Nuclear Astrophysics

Nuclear Astrophysics is the study of nuclear processes in space. This includes fusion processes in stars, known as nucleosynthesis. However, there are many other questions of interest, including: 

Course requirements for Module 3: Nuclear Astrophysics

The Origin of Elements

Join astrophysicist Kate Womack from the University of Hull to explore the astrophysical processes that form the elements.

As Kate explained in the video above, many of the chemical elements are formed during stellar evolution - the lifecycles of stars. The image in figure 1 shows the main pathways for stellar evolution.

Figure 1: Copyright: ESA / AOES Medialab 

Stars are formed from nebulae. These are giant clouds of gas and dust composed of the remnants of previous stars. When nebulae reach sufficiently large mass, gravitational attraction causes them to collapse to form protostars. The gas and dust in the centre starts to heat up until the temperatures and pressures are high enough for fusion of hydrogen into helium to commence. This is called hydrogen burning, and these stars are called main sequence stars.

Low-mass stars

The Sun is a fairly average-sized star. Its temperature is approximately 1.5x107K in its core and, 6000K at its surface, and it has a lifetime of approximately 10 billion years (of which we are about halfway through). When the cores of stars such as our Sun run out of hydrogen fuel, the radiation pressure keeping it stable will decrease, and the star begins to collapse under gravity. This causes the core of the star to get hotter and the fusion of helium begins. This increases the radiation pressure, to balance the increase in gravity, and the star rapidly expands to form a red giant.

Red giants are cooler in the outer layers (hence appearing red) where they fuse hydrogen, but hotter in the core where helium and other elements up to carbon are created in a process called nucleosynthesis. When they run out of fuel, these stars form planetary nebulae - the outer layers are blown off the core and create the characteristic rings seen in the photo above. It is worth noting that planetary nebulae have nothing to do with planets - this was a mistake in their naming! The remaining compact core is known as a white dwarf. These are made of highly compressed carbon and oxygen, and nuclear reactions no longer occur within them.  

Very low-mass stars

The smallest and coolest stars in the main sequence are called red dwarfs. Red dwarfs are extremely long-lived because they burn through their hydrogen very slowly. Consequently, we haven’t seen the death of a red dwarf. We expect it to follow a similar path to stars such as our Sun, except they may never reach sufficiently high temperatures for helium fusion.

Massive stars

Massive stars burn much hotter than Sun-like stars (hence their blue colour) and consequently have much shorter lifetimes. Their evolution follows a similar process, however: when they begin to run out of hydrogen to burn, they form red supergiants. These can create elements up to iron-56 in their cores. When massive stars run out of fuel, they collapse under gravity. This results in a giant explosion called a supernova. The temperatures of supernovae (several billion K) are such that fusion of heavier elements can take place. What remains of the core will become either a neutron star (first discovered as pulsars) or a black hole, depending on the original mass of the star. 

We will be talking in more detail about neutron stars later in this module, and it turns out that neutron stars are really important in astrophysics and the creation of elements. We have realised very recently that colliding neutron stars are responsible for creating some of the heaviest elements in the Universe, including uranium. This is explained by nuclear astrophysicist Sophie Abrahams from the University of York in the video below:

Find out more about the lifecycles of stars with the 'Star in a Box' activity in the 'Find Out More' section at the bottom of the page. 

Activity 3.1

Stellar Evolution and Neutron Star Mergers

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The Hot CNO Cycle

There are many different nuclear processes taking place within stars at different points in their life cycles.  Understanding this allows us to determine how stars generate their light, how they evolve throughout their life, and how they die - whether in violent explosions or otherwise. It also allows us to predict which chemical elements are produced during the life and death of stars through the so-called nucleosynthesis processes.

Watch the video below to find out about one such process - the Hot CNO cycle - in which four protons are converted into a helium nucleus through alpha decay. This is an example of hydrogen fusion, but is a catalytic process where carbon (C), nitrogen (N), and oxygen (O) are used to speed up the fusion processes at the high temperatures of the Hot CNO cycle. In the same way as for chemical catalytic processes, the catalysts (CNO) are recovered at the end of each cycle, and can be reused again and again.

This process takes place in novae. This is a type of (repeatedly) exploding star, which can occur at the end of the life of a typical main-sequence star if the white dwarf is in a binary system with another star. In the video below, Dr James Keegans from the University of Hull take you through the astrophysical scenarios and the nuclear physics reactions that are key to making this process happen in stars.

Modelling the Hot CNO Cycle

We can model the Hot CNO cycle in a classroom or group situation, as explained in the video below by Jenn and Bethany from the University of York. After you watch the video, follow the instructions below to have a go in an individual format.

