Professor Simon Jeffery

Simon’s astronomical life started in Edinburgh, and has taken him via a Physics degree at Imperial College, London, to St Andrews, Scotland and Kiel, Germany, before finally moving to the Armagh Observatory and Planetarium in Northern Ireland, where he works as a senior research astronomer. He is also adjunct Professor of Physics at Trinity College Dublin and recently held a Visiting Byfellowship at Churchill College, Cambridge.

Following a lifelong interest in how stars work and how they vary over time, Simon's PhD in theoretical stellar structure and evolution was followed by pursuing observational and theoretical work on stellar pulsations and atmospheres. Most stars never fully exhaust their initial hydrogen store, but retain a hydrogen surface to the very end. However, in rare and extreme cases, some stars become true ‘helium’ stars. Simon’s goal is ultimately to demonstrate their elusive origins. The surprising conclusion is that the great majority appear to have formed from the merger of two very old and faint stars … a double white dwarf. His favourite is the pulsating V652 Herculis -- the ‘born-again rocket star’.

Simon and his wife Angela have three children. Simon would like to spend more time dinghy racing, sings baritone, and hunts big game and wild seascapes with a camera.


Hydrogen-deficient stars

All stars are born from a mixture dominated by hydrogen, with some 30% helium (by mass) and a sprinkling of heavier elements. They shine by converting hydrogen to helium and helium to carbon in their dieppe interiors. Whilst the interior chemistry is radially change,  their surfaces normally remain pristine and hydrogen-rich.

However, a few classes of star have somehow lost some or all of their original hydrogen. Our research explores the traumatic evolution of these stellar exotics by measuring surface chemistries with spectroscopy and probing their interiors with pulsation.


Like all physical objects, stars posses natural oscillatory modes which may be excited and become visible as pulsations.  In hot stars the excitation of pulsations is usually  a valve mechanism involving the opacity of one or more ionizing elements. The combination of a unique spectrum of oscillations and the chemistry necessary to drive them underpins the study of stellar interiors by asteroseismology.

New classes of pulsating stars continue to be discovered in all parts of the Hetzspring-Russell diagrams. Our research explores the mechanisms responsible for driving pulsations in evolved hot stars.