  In galactic nuclei, stellar densities are so high that stars can physically
collide with each other. In this work we focus on the collision of red giants
and in particular on the formation of non-thermal processes through collisions
and their properties. We analytically address these points by evaluating
head-on collisions but also take into account scenarios with a deviation from
the radial orbit, which we treat in a perturbative fashion. The collisions
produce internal shocks with supersonic Mach numbers. Almost immediately,
jet-like structures with important Lorentz factors form. The debris from the
collision produces another shock wave which, when interacting with the
interstellar medium of a galactic nucleus, leads to particle acceleration. We
estimate the background flux in X- and gamma rays created by the background of
these collisions by deriving the spectral index within a radius of 100 Mpc and
find that they are high. Additionally, we make an estimate of the neutrino
production and find about $10^{11}$ neutrinos per square meter per second for a
collision at 100 Mpc from Earth. Also, we derive that there is a non-negligible
chance to ignite fusion during the collision, due to the squeezing of the
material. We investigate the possibility that the degenerate cores collide with
each other, leading to a high afterglow luminosity, and find that it is
non-negligible, although this should be addressed with dedicated numerical
simulations. Colliding red giants in galactic nuclei trigger a plethora of
high-energy phenomena, and have a particular gravitational wave emission
associated, as shown by us, so that their detection will allow us to rule out
alternatives.
