nova) by studying the microscopic nuclear reactions involved (top right) by experiment (bottom right)

Stars are big balls of high temperature ‘burning’ gas held together by gravity, and mostly hydrogen and helium, the most abundant elements in the universe. Many of you will know that our sun generates energy by fusing together hydrogen nuclei (we talk about nuclei here since in the hot, dense plasma of stars electrons move freely rather than being bound in atoms) into helium nuclei. Although this is a rare occurrence, it is made possible by virtue of the incredible density and number of particles within the sun’s core, as well as the spooky but real world of quantum mechanics, which allows two protons to get close enough to ‘fuse’ whereas classical physics rules would prohibit them from doing so.

Some of you will also know that everything we and the planet Earth are made of, right up to the heaviest natural element uranium, is synthesized within successive evolutionary stages of stars through a series of nuclear fusion reactions. Stars can then spread this heavy material around by either ejecting it in powerful winds, or by exploding and spewing the material into space. (The latter would include a supernova, probably the most well known type of ‘exploding star’). 

Some of the stages of this ‘nucleosynthesis’ have been known in outline form for a long time, for example the carbon-nitrogen-oxygen cycle originally outlined in a paper by Hans Bethe in 1939, while the first step of the main set of nuclear reactions that generate energy in the sun was first properly treated in a 1938 paper by Bethe and Critchfield* but was first suggested by Eddington in 1920. The opening lines of Bethe and Critchfield’s paper are quoted here:

It seems now generally accepted that the energy production in most stars is due to nuclear reactions involving light elements. Of all the elements, hydrogen is favored by its large abundance, by its large internal energy which makes a considerable energy evolution possible, and by its small charge and mass which enable it to penetrate easily through nuclear potential barriers. Again, the most primitive is the combination of two protons to form a deuteron, with the positron emission:

                                                H + H = D + e

Excuse the tech language there. It is important to realize that this was before the time of satellites, space exploration, rockets, the atomic bomb, electronics, computers. The transistor had only just been developed, and the field of nuclear reactions had only been alive for about 6 years since Cockroft and Walton’s accelerator experiment at the Cavendish Laboratory in 1932. Moreover, it had only been confirmed that the sun was composed mainly of hydrogen less than 15 year previously. However, the field of quantum mechanics was well established, and quite frankly, the scientists of the time were intellectual giants. It always amazes me that from so little information, and in so few decades, we went from not even knowing what atoms were to knowing what powers our own sun!

Anyway, the point is all of energy generated in stars occurs from nuclear processes, and that it what drives the evolution of stars forward, synthesizing heavier and heavier elements. Most of the details of the reactions involving the light elements are known very well from a long 50+ years of study. However, the processes that take place in more exotic scenarios like dying stars, involving the heavier elements, are not so well known. It is the goal of the field of Nuclear Astrophysics to understand exactly how all the elements were made, and also to understand the exploding, dying stars that produce them.

Since we cannot make an entire star, you might think we can make a small piece of one. That too however is problematic. Instead we say to ourselves that we understand quite well the physics of hot gases, and to some extent the physics of magnetic fields and turbulent motion. We can create a computational model (now using powerful computers) of a star, whose properties such as composition, temperature, density etc depend on the energy deposited by individual nuclear reactions. Since it is extremely difficult to calculate what the ‘strength’ of each reaction is, we must measure them individually by experiment.

So in essence, we measure each nuclear reaction of importance and that becomes another ‘building block’ to add to our model of stars, each time refining the model. The model then gives us predictions, such as the luminosity of the star, or the synthesis of certain types of nuclei within it, and we can then compare what we actually see from the real stars (by a satellite-based telescope for example), to our model to see if we get it right. So by combining astronomy with these laboratory nuclear physics experiments, we can actually understand the microscopic goings-on in the stars of our universe, including the gigantic supernova explosions and powerful x-ray bursters (Google those)!!

What’s so special about what we do at TRIUMF? Well, in order to study a nuclear reaction directly, for example a proton fusing to some…oxygen let’s say, you would usually take some oxygen gas and fire a proton at it using a particle accelerator, observing the high-energy photons (gamma rays) that are produced in the process as your signal that fusion has taken place. However the vast majority of nuclear reactions that take place in stars involve radioactive nuclei that are so short-lived that we can’t make a ‘gas target’ out of them to fire protons (or anything else) at. Therefore we do things in reverse: we actually create the short lived particles in a high-energy nuclear reaction with TRIUMF’s cyclotron accelerator, then accelerate those exotic particles to the energy that they would typically have inside the star of interest, and then we hit a hydrogen (or helium) gas target with them, before they have the chance to decay, to observe the fusion. Thus we need a ‘Radioactive Ion Beam Facility’, which is what ISAC at TRIUMF is.

In the interests of keeping each post from stretching into an essay, I will stop here for now but next time, I will tell you about just how we perform the fusion reactions using the unique DRAGON instrument here at TRIUMF. Any questions feel free to leave a comment 🙂

 

 


 *‘The Formation of Deuterons by Proton Combination’, H.A. Bethe & C. L. Critchfield, Physical Review 54, 1938

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