An intergalactic star, also known as a rogue star, is a star that is not located within a galaxy. These stars were a source of much discussion in the scientific community during the late 1990s and are generally thought to be the result of galaxies colliding.
The common belief 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.
Although the way in which these stars form is still a mystery, the most common theory is that the collision of two or more galaxies can toss certain stars out into the vast regions of empty space.
Observation history 
In 1997, the Hubble telescope discovered a large number of intergalactic stars in the Virgo cluster of galaxies. Later in the 1990s scientists discovered another group of intergalactic stars in the Fornax cluster of galaxies.
Although the precise mass of these stars cannot be known, it is estimated that they take up 10 percent of the mass of the Virgo cluster of galaxies. This means that most likely these stars have a larger mass than any particular one of the 2500 galaxies that form the Virgo cluster.
The first intergalactic stars were discovered in the Virgo cluster of galaxies. These stars form a massive group approximately 300,000 light years away from the nearest galaxy.
See also 
- "NewsCenter - Hubble Finds Intergalactic Stars (01/14/1997) - Introduction". HubbleSite. 1997-01-14. Retrieved 2010-12-09.
- "NewsCenter - Hubble Finds Intergalactic Stars (01/14/1997) - Release Text". HubbleSite. 1997-01-14. Retrieved 2010-12-09.