When a massive star is ejected from its birthplace, it crosses the interstellar environment and the interaction of its stellar wind with the matter which makes up this environment leads to the formation of a hydrodynamic bow shock in front of the star. The material along this arc is somewhat compressed and its temperature is somewhat higher than the material which has not yet crossed the bowshock. This leads to structures located notably in the infrared close to some of these runaway stars.  These bow shocks are also the site of interesting physical processes, including the acceleration of particles to speeds approaching that of light. A theoretical model predicted that the bow shocks associated with these runaway stars could accelerate particles (electrons and protons) to the point where they become capable of emitting high energy radiation such as gamma rays. For Michaël De Becker, of the Department of Astrophysics, Geophysics and Oceanography of the University of Liege, the star HD195592 conforms to this model which makes it the first gamma-ray emitting runaway star ever to be discovered.

The principal theme of the research of Michaël De Becker, senior lecturer at the Department of Astrophysics, Geophysics and Oceanography of the University of Liege, is the acceleration of particles in massive stars. More often, this involves systems made up of two massive stars as these usually form a binary system where two stars (sometimes three) orbit around their common center of mass. A star is said to be massive if its mass is equivalent to at least ten times that of the Sun. These are the least common stars and represent only a tiny fraction of a galaxy such as ours. One might therefore assume that they are of little importance. On the contrary, they are the most luminous, contributing greatly to the overall brightness of a galaxy, and they go further than the nuclear fusion reactions which occur in the core of stars because the temperature of their core is much higher than that of the Sun for example.  On the other hand, because these stars are very bright, the light exerts a high radiation pressure to the point where it progressively expels their outer layers which leads to the formation of stellar winds (see article: The stellar wind reveals its secrets). These stellar winds enrich the interstellar environment with chemical elements and are a source of mechanical energy. The interstellar environment is therefore not fixed. These stars have another characteristic: they generally finish their evolution by an explosion (supernova). During this explosion, great quantities of matter are released into the interstellar environment and, as other types of nuclear reactions are then produced; new chemical elements are deposited there. 

“My research in high energy astrophysics focuses on another particularity of massive stars. We must remember that our galaxy is crossed by a high-energy particle flux, cosmic rays. These are charged particles, most often protons, helium nuclei and to a lesser extent, nuclei of other chemical elements. These are accelerated almost to the speed of light but their origins are varied. While some of these cosmic rays, those with very high energy, are produced outside our galaxy, those with less energy come from massive stars in our own galaxy at different stages of their evolution”, explains Michaël De Becker.