An intergalactic star or a rogue star, is a star not gravitationally bound to any galaxy. Although a source of much discussion in the scientific community during the late 1990s, intergalactic stars are now generally thought to have originated in galaxies, like other stars, but later expelled as the result of either colliding galaxies or of a multiple-star system travelling too close to a supermassive black hole, which are found at the center of many galaxies.
The hypothesis that stars exist only in galaxies was disproven in 1997 with the discovery of intergalactic stars. The first to be discovered were in the Virgo cluster of galaxies, where some one trillion are now surmised to exist.
These are largely approximately 300,000 light years away from the nearest galaxy. Although the precise mass of the intergalactic star population cannot be known exactly, it is estimated that locally they make up 10 percent of the mass of the Virgo cluster of galaxies (and most likely, this total outweighs any of its 2500 galaxies).
The way these stars arise is still a mystery, but several scientifically credible hypotheses have been suggested and published by astrophysicists.
The most common hypothesis is that the collision of two or more galaxies can toss some stars out into the vast empty regions of intergalactic space. Although stars normally reside within galaxies, they can be expelled by gravitational forces when galaxies collide.
It is commonly believed that intergalactic stars may primarily have originated from extremely small galaxies, since it is easier for stars to escape a smaller galaxy’s gravitational pull, than that of a large galaxy. However, when large galaxies collide, some of the gravitational disturbances might also expel stars.
Another hypothesis, that is not mutually exclusive to the galactic collisions hypothesis, is that intergalactic stars were ejected from their galaxy of origin by a close encounter with the supermassive black hole in the galaxy center, should there be one. In such a scenario, it is likely that the intergalactic star(s) was originally part of a multiple star system where the other stars were pulled into the supermassive black hole and the soon-to-be intergalactic star was accelerated and ejected away at very high speeds.
Such an event could theoretically accelerate a star to such high speeds that it becomes a hypervelocity star, thereby escaping the gravitational well of the entire galaxy. In this respect, model calculations (from 1988) predicts the supermassive black hole in the center of our Milky Way galaxy to expel one star every 100,000 years on average.
In 2005, at the Smithsonian Center for Astrophysics, Warren Brown and his team attempted to measure the speeds of hypervelocity stars by using the Doppler Technique, by which light is observed for the similar changes that occur in sound when an object is moving away or toward something. But the speeds found are only estimated minimums, as in reality their speeds may be larger than the speeds found by the researchers.
“One of the newfound exiles is moving in the direction of the constellation Ursa Major at about 1.25 million mph with respect to the galaxy. It is 240,000 light-years away. The other is headed toward the constellation Cancer, outbound at 1.43 million miles per hour and 180,000 light-years away.”
As of 2015, approximately 675 rogue stars have been discovered at the edge of the Milky Way, towards the Andromeda Galaxy.