Nuclear Astrophysics

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.

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. 

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.

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.