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An intergalactic star is one that does not belong to and is not in a galaxy. These are also known as stellar outcasts, and an additional, unofficial name is tramp stars. Intergalactic stars probably formed in a galaxy, but an event of some sort could have flung the stars out, leaving them on their own. The concept of an intergalactic star was hypothetical until 1997, when the Hubble telescope observed several in a region of the universe known as the Virgo cluster, a group of galaxies that, from the Earth, look like they are in the constellation of Virgo.
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. One is that the stars were part of colliding galaxies that ejected the stars in the process of coming together. Another hypothetical process is a multistar system coming too close to a black hole, with one of the stars in the system crossing the event horizon and falling into the black hole, and the others being repelled somehow, eventually combining to form an intergalactic star.
The stars that the Hubble telescope observed were red giants. The first hint that the intergalactic stars might actually exist came when astronomers found planetary nebulae outside of galaxies in the Virgo cluster. A planetary nebula forms as part of the process that occurs when a star is near the end of its lifespan, and if planetary nebulae were outside galaxies, it implied there had been stars outside those galaxies previously. Astronomers then compared shots of the Hubble Deep Field (HDF), a deep-space image of galaxies, to an image taken of a relatively dark section of the Virgo cluster. If intergalactic stars existed there, the astronomers thought they would find additional but faint spots of light, and they did, confirming the existence of intergalactic stars.