The scenario you're describing, a collision between two massive stars like UY Scuti and Stephenson 2-18 at near light speed, would have catastrophic and highly complex consequences. It's important to note that such a scenario is purely hypothetical, as there are currently no known mechanisms that would allow stars to travel at or near the speed of light.
However, let's consider the hypothetical situation for the sake of discussion. UY Scuti and Stephenson 2-18 are both red supergiant stars and among the largest known stars in the universe. If they were to collide at near light speed, several things would likely happen:
Gravitational disruption: The gravitational forces between these massive stars would be immense. As they approach each other, their gravitational interactions would cause significant distortions in their shapes and tidal forces on their surfaces.
Shockwave and energy release: The collision would generate an enormous amount of energy in the form of intense shockwaves, radiation, and thermal energy. The release of this energy would likely be cataclysmic and could result in the ejection of vast amounts of matter into space.
Stellar merger or destruction: Depending on the angle and velocity of the collision, the stars could potentially merge into a single, more massive star or undergo a partial or complete disruption. The exact outcome would depend on various factors such as their masses, velocities, and the impact angle.
Formation of new objects: The collision could lead to the formation of new objects, such as a binary system, an accretion disk, or even trigger the birth of new stars if the conditions are favorable.
Given the extreme nature of this hypothetical scenario, it is challenging to predict the exact outcome with certainty. Complex hydrodynamic simulations would be necessary to model the physics and dynamics of such an event accurately.
It's worth reiterating that stars cannot travel at or near the speed of light based on our current understanding of physics. The speed of light is an absolute speed limit in the universe according to Einstein's theory of relativity.