Intergalactic star

An intergalactic star, also known as a rogue star, is a star not gravitationally bound to 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 either of galaxies colliding, or of a binary star pair travelling too close to a black hole (which can be found at the center of many galaxies).

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.

The discovery of intergalactic stars addresses the "missing baryon problem"—baryons are a class of subatomic particles including the protons and neutrons that make up the cores of the atoms in normal matter, but astrophysicists and cosmologists usually refer to any ordinary matter as baryons, thus including electrons. The missing baryon problem is identified by many theories of formation and evolution of the universe, which point out that there should be many more baryons than those that scientists have found.

The way these stars arise is still a mystery, but the most common theory 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 extracted by gravitational forces when galaxies collide. It is commonly believed that these intergalactic stars may also have come from extremely small galaxies, since it is easier for stars to escape a smaller galaxy's gravitational pull.

Another theory (an example of which is shown in the image below) states that stars can be ejected from their galaxy by supermassive black holes. In this theory, it's likely that the soon-to-be intergalactic star is a part of a binary star system where one of the stars is pulled into the supermassive black hole at the center of a galaxy and the other is ejected away after travelling too close to the aforementioned black hole. The star that has been ejected from the galaxy's center is travelling at high speeds to escape the gravitational pull of the galaxy, and is now called a hypervelocity star.

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