What are the Intergalactic stars?

An intergalactic star, also known as an intracluster 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.

Stellar outcasts apparently aren’t that rare. The estimated number of intergalactic stars in the Virgo cluster alone may be over one trillion. Despite the vast amount of these stars, astronomers think the night-sky view from a planet orbiting an intergalactic star wouldn’t be very exciting. The star isn’t in a galaxy, so while there might be a few far-off galaxies that would be visible, inhabitants wouldn’t have the crowded, starry skies humans can see from Earth. The effect would be even worse if the planet does not have a moon.

How the stars became intergalactic isn’t exactly known, but there may be a couple of possible ways.

The hypothesis that stars exist only in galaxies was eventually disproven in 1997 with the discovery of intergalactic stars. The first to be discovered were within the Virgo cluster of galaxies, where about one trillion are now surmised to exist.

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.

This illustration shows one possible mechanism for how the star HE 0437-5439 acquired enough energy to be ejected from our Milky Way galaxy. It is likely that the intergalactic star would have originally been part of a multiple star system where the other star or stars were pulled into the supermassive black hole and the soon-to-be intergalactic star was accelerated and ejected away at very high speeds.

Collectively, intergalactic stars are referred to as the intracluster stellar population, or IC population for short.

Illustration Credit: NASA, ESA, O. Gnedin (University of Michigan, Ann Arbor), and W. Brown (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.)