In dense stellar clusters like galactic nuclei and globular clusters stellar
densities are so high that stars might physically collide with each other. In
galactic nuclei the energy and power output can be close, and even exceed, to
those from supernovae events. We address the event rate and the electromagnetic characteristics of collisions of main sequence stars (MS) and red giants (RG).
We also investigate the case in which the cores form a binary and emit
gravitational waves. In the case of RGs this is particularly interesting
because the cores are degenerate. We find that MS event rate can be as high as tens per year, and that of RGs one order of magnitude larger. The collisions
are powerful enough to mimic supernovae- or tidal disruptions events. We find
Zwicky Transient Facility observational data which seem to exhibit the features
we describe. The cores embedded in the gaseous debris experience a friction
force which has an impact on the chirping mass of the gravitational wave. As a
consequence, the two small cores in principle mimic two supermassive black
holes merging. However, their evolution in frequency along with the precedent
electromagnetic burst and the ulterior afterglow are efficient tools to reveal
the impostors. In the particular case of RGs, we derive the properties of the
degenerate He cores and their H-burning shells to analyse the formation of the
binaries. The merger is such that it can be misclassified with SN Ia events.
Because the masses and densities of the cores are so dissimilar in values
depending on their evolutionary stage, the argument about standard candles and cosmic ladder should be re-evaluated.