Why does this galaxy have such a long curvy tail coming off it? That river of sunlike stars, going into deep space, was pulled away from NGC 7714 due to the gravitational pull of a galaxy passing by.

This galaxy was spotted by the Hubble Space Telescope in four separate exposures. NASA writes:

Though the universe is full of such colliding galaxies that are distorted in a gravitational taffy-pull, NGC 7714 is particularly striking for the seeming fluidity of the stars along a vast arc. The near-collision between the galaxies happened at least 100 million years ago.

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More details, including a way bigger image, at the link. [HubbleSite]