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Yeah, so not impossible, but still, unlikely... Its unlikely that you will win the lotto, but every week, somebody does...
In a close cluster of galaxies, there are going to be collisions which will no doubt send stars out of the reach of their progenitor galaxy... And in a large cluster of galaxies, this will happen "Often". How many stars get sent out astray when two galaxies collide? I would say definitely more than one... The combined gravitational influence of a galaxy cluster will hold them "near by" (in intergalactic terms)...
My comments regarding how likely it is were more based on the fact that there are roughly 10^11 ~ 10^12 stars in our galaxy, and perhaps 10^11 ~ 10^12 galaxies in the universe. Using these somewhat arbitrary, although according to ESO, somewhat accurate numbers, there are roughly 10^22 ~ 10^24 (1,000,000,000,000,000,000,000,000) stars in the universe... There would need to be a hell of a lot of intergalactic stars for it to be considered statistically significant... and a hell of a lot more than that for it to be considered "common"
Do stars that are still bound to their parent galaxy cluster's gravitational influence still count as intergalactic stars? They may not be in a galaxy, but a group of galaxies still controls its velocity and location, I would assume that the stars orbit around the cluster will eventually decay, and it will once again become a part of one of the galaxies within the cluster.. Perhaps not before another collision within the cluster sends more orphans out..
My thoughts are that a true intergalactic star, not under the gravitational influence of any galaxy and or galaxy cluster would be a rarity... In my uninformed opinion anyway..
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