Would a head-on collision between two stars create heavier elements? I was thinking about Przybylski's Star, and I was wondering how it was possible that so many heavy elements ended up in the star, such as einsteinium, californium, berkelium, etc. But there is unusually low amounts of iron and nickel. Also, I read about the LIGO detection of a neutron star merger. In my layman mind, it seems like a much more likely scenario for a neutron star merger is that the two become ensnared by each others' gravity and the two slowly collide, losing momentum from Chandrasekhar friction. This seems like a much more likely event than two neutron stars heading in opposite directions, each traveling 200km/s, meeting head-on. It's my guess that this is less likely, because I heard that when the Milky Way and Andromeda merge, it's likely not a single star will collide. So, all this together, I am wondering if two neutron stars met in this high speed, head-on manner, if that would produce ultra-heavy elements like seen in Przybylski's Star?
 A: I looked at the Gopka paper that claims the detection of Einsteinium and I don't find it compelling. Strange unexplained lines are quite common in stellar spectroscopy and finding one that matches one identified transition in a particular ionisation state does not (yet) convince me.
That aside, heavy elements get into new stars in the same way. By being produced in earlier generations, mixed into the interstellar medium and then incorporated into a new generation of stars.
Very heavy elements are produced by the r-process. This is very active during neutron star mergers and the resultant "kilonova" explosions. There is some observational evidence that kilonovae do indeed result in the production of heavy r-process elements in their ejecta.
It is also probable that supernovae make some contribution.
Note these neutron star merger events are not head-on collisions of two random neutron stars, but the result of the orbital evolution and merger of two neutron stars most likely born in a binary system. The merger is not slow; a simple application of Kepler's third law will tell you that when the surface of two $\sim 1.5$ solar mass neutron stars touch, when they are $\sim 20$ km apart, they will be travelling at a large fraction of the speed of light.
The difficulty with Einsteinium is the short half life. If its existence could be confirmed then, because of its short half-life, it would have to have been produced within the star. A suggestion is that it could be the product of decay from much longer-lived nuclei, perhaps associated with an "island of stability", hypothesised to exist at even larger atomic number.
The crusts of neutron stars are full of exotic, neutron-rich nuclei; most of which have never been seen in a lab. They are stabilised by electron degeneracy pressure. The neutron star merger will liberate some of this crustal material and some of it will undergo further neutron capture in the explosion. Mostly it will decay into "normal" heavy metals like iron and lead, but if there were a plausible site to produce new "island of stability" elements, this would be it.
A: Whether we consider the usual gravitational in-spiral case or an unlikely head-on collision, two neutron stars that are close enough to collide will also have powerful gravitational forces between them, accelerating them to high speeds.
By conservation of energy, you can expect their velocity to be roughly the escape velocity $v^{2} \sim \frac{GM}{r}$ where $M$ is the mass of a neutron star and $r$ is the distance between them. When they collide, $r$ will simply become the radius of the neutron star (when they are touching, their centers of mass will be roughly 2r apart). For the mass and radius of neutron stars, the escape velocity is tenths of the speed of light.
So they slam together at a speed tenths the speed of light, providing the energy to eject some of the material. This neutron-rich material, freed from the pressures of being in a neutron star, undergoes the r-process and indeed creates heavier elements.
I don't know about Przybylski's star in particular.
