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4.4 Nuclear Astrophysics
4.4.1 Introduction
The scope of nuclear astrophysics
From the first few seconds of the Big Bang which created
the seed material for our universe, through to the
present energy generation in our Sun which keeps us
alive, nuclear physics has shaped the evolution of the
universe and our place in it. Along the way, nuclear
reactions have controlled the evolution and death of
stars forming the most compact objects in the Universe,
determined the chemical evolution of galaxies and produced
the elements from which we ourselves are built.
Our understanding of this complex evolution has developed
as a result of nuclear physicists working closely
with cosmologists, astrophysicists and astronomers in
a hugely productive collaborative effort to understand
the development of the universe and our place in it. Some
of the key questions for the field are:
• How and where are the elements made
• Can we understand, and recreate on Earth, the critical
reactions that drive the energy generation and the
associated synthesis of new elements in Stars
• How does the fate of a star depend on the nuclear
reactions that control its evolution
• What are the properties of dense matter in a compact
star such as a neutron star or a hypothetical quark
star
Over the last decade our understanding has expanded
enormously as the result of intensive experimental and
theoretical activity. New highly sensitive techniques have
been developed to analyse specks of stardust which
arrive at the Earth in meteorites. X-ray and γ-ray detectors
onboard satellites have revealed detailed information
on the ongoing nucleosynthesis in the universe and will
soon provide direct measurements of element production
in Novae, X-ray Bursters and Supernovae. Detectors
in deep underground laboratories have picked up the
elusive neutrinos from our sun and supernova, directly
probing the nuclear reactions in the sun’s core and confirming
the general features of current supernova models.
A rich variety of neutron stars have been discovered,
revealing remarkable phenomena, which in turn have triggered
a burst of theoretical studies. Theoretical models
have been developed to the stage where we can begin
to elaborate realistic simulations of neutron stars. More
understanding will come with the advent of new powerful
instruments and the development of gravitational-wave
astronomy. In parallel we have developed a new generation
of facilities which for the first time allow us to
directly measure the key nuclear reactions involved in
the dramatic outbursts in explosive astrophysical sites.
These facilities also provide new experimental tools for
Figure 1. This image is of a speck of star dust which condensed in
the ejecta of a distant Nova and which has been brought to Earth
as an inclusion in a meteorite. Having these original materials
available here on Earth enables us to use the extremely sensitive
techniques like AMS to proble the detailed isotopic composition,
revealing clues as to the environment in which the nucleosynthesis
occurred.
probing the properties of dense nuclear matter that can
be found in the interior of neutron stars.
As this understanding has developed, we have come
to recognize three main classes of nucleosynthesis; the
nuclear reactions which occurred in the first few minutes
of the universe and led to the production of the
primordial hydrogen and helium (Big Bang nucleosynthesis),
the reactions which occur during the life of a
star and which provide the energy which stabilises the
star against gravitational collapse (Hydrostatic equilibrium
nucleosynthesis) and the more complex reactions
which occur in more dramatic objects like Novae, X-ray
Bursters, Supernovae etc. when stars reach the end of
their life cycles (Explosive nucleosynthesis). In recent
years increasing attention is being directed at understanding
the possible existence of novel states of matter
in compact objects such as neutron stars. A detailed
understanding of all these objects is required to follow
Galactic Chemical Evolution throughout the history of
the Universe. Later in this chapter we review the present
state of knowledge, and identify the key areas where new
measurements are needed to enable our understanding
to advance.
The role of nuclear experiment and theory
Whether it is the steady release of energy in the core
of a star which stabilises it against the crushing grip of
gravity, or the violent, explosive energy release in Novae,
X-ray Bursters and Supernovae, it is nuclear energy that
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