Hypervelocity stars are stars with sufficiently large velocities that they are no longer gravitationally bound to the galaxy. While ordinary stars have velocities on the order of 100 kilometers per second, hypervelocity stars have velocities on the order of 1000 kilometers per second. At such velocities, hypervelocity stars will escape their home galaxy forever and become lone wanderers of intergalactic space. In February 2010, I wrote and posted a short article about hypervelocity stars, including possible mechanisms that can lead to such stars.
For a moment, imagine a Sun-like hypervelocity star with an entire system of planets orbiting it, where one of the planets is an Earth-like world that is not too different from ours. The Sun-like star and its system of planets are traveling in excess of 1000 kilometers per second, on a trajectory that has already taken them far from the home galaxy. How will the night sky from the surface of such an Earth-like world appear as it wanders the dark and immense distances of intergalactic space?
In the vast and starless voids of intergalactic space, the night sky from the surface of this Earth-like planet will be totally devoid of any stars. Assuming that the Earth-like planet and its parent star left their home galaxy a hundred million years ago, an alien observer on the surface of the Earth-like planet will be able to see the entire galaxy as a disk of wispy arms spiraling out from a glowing central bulge. The galaxy will span across a huge swath of the starless night sky and none of the hundreds of billions of stars that make up the galaxy will be individually distinguishable with unaided eyes. Every several years or so, a star in the galaxy will end its life in a supernova explosion and it will appear as a brilliant point of light which will suddenly appear and gradually fade.
It may take a huge leap of imagination and ingenuity for an extraterrestrial civilization living on this planet to realize that each of the hundreds of billions of miniscule points of light that comprise the galaxy are actually stars not too different from their yellow Sun. Imagine the idea of interstellar space travel in such a scenario where instead of a mere 4.37 light years away, the nearest stars are many thousands of light years away! Additionally, this extraterrestrial civilization might even contemplate about the possibilities of other extraterrestrial civilizations living on worlds within the galaxy and how these civilizations might possibly figure out the shape of the galaxy in which they live in!