Activity 3.2

Modelling the Hot CNO Cycle

Have a go at modelling the Hot CNO cycle by following the instructions below:

You will need a coin, a timer (such as that on your mobile), this Hot CNO Cycle worksheet (which you can either print out or use on screen), and some way of recording your path through the Hot CNO cycle (coloured pens to write on the printed chart work best, but if you don’t have a printer, have a notebook and pen handy to keep track of your path).  

How to play

1. Start on any of the carbon or nitrogen isotopes.

2. Black squares with white text are stable isotopes. Some of them will be able to capture protons and do one or both of the following things:

If an isotope is able to do both proton capture mechanisms, then the coin is flipped once to decide which mechanism will occur

3. White squares with black text are unstable isotopes. These isotopes will decay, and some can also capture a proton if they have a long enough half-life.

Each unstable isotope has their decay mode and half-life indicated on their square. If the isotope has a short half-life, less than 180 seconds, use a stopwatch to time one whole half-life before flipping the coin. If heads is shown, the isotope decays. If tails is shown, use the stopwatch to wait another half-life before flipping the coin again. This continues until heads is flipped.

Some of the unstable isotopes have long half-lives, more than 180 seconds. In these cases the isotope will probably have captured a proton before the first half-life has elapsed. Therefore, continuously flip the coin until a heads or tails appears, depending on which proton capture mechanism can be performed. 

4. Your journey through the CNO cycle is complete when you either return to your starting point or emit an alpha particle.

5. There may be some sticking points on the table. This will be due to long half-lives and is absolutely normal and helps to understand how the CNO cycle works.  If you find yourself stuck, make a note of where and then ‘cheat’ to save sitting around for long periods of time! 

6. There are two main cycles to be found.  Try starting in different places or following different processes to see if you can find them both. 

Activity 3.3

Hot CNO Cycle Questions

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Modelling Neutron Stars

Neutron stars are extremely compact objects. The neutron stars we have observed so far are typically about 20km in diameter but about as heavy as the Sun! In the following activity, you will determine key parameters in a nuclear model called the ‘liquid drop model’. This is based on experimental mass measurements of oxygen isotopes. You will then use this to investigate what the smallest possible size of a neutron star might be. Through your investigation, you will hopefully find that your model will allow the existence of neutron stars of the size we have actually observed in the Universe.

The Liquid Drop (LD) model was the first formula designed to compute the binding energies as a function of the number of protons, neutrons and total number of nucleons. 

Activity 3.4

Modelling Neutron Stars 

Use this online tool and work through the activities given, in order to:

[Please note that this tool was originally developed for a FutureLearn course. Where it indicates that you should make a note of your value, you should do this in order to help you with the questions below (3.5 Modelling Neutron Stars).] 

Activity 3.5

Modelling Neutron Stars

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Find out more (optional)

An Astronomical Podcast

Interested in hearing more about astrophysics? Have a listen to the fantastic podcast, Syzygy, where astronomer Dr Emily Brunsden from the University of York and not-astronomer but enthusiastic science nerd Dr Chris Stewart explore the universe. 

If you've particularly enjoyed the nuclear astrophysics, you might want to start by listening to these related podcasts:

Stellar Evolution: Star in a box

Investigate the lifecycles of stars with your own Star in a Box. This great resource was created by Las Cumbres Observatory and Cardiff University. There are also lots of accompanying resources available from both Las Cumbres Observatory and Cardiff University 

Case Study: Fluorine-18

Fluorine-18 is of particular interest both in nuclear medicine and nuclear astrophysics. In nature, fluorine-18 is produced in one of the most violent explosions in our galaxy - a novae explosion, and indeed fluorine-18 production in the universe has now been observed using space telescopes. Dr Alison Laird has led experiments which determine how fluorine-18 is produced and destroyed in nova explosions, and she has shown how this can be used to model the explosions and how far away we will be able to detect individual explosions. Artificially produced fluorine-18 is critical in medical imaging, specifically in Positron Emission Tomography (PET) scanners. These scans exploit fluorine-18’s beta+ decay  to correlate the location of objects of interest in the body from the photons emitted in the annihilation of the positron (from the beta+ decay) and an electron in the body.

Currently, attempts are being made to integrate PET and Magnetic Resonance Imaging (MRI) into one machine. This would have significant medical advantages because both functional and anatomical information can then be gathered simultaneously, improving diagnostics and leading to more effective treatment. To achieve this, the scanner would require light sensors (scintillators) which are insensitive to magnetic fields. Such sensors are being developed at the University of York by Prof. David Jenkins and his team of researchers.

References: A.M. Laird, et al., Is γ-ray emission from novae affected by interference effects in the 18F(p,α)15O reaction? ,Phys. Rev. Lett. 110, 032502 (2013); D. Jenkins, Novel Scintillators and Silicon Photomultipliers for Nuclear Physics and Applications, 2015 J. Phys.: Conf. Ser. 620 012001 (2015).