Stars are the most important objects in the Universe. Even though they make up less than 3% of the matter they make up 100% of the visible matter. They are a major apparatus for studying the universe and its evolution. This includes the remnants left from their deaths and the discarded material from stellar winds during their life. The evolution of a star is qualitatively understood. It begins on the main sequence, burning hydrogen to helium. There it spends the largest fraction of its lifetime. Once a helium core has formed, hydrogen continues to burn in a shell around the core and the stellar radius swells and the star becomes a red giant or supergiant. In the centre of the core helium burning then ignites forming a carbon and oxygen.
Stars with initial masses less than about 8 times the mass of the sun eventually form a carbon-oxygen core is and become asymptotic-giant branch stars. No further nuclear reactions occur in the centre of the star but the two burning shells of hydrogen and helium continue to burn by a series of thermal pulses in which a large amount of nucleosynthesis of heavy elements occurs. Eventually these stars lose their hydrogen envelopes by stellar winds and the carbon-oxygen core is exposed and becomes a white dwarf.
For stars with initial masses greater than 8 times the mass of the sun later burning stages occur. After the formation of a carbon-oxygen core progressively heavier elements burn preventing stellar collapse until an iron core is formed. With no further energy source to prevent the core from collapsing, a neutron star or a black hole is formed and the star explodes as a supernova or long gamma-ray burst. These events, and the strong stellar winds these stars have, make them the an important source of energy and nucleosynthesis products in the interstellar medium. Thus massive stars are important to understand to understand the structure and evolution of galaxies.
In addition the evolution of any star can be strongly affected if it is in a binary system. The evolution of a binary only differs from that of single stars if the size of the stars is similar to that of the orbit. If this occurs stars can lose mass, possibly transferring some to a companion. Mass-transfer can have dramatic consequences for both stars.
The research into stellar evolution at the IoA encompasses all aspects of stellar evolution but the main themes currently are binary stars, magnetic fields, stellar winds, supernova progenitors, progenitors of gamma-ray bursts, spectral synthesis of resolved and unresolved stellar populations and asymptotic-giant branch stars. The research is primarily theoretical in nature. Our main code is the novel and versatile Cambridge STARS code to produce stellar models to study the above phenomena. In addition we also have strong links with observational groups around the world for which we provide theoretical input.
Sverre Aarseth's research into numerical simulations of many-body (N-body) gravitational interactions has developed into a set of FORTRAN codes which describe the dynamics very closely, and freely downloadable.